Category Archives: qt

Doing It Right examples on autotools, qmake, cmake and meson

About

I finished my earlier work on build environment examples. Illustrating how to do versioning on shared object files right with autotools, qmake, cmake and meson. You can find it here.

The DIR examples are examples for various build environments on how to create a good project structure that will build libraries that are versioned with libtool or have versioning that is equivalent to what libtool would deliver, have a pkg-config file and have a so called API version in the library’s name.

What is right?

Information on this can be found in the autotools mythbuster docs, the libtool docs on versioning and freeBSD’s chapter on shared libraries. I tried to ensure that what is written here works with all of the build environments in the examples.

libpackage-4.3.so.2.1.0, what is what?

You’ll notice that a library called ‘package’ will in your LIBDIR often be called something like libpackage-4.3.so.2.1.0

We call the 4.3 part the APIVERSION, and the 2.1.0 part the VERSION (the ABI version).

I will explain these examples using semantic versioning as APIVERSION and either libtool’s current:revision:age or a semantic versioning alternative as field for VERSION (like in FreeBSD and for build environments where compatibility with libtool’s -version-info feature ain’t a requirement).

Noting that with libtool’s -version-info feature the values that you fill in for current, age and revision will not necessarily be identical to what ends up as suffix of the soname in LIBDIR. The formula to form the filename’s suffix is, for libtool, “(current – age).age.revision”. This means that for soname libpackage-APIVERSION.so.2.1.0, you would need current=3, revision=0 and age=1.

The VERSION part

In case you want compatibility with or use libtool’s -version-info feature, the document libtool/version.html on autotools.io states:

The rules of thumb, when dealing with these values are:

  • Increase the current value whenever an interface has been added, removed or changed.
  • Always increase the revision value.
  • Increase the age value only if the changes made to the ABI are backward compatible.

The libtool’s -version-info feature‘s updating-version-info part of libtool’s docs states:

  1. Start with version information of ‘0:0:0’ for each libtool library.
  2. Update the version information only immediately before a public release of your software. More frequent updates are unnecessary, and only guarantee that the current interface number gets larger faster.
  3. If the library source code has changed at all since the last update, then increment revision (‘c:r:a’ becomes ‘c:r+1:a’).
  4. If any interfaces have been added, removed, or changed since the last update, increment current, and set revision to 0.
  5. If any interfaces have been added since the last public release, then increment age.
  6. If any interfaces have been removed or changed since the last public release, then set age to 0.

When you don’t care about compatibility with libtool’s -version-info feature, then you can take the following simplified rules for VERSION:

  • SOVERSION = Major version
  • Major version: increase it if you break ABI compatibility
  • Minor version: increase it if you add ABI compatible features
  • Patch version: increase it for bug fix releases.

Examples when these simplified rules are or can be applicable is in build environments like cmake, meson and qmake. When you use autotools you will be using libtool and then they ain’t applicable.

The APIVERSION part

For the API version I will use the rules from semver.org. You can also use the semver rules for your package’s version:

Given a version number MAJOR.MINOR.PATCH, increment the:

  1. MAJOR version when you make incompatible API changes,
  2. MINOR version when you add functionality in a backwards-compatible manner, and
  3. PATCH version when you make backwards-compatible bug fixes.

When you have an API, that API can change over time. You typically want to version those API changes so that the users of your library can adopt to newer versions of the API while at the same time other users still use older versions of your API. For this we can follow section 4.3. called “multiple libraries versions” of the autotools mythbuster documentation. It states:

In this situation, the best option is to append part of the library’s version information to the library’s name, which is exemplified by Glib’s libglib-2.0.so.0 > soname. To do so, the declaration in the Makefile.am has to be like this:

lib_LTLIBRARIES = libtest-1.0.la

libtest_1_0_la_LDFLAGS = -version-info 0:0:0

The pkg-config file

Many people use many build environments (autotools, qmake, cmake, meson, you name it). Nowadays almost all of those build environments support pkg-config out of the box. Both for generating the file as for consuming the file for getting information about dependencies.

I consider it a necessity to ship with a useful and correct pkg-config .pc file. The filename should be /usr/lib/pkgconfig/package-APIVERSION.pc for soname libpackage-APIVERSION.so.VERSION. In our example that means /usr/lib/pkgconfig/package-4.3.pc. We’d use the command pkg-config package-4.3 –cflags –libs, for example.

Examples are GLib’s pkg-config file, located at /usr/lib/pkgconfig/glib-2.0.pc

The include path

I consider it a necessity to ship API headers in a per API-version different location (like for example GLib’s, at /usr/include/glib-2.0). This means that your API version number must be part of the include-path.

For example using earlier mentioned API-version 4.3, /usr/include/package-4.3 for /usr/lib/libpackage-4.3.so(.2.1.0) having /usr/lib/pkg-config/package-4.3.pc

What will the linker typically link with?

The linker will for -lpackage-4.3 typically link with /usr/lib/libpackage-4.3.so.2 or with libpackage-APIVERSION.so.(current – age). Noting that the part that is calculated as (current – age) in this example is often, for example in cmake and meson, referred to as the SOVERSION. With SOVERSION the soname template in LIBDIR is libpackage-APIVERSION.so.SOVERSION.

What is wrong?

Not doing any versioning

Without versioning you can’t make any API or ABI changes that wont break all your users’ code in a way that could be managable for them. If you do decide not to do any versioning, then at least also don’t put anything behind the .so part of your so’s filename. That way, at least you wont break things in spectacular ways.

Coming up with your own versioning scheme

Knowing it better than the rest of the world will in spectacular ways make everything you do break with what the entire rest of the world does. You shouldn’t congratulate yourself with that. The only thing that can be said about it is that it probably makes little sense, and that others will probably start ignoring your work. Your mileage may vary. Keep in mind that without a correct SOVERSION, certain things will simply not work correct.

In case of libtool: using your package’s (semver) release numbering for current, revision, age

This is similarly wrong to ‘Coming up with your own versioning scheme’.

The Libtool documentation on updating version info is clear about this:

Never try to set the interface numbers so that they correspond to the release number of your package. This is an abuse that only fosters misunderstanding of the purpose of library versions.

This basically means that once you are using libtool, also use libtool’s versioning rules.

Refusing or forgetting to increase the current and/or SOVERSION on breaking ABI changes

The current part of the VERSION (current, revision and age) minus age, or, SOVERSION is/are the most significant field(s). The current and age are usually involved in forming the so called SOVERSION, which in turn is used by the linker to know with which ABI version to link. That makes it … damn important.

Some people think ‘all this is just too complicated for me’, ‘I will just refuse to do anything and always release using the same version numbers’. That goes spectacularly wrong whenever you made ABI incompatible changes. It’s similarly wrong to ‘Coming up with your own versioning scheme’.

That way, all programs that link with your shared library can after your shared library gets updated easily crash, can corrupt data and might or might not work.

By updating the current and age, or, SOVERSION you will basically trigger people who manage packages and their tooling to rebuild programs that link with your shared library. You actually want that the moment you made breaking ABI changes in a newer version of it.

When you don’t want to care about libtool’s -version-info feature, then there is also a set of more simple to follow rules. Those rules are for VERSION:

  • SOVERSION = Major version (with these simplified set of rules, no subtracting of current with age is needed)
  • Major version: increase it if you break ABI compatibility
  • Minor version: increase it if you add ABI compatible features
  • Patch version: increase it for bug fix releases.

What isn’t wrong?

Not using libtool (but nonetheless doing ABI versioning right)

GNU libtool was made to make certain things more easy. Nowadays many popular build environments also make things more easy. Meanwhile has GNU libtool been around for a long time. And its versioning rules, commonly known as the current:revision:age field as parameter for -verison-info, got widely adopted.

What GNU libtool did was, however, not really a standard. It’s is one interpretation of how to do it. And a rather complicated one, at that.

Please let it be crystal clear that not using libtool does not mean that you can do ABI versioning wrong. Because very often people seem to think that they can, and think they’ll still get out safely while doing ABI versioning completely wrong. This is not the case.

Not having a APIVERSION at all

It isn’t wrong not to have an APIVERSION in the soname. It however means that you promise to not ever break API. Because the moment you break API, you disallow your users to stay on the old API for a little longer. They might both have programs that use the old and that use the new API. Now what?

When you have an APIVERSION then you can allow the introduction of a new version of the API while simultaneously the old API remains available on a user’s system.

Using a different naming-scheme for APIVERSION

I used the MAJOR.MINOR version numbers from semver to form the APIVERSION. I did this because only the MAJOR and the MINOR are technically involved in API changes (unless you are doing semantic versioning wrong – in which case see ‘Coming up with your own versioning scheme’).

Some projects only use MAJOR. Examples are Qt which puts the MAJOR number behind the Qt part. For example libQt5Core.so.VERSION (so that’s “Qt” + MAJOR + Module). The GLib world, however, uses “g” + Module + “-” + MAJOR + “.0″ as they have releases like 2.2, 2.3, 2.4 that are all called libglib-2.0.so.VERSION. I guess they figured that maybe someday in their 2.x series, they could use that MINOR field?

DBus seems to be using a similar thing to GLib, but then without the MINOR suffix: libdbus-1.so.VERSION. For their GLib integration they also use it as libdbus-glib-1.so.VERSION.

Who is right, who is wrong? It doesn’t matter too much for your APIVERSION naming scheme. As long as there is a way to differentiate the API in a) the include path, b) the pkg-config filename and c) the library that will be linked with (the -l parameter during linking/compiling). Maybe someday a standard will be defined? Let’s hope so.

Differences in interpretation per platform

FreeBSD

FreeBSD’s Shared Libraries of Chapter 5. Source Tree Guidelines and Policies states:

The three principles of shared library building are:

  1. Start from 1.0
  2. If there is a change that is backwards compatible, bump minor number (note that ELF systems ignore the minor number)
  3. If there is an incompatible change, bump major number

For instance, added functions and bugfixes result in the minor version number being bumped, while deleted functions, changed function call syntax, etc. will force the major version number to change.

I think that when using libtool on a FreeBSD (when you use autotools), that the platform will provide a variant of libtool’s scripts that will convert earlier mentioned current, revision and age rules to FreeBSD’s. The same goes for the VERSION variable in cmake and qmake. Meaning that with those tree build environments, you can just use the rules for GNU libtool’s -version-info.

I could be wrong on this, but I did find mailing list E-mails from ~ 2011 stating that this SNAFU is dealt with. Besides, the *BSD porters otherwise know what to do and you could of course always ask them about it.

Note that FreeBSD’s rules are or seem to be compatible with the rules for VERSION when you don’t want to care about libtool’s -version-info compatibility. However, when you are porting from a libtoolized project, then of course you don’t want to let newer releases break against releases that have already happened.

Modern Linux distributions

Nowadays you sometimes see things like /usr/lib/$ARCH/libpackage-APIVERSION.so linking to /lib/$ARCH/libpackage-APIVERSION.so.VERSION. I have no idea how this mechanism works. I suppose this is being done by packagers of various Linux distributions? I also don’t know if there is a standard for this.

I will update the examples and this document the moment I know more and/or if upstream developers need to worry about it. I think that using GNUInstallDirs in cmake, for example, makes everything go right. I have not found much for this in qmake, meson seems to be doing this by default and in autotools you always use platform variables for such paths.

As usual, I hope standards will be made and that the build environment and packaging community gets to their senses and stops leaving this into the hands of developers. I especially think about qmake, which seems to not have much at all to state that standardized installation paths must be used (not even a proper way to define a prefix).

Questions that I can imagine already exist

Why is there there a difference between APIVERSION and VERSION?

The API version is the version of your programmable interfaces. This means the version of your header files (if your programming language has such header files), the version of your pkgconfig file, the version of your documentation. The API is what software developers need to utilize your library.

The ABI version can definitely be different and it is what programs that are compiled and installable need to utilize your library.

An API breaks when recompiling the program without any changes, that consumes a libpackage-4.3.so.2, is not going to succeed at compile time. The API got broken the moment any possible way package’s API was used, wont compile. Yes, any way. It means that a libpackage-5.0.so.0 should be started.

An ABI breaks when without recompiling the program, replacing a libpackage-4.3.so.2.1.0 with a libpackage-4.3.so.2.2.0 or a libpackage-4.3.so.2.1.1 (or later) as libpackage-4.3.so.2 is not going to succeed at runtime. For example because it would crash, or because the results would be wrong (in any way). It implies that libpackage-4.3.so.2 shouldn’t be overwritten, but libpackage-4.3.so.3 should be started.

For example when you change the parameter of a function in C to be a floating point from a integer (and/or the other way around), then that’s an ABI change but not neccesarily an API change.

What is this SOVERSION about?

In most projects that got ported from an environment that uses GNU libtool (for example autotools) to for example cmake or meson, and in the rare cases that they did anything at all in a qmake based project, I saw people converting the current, revision and age parameters that they passed to the -version-info option of libtool to a string concatenated together using (current – age), age, revision as VERSION, and (current – age) as SOVERSION.

I wanted to use the exact same rules for versioning for all these examples, including autotools and GNU libtool. When you don’t have to (or want to) care about libtool’s set of (for some people, needlessly complicated) -version-info rules, then it should be fine using just SOVERSION and VERSION using these rules:

  • SOVERSION = Major version
  • Major version: increase it if you break ABI compatibility
  • Minor version: increase it if you add ABI compatible features
  • Patch version: increase it for bug fix releases.

I, however, also sometimes saw variations that are incomprehensible with little explanation and magic foo invented on the spot. Those variations are probably wrong.

In the example I made it so that in the root build file of the project you can change the numbers and calculation for the numbers. However. Do follow the rules for those correctly, as this versioning is about ABI compatibility. Doing this wrong can make things blow up in spectacular ways.

The examples

qmake in the qmake-example

Note that the VERSION variable must be filled in as “(current – age).age.revision” for qmake (to get 2.1.0 at the end, you need VERSION=2.1.0 when current=3, revision=0 and age=1)

To try this example out, go to the qmake-example directory and type

$ cd qmake-example
$ mkdir=_test
$ qmake PREFIX=$PWD/_test
$ make
$ make install

This should give you this:

$ find _test/
_test/
├── include
│   └── qmake-example-4.3
│       └── qmake-example.h
└── lib
    ├── libqmake-example-4.3.so -> libqmake-example-4.3.so.2.1.0
    ├── libqmake-example-4.3.so.2 -> libqmake-example-4.3.so.2.1.0
    ├── libqmake-example-4.3.so.2.1 -> libqmake-example-4.3.so.2.1.0
    ├── libqmake-example-4.3.so.2.1.0
    ├── libqmake-example-4.la
    └── pkgconfig
        └── qmake-example-4.3.pc

When you now use pkg-config, you get a nice CFLAGS and LIBS line back (I’m replacing the current path with $PWD in the output each time):

$ export PKG_CONFIG_PATH=$PWD/_test/lib/pkgconfig
$ pkg-config qmake-example-4.3 --cflags
-I$PWD/_test/include/qmake-example-4.3
$ pkg-config qmake-example-4.3 --libs
-L$PWD/_test/lib -lqmake-example-4.3

And it means that you can do things like this now (and people who know about pkg-config will now be happy to know that they can use your library in their own favorite build environment).

$ export LD_LIBRARY_PATH=$PWD/_test/lib
$ echo -en "#include <qmake-example.h>\nmain() {} " > test.cpp
$ g++ -fPIC test.cpp -o test.o `pkg-config qmake-example-4.3 --libs --cflags`

You can see that it got linked to libqmake-example-4.3.so.2, where that 2 at the end is (current – age).

$ ldd test.o 
    linux-gate.so.1 (0xb77b0000)
    libqmake-example-4.3.so.2 => $PWD/_test/lib/libqmake-example-4.3.so.2 (0xb77a6000)
    libstdc++.so.6 => /usr/lib/i386-linux-gnu/libstdc++.so.6 (0xb75f5000)
    libm.so.6 => /lib/i386-linux-gnu/libm.so.6 (0xb759e000)
    libgcc_s.so.1 => /lib/i386-linux-gnu/libgcc_s.so.1 (0xb7580000)
    libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb73c9000)
    /lib/ld-linux.so.2 (0xb77b2000)

cmake in the cmake-example

Note that the VERSION property on your library target must be filled in with “(current – age).age.revision” for cmake (to get 2.1.0 at the end, you need VERSION=2.1.0 when current=3, revision=0 and age=1. Note that in cmake you must also fill in the SOVERSION property as (current – age), so SOVERSION=2 when current=3 and age=1).

To try this example out, go to the cmake-example directory and do

$ cd cmake-example
$ mkdir _test
$ cmake -DCMAKE_INSTALL_PREFIX:PATH=$PWD/_test
-- Configuring done
-- Generating done
-- Build files have been written to: .
$ make
[ 50%] Building CXX object src/libs/cmake-example/CMakeFiles/cmake-example.dir/cmake-example.cpp.o
[100%] Linking CXX shared library libcmake-example-4.3.so
[100%] Built target cmake-example
$ make install
[100%] Built target cmake-example
Install the project...
-- Install configuration: ""
-- Installing: $PWD/_test/lib/libcmake-example-4.3.so.2.1.0
-- Up-to-date: $PWD/_test/lib/libcmake-example-4.3.so.2
-- Up-to-date: $PWD/_test/lib/libcmake-example-4.3.so
-- Up-to-date: $PWD/_test/include/cmake-example-4.3/cmake-example.h
-- Up-to-date: $PWD/_test/lib/pkgconfig/cmake-example-4.3.pc

This should give you this:

$ tree _test/
_test/
├── include
│   └── cmake-example-4.3
│       └── cmake-example.h
└── lib
    ├── libcmake-example-4.3.so -> libcmake-example-4.3.so.2
    ├── libcmake-example-4.3.so.2 -> libcmake-example-4.3.so.2.1.0
    ├── libcmake-example-4.3.so.2.1.0
    └── pkgconfig
        └── cmake-example-4.3.pc

When you now use pkg-config, you get a nice CFLAGS and LIBS line back (I’m replacing the current path with $PWD in the output each time):

$ pkg-config cmake-example-4.3 --cflags
-I$PWD/_test/include/cmake-example-4.3
$ pkg-config cmake-example-4.3 --libs
-L$PWD/_test/lib -lcmake-example-4.3

And it means that you can do things like this now (and people who know about pkg-config will now be happy to know that they can use your library in their own favorite build environment):

$ echo -en "#include <cmake-example.h>\nmain() {} " > test.cpp
$ g++ -fPIC test.cpp -o test.o `pkg-config cmake-example-4.3 --libs --cflags`

You can see that it got linked to libcmake-example-4.3.so.2, where that 2 at the end is the SOVERSION. This is (current – age).

$ ldd test.o
    linux-gate.so.1 (0xb7729000)
    libcmake-example-4.3.so.2 => $PWD/_test/lib/libcmake-example-4.3.so.2 (0xb771f000)
    libstdc++.so.6 => /usr/lib/i386-linux-gnu/libstdc++.so.6 (0xb756e000)
    libm.so.6 => /lib/i386-linux-gnu/libm.so.6 (0xb7517000)
    libgcc_s.so.1 => /lib/i386-linux-gnu/libgcc_s.so.1 (0xb74f9000)
    libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb7342000)
    /lib/ld-linux.so.2 (0xb772b000)

autotools in the autotools-example

Note that you pass -version-info current:revision:age directly with autotools. The libtool will translate that to (current – age).age.revision to form the so’s filename (to get 2.1.0 at the end, you need current=3, revision=0, age=1).

To try this example out, go to the autotools-example directory and do

$ cd autotools-example
$ mkdir _test
$ libtoolize
$ aclocal
$ autoheader
$ autoconf
$ automake --add-missing
$ ./configure --prefix=$PWD/_test
$ make
$ make install

This should give you this:

$ tree _test/
_test/
├── include
│   └── autotools-example-4.3
│       └── autotools-example.h
└── lib
    ├── libautotools-example-4.3.a
    ├── libautotools-example-4.3.la
    ├── libautotools-example-4.3.so -> libautotools-example-4.3.so.2.1.0
    ├── libautotools-example-4.3.so.2 -> libautotools-example-4.3.so.2.1.0
    ├── libautotools-example-4.3.so.2.1.0
    └── pkgconfig
        └── autotools-example-4.3.pc

When you now use pkg-config, you get a nice CFLAGS and LIBS line back (I’m replacing the current path with $PWD in the output each time):

$ export PKG_CONFIG_PATH=$PWD/_test/lib/pkgconfig
$ pkg-config autotools-example-4.3 --cflags
-I$PWD/_test/include/autotools-example-4.3
$ pkg-config autotools-example-4.3 --libs
-L$PWD/_test/lib -lautotools-example-4.3

And it means that you can do things like this now (and people who know about pkg-config will now be happy to know that they can use your library in their own favorite build environment):

$ echo -en "#include <autotools-example.h>\nmain() {} " > test.cpp
$ export LD_LIBRARY_PATH=$PWD/_test/lib
$ g++ -fPIC test.cpp -o test.o `pkg-config autotools-example-4.3 --libs --cflags`

You can see that it got linked to libautotools-example-4.3.so.2, where that 2 at the end is (current – age).

$ ldd test.o 
    linux-gate.so.1 (0xb778d000)
    libautotools-example-4.3.so.2 => $PWD/_test/lib/libautotools-example-4.3.so.2 (0xb7783000)
    libstdc++.so.6 => /usr/lib/i386-linux-gnu/libstdc++.so.6 (0xb75d2000)
    libm.so.6 => /lib/i386-linux-gnu/libm.so.6 (0xb757b000)
    libgcc_s.so.1 => /lib/i386-linux-gnu/libgcc_s.so.1 (0xb755d000)
    libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb73a6000)
    /lib/ld-linux.so.2 (0xb778f000)

meson in the meson-example

Note that the version property on your library target must be filled in with “(current – age).age.revision” for meson (to get 2.1.0 at the end, you need version=2.1.0 when current=3, revision=0 and age=1. Note that in meson you must also fill in the soversion property as (current – age), so soversion=2 when current=3 and age=1).

To try this example out, go to the meson-example directory and do

$ cd meson-example
$ mkdir -p _build/_test
$ cd _build
$ meson .. --prefix=$PWD/_test
$ ninja
$ ninja install

This should give you this:

$ tree _test/
_test/
├── include
│   └── meson-example-4.3
│       └── meson-example.h
└── lib
    └── i386-linux-gnu
        ├── libmeson-example-4.3.so -> libmeson-example-4.3.so.2.1.0
        ├── libmeson-example-4.3.so.2 -> libmeson-example-4.3.so.2.1.0
        ├── libmeson-example-4.3.so.2.1.0
        └── pkgconfig
            └── meson-example-4.3.pc

When you now use pkg-config, you get a nice CFLAGS and LIBS line back (I’m replacing the current path with $PWD in the output each time):

$ export PKG_CONFIG_PATH=$PWD/_test/lib/i386-linux-gnu/pkgconfig
$ pkg-config meson-example-4.3 --cflags
-I$PWD/_test/include/meson-example-4.3
$ pkg-config meson-example-4.3 --libs
-L$PWD/_test/lib -lmeson-example-4.3

And it means that you can do things like this now (and people who know about pkg-config will now be happy to know that they can use your library in their own favorite build environment):

$ echo -en "#include <meson-example.h>\nmain() {} " > test.cpp
$ export LD_LIBRARY_PATH=$PWD/_test/lib/i386-linux-gnu
$ g++ -fPIC test.cpp -o test.o `pkg-config meson-example-4.3 --libs --cflags`

You can see that it got linked to libmeson-example-4.3.so.2, where that 2 at the end is the soversion. This is (current – age).

$ ldd test.o 
    linux-gate.so.1 (0xb772e000)
    libmeson-example-4.3.so.2 => $PWD/_test/lib/i386-linux-gnu/libmeson-example-4.3.so.2 (0xb7724000)
    libstdc++.so.6 => /usr/lib/i386-linux-gnu/libstdc++.so.6 (0xb7573000)
    libm.so.6 => /lib/i386-linux-gnu/libm.so.6 (0xb751c000)
    libgcc_s.so.1 => /lib/i386-linux-gnu/libgcc_s.so.1 (0xb74fe000)
    libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb7347000)
    /lib/ld-linux.so.2 (0xb7730000)

Doing it right, making libraries using popular build environments

Enough with the political posts!

Making libraries that are both API and libtool versioned with qmake, how do they do it?

I started a project on github that will collect what I will call “doing it right” project structures for various build environments.

With right I mean that the library will have a API version in its Library name, that the library will be libtoolized and that a pkg-config .pc file gets installed for it.

I have in mind, for example, autotools, cmake, meson, qmake and plain make. First example that I have finished is one for qmake.

Let’s get started working on a libqmake-example-3.2.so.3.2.1

We get the PREFIX, MAJOR_VERSION, MINOR_VERSION and PATCH_VERSION from a project-wide include

include(../../../qmake-example.pri)

We will use the standard lib template of qmake

TEMPLATE = lib

We need to set VERSION to a semver.org version for compile_libtool (in reality it should use what is called current, revision and age to form an API and ABI version number. In the actual example it’s explained in the comments, as this is too much for a small blog post).

VERSION = $${MAJOR_VERSION}"."$${MINOR_VERSION}"."$${PATCH_VERSION}

According section 4.3 of Autotools’ mythbusters we should have as target-name the API version in the library’s name

TARGET = qmake-example-$${MAJOR_VERSION}"."$${MINOR_VERSION}

We will write a define in config.h for access to the semver.org version as a double quoted string

QMAKE_SUBSTITUTES += config.h.in

Our example happens to use QDebug, so we need QtCore here

QT = core

This is of course optional

CONFIG += c++14

We will be using libtool style libraries

CONFIG += compile_libtool
CONFIG += create_libtool

These will create a pkg-config .pc file for us

CONFIG += create_pc create_prl no_install_prl

Project sources

SOURCES = qmake-example.cpp

Project’s public and private headers

HEADERS = qmake-example.h

We will install the headers in a API specific include path

headers.path = $${PREFIX}/include/qmake-example-$${MAJOR_VERSION}"."$${MINOR_VERSION}

Here put only the publicly installed headers

headers.files = $${HEADERS}

Here we will install the library to

target.path = $${PREFIX}/lib

This is the configuration for generating the pkg-config file

QMAKE_PKGCONFIG_NAME = $${TARGET}
QMAKE_PKGCONFIG_DESCRIPTION = An example that illustrates how to do it right with qmake
# This is our libdir
QMAKE_PKGCONFIG_LIBDIR = $$target.path
# This is where our API specific headers are
QMAKE_PKGCONFIG_INCDIR = $$headers.path
QMAKE_PKGCONFIG_DESTDIR = pkgconfig
QMAKE_PKGCONFIG_PREFIX = $${PREFIX}
QMAKE_PKGCONFIG_VERSION = $$VERSION
# These are dependencies that our library needs
QMAKE_PKGCONFIG_REQUIRES = Qt5Core

Installation targets (the pkg-config seems to install automatically)

INSTALLS += headers target

This will be the result after make-install

├── include
│   └── qmake-example-3.2
│       └── qmake-example.h
└── lib
    ├── libqmake-example-3.2.so -> libqmake-example-3.2.so.3.2.1
    ├── libqmake-example-3.2.so.3 -> libqmake-example-3.2.so.3.2.1
    ├── libqmake-example-3.2.so.3.2 -> libqmake-example-3.2.so.3.2.1
    ├── libqmake-example-3.2.so.3.2.1
    ├── libqmake-example-3.la
    └── pkgconfig
        └── qmake-example-3.pc

ps. Dear friends working at their own customers: when I visit your customer, I no longer want to see that you produced completely stupid wrong qmake based projects for them. Libtoolize it all, get an API version in your Library’s so-name and do distribute a pkg-config .pc file. That’s the very least to pass your exam. Also read this document (and stop pretending that you don’t need to know this when at the same time you charge them real money pretending that you know something about modern UNIX software development).

Eigenlijk he

Eigenlijk moet ik nog eens iets posten over hoe we allerlei dingen met Qt en QML doen he?

Na zoveel jaren recruiters te vragen om naar een Duitstalige Qt/QML ontwikkelaar in Eindhoven te zoeken, zou Heidenhain er goed aan doen eens wat te laten zien waar wij aan toe zijn. Vind ik. Maar ja. Het is en blijft een bedrijf dat zelf een beetje geheimzinnig wil zijn.

De laatste tijd zijn het de details van die Klartext-editor die aan beurt zijn. M.a.w. dat wat er wel toe doet: dat wat werkers moeten gebruiken om een werkstuk in te geven in een CNC machine. Dat duurt even. Want de TNC640 heeft best wel veel specifieke dingetjes die al vele jaren ingeleerd zijn bij gebruikers van de machines.

Ik heb m.a.w. veel dingen die ik zou kunnen posten. Maar ik ben lui, plus ik moet wat opletten wat ik wel en wat ik niet publiek maak. Het grappigste daarvan is dat Nokia veel stricter was. Maar ook veel duidelijker. Daarom kon ik veel meer tijdens Nokia zeggen als nu. Nokia was kei streng in haar NDA’s (tot 180000 Euro schadevergoeding, en meer), maar wel duidelijk: dit mag je zeggen, dit niet.

Asynchronous commands

With asynchronous commands we have typical commands from the Model View ViewModel world that return asynchronously.

Whenever that happens we want result reporting and progress reporting. We basically want something like this in QML:

Item {
  id: container
  property ViewModel viewModel: ViewModel {}

  Connections {
    target: viewModel.asyncHelloCommand
    onExecuteProgressed: {
        progressBar.value = value
        progressBar.maximumValue = maximum
    }
  }
  ProgressBar {
     id: progressBar
  }
  Button {
    enabled: viewModel.asyncHelloCommand.canExecute
    onClicked: viewModel.asyncHelloCommand.execute()
  }
}

How do we do this? First we start with defining a AbstractAsyncCommand (impl. of protected APIs here):

class AbstractAsyncCommand : public AbstractCommand {
    Q_OBJECT
public:
    AbstractAsyncCommand(QObject *parent=0);

    Q_INVOKABLE virtual QFuture<void*> executeAsync() = 0;
    virtual void execute() Q_DECL_OVERRIDE;
signals:
    void executeFinished(void* result);
    void executeProgressed(int value, int maximum);
protected:
    QSharedPointer<QFutureInterface<void*>> start();
    void progress(QSharedPointer<QFutureInterface<void*>> fut, int value, int total);
    void finish(QSharedPointer<QFutureInterface<void*>> fut, void* result);
private:
    QVector<QSharedPointer<QFutureInterface<void*>>> m_futures;
};

After that we provide an implementation:

#include <QThreadPool>
#include <QRunnable>

#include <MVVM/Commands/AbstractAsyncCommand.h>

class AsyncHelloCommand: public AbstractAsyncCommand
{
    Q_OBJECT
public:
    AsyncHelloCommand(QObject *parent=0);
    bool canExecute() const Q_DECL_OVERRIDE { return true; }
    QFuture<void*> executeAsync() Q_DECL_OVERRIDE;
private:
    void* executeAsyncTaskFunc();
    QSharedPointer<QFutureInterface<void*>> current;
    QMutex mutex;
};

#include "asynchellocommand.h"

#include <QtConcurrent/QtConcurrent>

AsyncHelloCommand::AsyncHelloCommand(QObject* parent)
    : AbstractAsyncCommand(parent) { }

void* AsyncHelloCommand::executeAsyncTaskFunc()
{
    for (int i=0; i<10; i++) {
        QThread::sleep(1);
        qDebug() << "Hello Async!";
        mutex.lock();
        progress(current, i, 10);
        mutex.unlock();
    }
    return nullptr;
}

QFuture<void*> AsyncHelloCommand::executeAsync()
{
    mutex.lock();
    current = start();
    QFutureWatcher<void*>* watcher = new QFutureWatcher<void*>(this);
    connect(watcher, &QFutureWatcher<void*>::progressValueChanged, this, [=]{
        mutex.lock();
        progress(current, watcher->progressValue(), watcher->progressMaximum());
        mutex.unlock();
    });
    connect(watcher, &QFutureWatcher<void*>::finished, this, [=]{
        void* result=watcher->result();
        mutex.lock();
        finish(current, result);
        mutex.unlock();
        watcher->deleteLater();
    });
    watcher->setFuture(QtConcurrent::run(this, &AsyncHelloCommand::executeAsyncTaskFunc));
    QFuture<void*> future = current->future();
    mutex.unlock();

    return future;
}

You can find the complete working example here.

The RelayCommand in Qt

A few days ago I explained how we can do MVVM techniques like ICommand in Qt.

Today I’ll explain how to make and use a simple version of the, in the XAML MVVM world quite famous, RelayCommand. In the Microsoft Prism4 & 5 world this is DelegateCommand. Both are equivalent. I will only show a non-templated RelayCommand, so no RelayCommand<T> for now. Perhaps I’ll add a templated one to that mvvm project some other day.

What people call a delegate in C# is what C++ people call a Functor. Obviously we will use functors, then. Note that for people actually reading all those links: in C# the Action<T> and Func<T,G> are basically also C# delegates (or, functors, if you fancy C++’s names for this more).

Here is the RelayCommand.h:

#include <functional>
#include <QSharedPointer>
#include <MVVM/Commands/AbstractCommand.h>

class RelayCommand : public AbstractCommand
{
    Q_OBJECT
public:
    RelayCommand(std::function<void()> executeDelegatep,
                 std::function<bool()> canExecuteDelegatep,
                 QObject *parent = 0)
    : AbstractCommand(parent)
    , executeDelegate(executeDelegatep)
    , canExecuteDelegate(canExecuteDelegatep) {}

    void execute() Q_DECL_OVERRIDE;
    bool canExecute() const Q_DECL_OVERRIDE;
public slots:
    void evaluateCanExecute();
private:
    std::function<void()> executeDelegate;
    std::function<bool()> canExecuteDelegate;
};

The implementation is too simple to be true:

#include "RelayCommand.h"

bool RelayCommand::canExecute() const
{
    return canExecuteDelegate();
}

void RelayCommand::evaluateCanExecute()
{
    emit canExecuteChanged( canExecute() );
}

void RelayCommand::execute()
{
    executeDelegate();
}

Okay, so how do we use this? First we make a ViewModel. Because in this case we will define the command in C++. That probably means you want a ViewModel.

I added a CompositeCommand in the mix. For a Q_PROPERTY isn’t a CommandProxy really needed, as ownership stays in C++ (when for example you pass this as parent). For a Q_INVOKABLE you would need it to wrap the QSharedPointer<AbstractCommand>.

Note. I already hear you think: wait a minute, you are not passing this to the QObject’s constructor, it’s not a QScopedPointer and you have a new but no delete. That’s because CommandProxy converts the ownership rules to QQmlEngine::setObjectOwnership (this, QQmlEngine::JavaScriptOwnership) for itself. I don’t necessarily recommend its usage here (for it’s not immediately clear), but at the same time this is just a demo. You can try printing a warning in the destructor and you’ll see that the QML garbage collector takes care of it.

#include <QObject>
#include <QScopedPointer>

#include <MVVM/Commands/CommandProxy.h>
#include <MVVM/Commands/CompositeCommand.h>
#include <MVVM/Commands/RelayCommand.h>
#include <MVVM/Models/CommandListModel.h>

class ViewModel: public QObject
{
    Q_OBJECT

    Q_PROPERTY(CommandProxy* helloCommand READ helloCommand CONSTANT)
public:
    ViewModel(QObject *parent=0):QObject(parent),
        helloCmd(new CompositeCommand()){

        QSharedPointer<CompositeCommand> cCmd = helloCmd.dynamicCast<CompositeCommand>();
        cCmd->add( new RelayCommand ([=] { qWarning() << "Hello1 from C++ RelayCommand"; },
                            [=]{ return true; }));
        cCmd->add( new RelayCommand ([=] { qWarning() << "Hello2 from C++ RelayCommand"; },
                            [=]{ return true; }));
        proxyCmd = new CommandProxy (helloCmd);
    }
    CommandProxy* helloCommand() {
        return proxyCmd;
    }
private:
    QSharedPointer<AbstractCommand> helloCmd;
    CommandProxy *proxyCmd;
};

Let’s also make a very simple View.qml that uses the ViewModel

import QtQuick 2.3
import QtQuick.Window 2.0
import QtQuick.Controls 1.2

import Example 1.0

Item {
    property ViewModel viewModel: ViewModel {}

    Button {
        enabled: viewModel.helloCommand.canExecute
        onClicked: viewModel.helloCommand.execute()
    }
}

AbstractCommand Model View ViewModel techniques

In the .NET XAML world, you have the ICommand, the CompositeCommand and the DelegateCommand. You use these commands to in a declarative way bind them as properties to XAML components like menu items and buttons. You can find an excellent book on this titled Prism 5.0 for WPF.

The ICommand defines two things: a canExecute property and an execute() method. The CompositeCommand allows you to combine multiple commands together, the DelegateCommand makes it possible to pass two delegates (functors or lambda’s); one for the canExecute evaluation and one for the execute() method.

The idea here is that you want to make it possible to put said commands in a ViewModel and then data bind them to your View (so in QML that’s with Q_INVOKABLE and Q_PROPERTY). Meaning that the action of the component in the view results in execute() being called, and the component in the view being enabled or not is bound to the canExecute bool property.

In QML that of course corresponds to a ViewModel.cpp for a View.qml. Meanwhile you also want to make it possible to in a declarative way use certain commands in the View.qml without involving the ViewModel.cpp.

So I tried making exactly that. I’ve placed it on github in a project I plan to use more often to collect MVVM techniques I come up with. And in this article I’ll explain how and what. I’ll stick to the header files and the QML file.

We start with defining a AbstractCommand interface. This is very much like .NET’s ICommand, of course:

#include <QObject>

class AbstractCommand : public QObject {
    Q_OBJECT
    Q_PROPERTY(bool canExecute READ canExecute NOTIFY canExecuteChanged)
public:
    AbstractCommand(QObject *parent = 0):QObject(parent){}
    Q_INVOKABLE virtual void execute() = 0;
    virtual bool canExecute() const = 0;
signals:
    void canExecuteChanged(bool canExecute);
};

We will also make a command that is very easy to use in QML, the EmitCommand:

#include <MVVM/Commands/AbstractCommand.h>

class EmitCommand : public AbstractCommand
{
    Q_OBJECT
    Q_PROPERTY(bool canExecute READ canExecute WRITE setCanExecute NOTIFY privateCanExecuteChanged)
public:
    EmitCommand(QObject *parent=0):AbstractCommand(parent){}

    void execute() Q_DECL_OVERRIDE;
    bool canExecute() const Q_DECL_OVERRIDE;
public slots:
    void setCanExecute(bool canExecute);
signals:
    void executes();
    void privateCanExecuteChanged();
private:
    bool canExe = false;
};

We make a command that allows us to combine multiple commands together as one. This is the equivalent of .NET’s CompositeCommand, here you have our own:

#include <QSharedPointer>
#include <QQmlListProperty>

#include <MVVM/Commands/AbstractCommand.h>
#include <MVVM/Commands/ListCommand.h>

class CompositeCommand : public AbstractCommand {
    Q_OBJECT

    Q_PROPERTY(QQmlListProperty<AbstractCommand> commands READ commands NOTIFY commandsChanged )
    Q_CLASSINFO("DefaultProperty", "commands")
public:
    CompositeCommand(QObject *parent = 0):AbstractCommand (parent) {}
    CompositeCommand(QList<QSharedPointer<AbstractCommand> > cmds, QObject *parent=0);
    ~CompositeCommand();
    void execute() Q_DECL_OVERRIDE;
    bool canExecute() const Q_DECL_OVERRIDE;
    void remove(const QSharedPointer<AbstractCommand> &cmd);
    void add(const QSharedPointer<AbstractCommand> &cmd);

    void add(AbstractCommand *cmd);
    void clearCommands();
    QQmlListProperty<AbstractCommand> commands();

signals:
    void commandsChanged();
private slots:
    void onCanExecuteChanged(bool canExecute);
private:
    QList<QSharedPointer<AbstractCommand> > cmds;
    static void appendCommand(QQmlListProperty<AbstractCommand> *lst, AbstractCommand *cmd);
    static AbstractCommand* command(QQmlListProperty<AbstractCommand> *lst, int idx);
    static void clearCommands(QQmlListProperty<AbstractCommand> *lst);
    static int commandCount(QQmlListProperty<AbstractCommand> *lst);
};

We also make a command that looks a lot like ListElement in QML’s ListModel:

#include <MVVM/Commands/AbstractCommand.h>

class ListCommand : public AbstractCommand
{
    Q_OBJECT
    Q_PROPERTY(AbstractCommand *command READ command WRITE setCommand NOTIFY commandChanged)
    Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged)
public:
    ListCommand(QObject *parent = 0):AbstractCommand(parent){}
    void execute() Q_DECL_OVERRIDE;
    bool canExecute() const Q_DECL_OVERRIDE;
    AbstractCommand* command() const;
    void setCommand(AbstractCommand *newCommand);
    void setCommand(const QSharedPointer<AbstractCommand> &newCommand);
    QString text() const;
    void setText(const QString &newValue);
signals:
    void commandChanged();
    void textChanged();
private:
    QSharedPointer<AbstractCommand> cmd;
    QString txt;
};

Let’s now also make the equivalent for QML’s ListModel, CommandListModel:

#include <QObject>
#include <QQmlListProperty>

#include <MVVM/Commands/ListCommand.h>

class CommandListModel:public QObject {
    Q_OBJECT
    Q_PROPERTY(QQmlListProperty<ListCommand> commands READ commands NOTIFY commandsChanged )
    Q_CLASSINFO("DefaultProperty", "commands")
public:
    CommandListModel(QObject *parent = 0):QObject(parent){}
    void clearCommands();
    int commandCount() const;
    QQmlListProperty<ListCommand> commands();
    void appendCommand(ListCommand *command);
    ListCommand* command(int idx) const;
signals:
    void commandsChanged();
private:
    static void appendCommand(QQmlListProperty<ListCommand> *lst, ListCommand *cmd);
    static ListCommand* command(QQmlListProperty<ListCommand> *lst, int idx);
    static void clearCommands(QQmlListProperty<ListCommand> *lst);
    static int commandCount(QQmlListProperty<ListCommand> *lst);

    QList<ListCommand* > cmds;
};

Okay, let’s now put all this together in a simple example QML:

import QtQuick 2.3
import QtQuick.Window 2.0
import QtQuick.Controls 1.2

import be.codeminded.mvvm 1.0

import Example 1.0 as A

Window {
    width: 360
    height: 360
    visible: true

    ListView {
        id: listView
        anchors.fill: parent

        delegate: Item {
            height: 20
            width: listView.width
            MouseArea {
                anchors.fill: parent
                onClicked: if (modelData.canExecute) modelData.execute()
            }
            Text {
                anchors.fill: parent
                text: modelData.text
                color: modelData.canExecute ? "black" : "grey"
            }
        }

        model: comsModel.commands

        property bool combineCanExecute: false

        CommandListModel {
            id: comsModel

            ListCommand {
                text: "C++ Lambda command"
                command:  A.LambdaCommand
            }

            ListCommand {
                text: "Enable combined"
                command: EmitCommand {
                    onExecutes: { console.warn( "Hello1");
                        listView.combineCanExecute=true; }
                    canExecute: true
                }
            }

            ListCommand {
                text: "Disable combined"
                command: EmitCommand {
                    onExecutes: { console.warn( "Hello2");
                        listView.combineCanExecute=false; }
                    canExecute: true
                }
            }

            ListCommand {
                text: "Combined emit commands"
                command: CompositeCommand {
                    EmitCommand {
                        onExecutes: console.warn( "Emit command 1");
                        canExecute: listView.combineCanExecute
                    }
                    EmitCommand {
                        onExecutes: console.warn( "Emit command 2");
                        canExecute: listView.combineCanExecute
                    }
                }
            }
        }
    }
}

I made a task-bug for this on Qt, here.

Colleague tells me I write blogs in chats while I explain how to write a producer-consumer

I’m at home now. I don’t do non-public unpaid work. So let’s blog the example I’m making for him.

workplace.h

#ifndef Workplace_H
#define Workplace_H

#include <QObject>
#include <QFuture>
#include <QWaitCondition>
#include <QMutex>
#include <QStack>
#include <QList>
#include <QThread>
#include <QFutureWatcher>

class Workplace;

typedef enum {
    WT_INSERTS,
    WT_QUERY
} WorkplaceWorkType;

typedef struct {
    WorkplaceWorkType type;
    QList<int> values;
    QString query;
    QFutureInterface<bool> insertIface;
    QFutureInterface<QList<QStringList> > queryIface;
} WorkplaceWork;

class WorkplaceWorker: public QThread {
    Q_OBJECT
public:
    WorkplaceWorker(QObject *parent = NULL)
        : QThread(parent), m_running(false) { }
    void run() Q_DECL_OVERRIDE;
    void pushWork(WorkplaceWork *a_work);
private:
    QStack<WorkplaceWork*> m_ongoing;
    QMutex m_mutex;
    QWaitCondition m_waitCondition;
    bool m_running;
};

class Workplace: public QObject {
    Q_OBJECT
public:
    explicit Workplace(QObject *a_parent=0) : QObject (a_parent) {}
    bool insert(QList<int> a_values);
    QList<QStringList> query(const QString &a_param);
    QFuture<bool> insertAsync(QList<int> a_values);
    QFuture<QList<QStringList> > queryAsync(const QString &a_param);
private:
    WorkplaceWorker m_worker;
};

class App: public QObject {
    Q_OBJECT
public slots:
    void perform();
    void onFinished();
private:
    Workplace m_workplace;
};

#endif// Workplace_H

workplace.cpp

#include "workplace.h"

void App::onFinished()
{
    QFutureWatcher<bool> *watcher = static_cast<QFutureWatcher<bool>* > ( sender() );
    delete watcher;
}

void App::perform()
{
    for (int i=0; i<10; i++) {
       QList<int> vals;
       vals.append(1);
       vals.append(2);
       QFutureWatcher<bool> *watcher = new QFutureWatcher<bool>;
       connect (watcher, &QFutureWatcher<bool>::finished, this, &App::onFinished);
       watcher->setFuture( m_workplace.insertAsync( vals ) );
    }

    for (int i=0; i<10; i++) {
       QList<int> vals;
       vals.append(1);
       vals.append(2);
       qWarning() << m_workplace.insert( vals );
       qWarning() << m_workplace.query("test");
    }
}

void WorkplaceWorker::pushWork(WorkplaceWork *a_work)
{
    if (!m_running) {
        start();
    }

    m_mutex.lock();
    switch (a_work->type) {
    case WT_QUERY:
        m_ongoing.push_front( a_work );
    break;
    default:
        m_ongoing.push_back( a_work );
    }
    m_waitCondition.wakeAll();
    m_mutex.unlock();
}

void WorkplaceWorker::run()
{
    m_mutex.lock();
    m_running = true;
    while ( m_running ) {
        m_mutex.unlock();
        m_mutex.lock();
        if ( m_ongoing.isEmpty() ) {
            m_waitCondition.wait(&m_mutex);
        }
        WorkplaceWork *work = m_ongoing.pop();
        m_mutex.unlock();

        // Do work here and report progress
        sleep(1);

        switch (work->type) {
        case WT_QUERY: {
            // Report result here
            QList<QStringList> result;
            QStringList row;
            row.append("abc"); row.append("def");
            result.append(row);
            work->queryIface.reportFinished( &result );
            } break;

        case WT_INSERTS:
        default: {
            // Report result here
            bool result = true;
            work->insertIface.reportFinished( &result );
            } break;
        }

        m_mutex.lock();
        delete work;
    }
    m_mutex.unlock();
}

bool Workplace::insert(QList<int> a_values)
{
    WorkplaceWork *work = new WorkplaceWork;;
    QFutureWatcher<bool> watcher;
    work->type = WT_INSERTS;
    work->values = a_values;
    work->insertIface.reportStarted();
    watcher.setFuture ( work->insertIface.future() );
    m_worker.pushWork( work );
    watcher.waitForFinished();
    return watcher.result();
}

QList<QStringList> Workplace::query(const QString &a_param)
{
    WorkplaceWork *work = new WorkplaceWork;
    QFutureWatcher<QList<QStringList> > watcher;
    work->type = WT_QUERY;
    work->query = a_param;
    work->queryIface.reportStarted();
    watcher.setFuture ( work->queryIface.future() );
    m_worker.pushWork( work );
    watcher.waitForFinished();
    return watcher.result();
}

QFuture<bool> Workplace::insertAsync(QList<int> a_values)
{
    WorkplaceWork *work = new WorkplaceWork;
    work->type = WT_INSERTS;
    work->values = a_values;
    work->insertIface.reportStarted();
    QFuture<bool> future = work->insertIface.future();
    m_worker.pushWork( work );
    return future;
}

QFuture<QList<QStringList> > Workplace::queryAsync(const QString &a_param)
{
    WorkplaceWork *work = new WorkplaceWork;
    work->type = WT_QUERY;
    work->query = a_param;
    work->queryIface.reportStarted();
    QFuture<QList<QStringList> > future = work->queryIface.future();
    m_worker.pushWork( work );
    return future;
}

How do they do it? Asynchronous undo and redo editors

Imagine we want an editor that has undo and redo capability. But the operations on the editor are all asynchronous. This implies that also undo and redo are asynchronous operations.

We want all this to be available in QML, we want to use QFuture for the asynchronous stuff and we want to use QUndoCommand for the undo and redo capability.

But how do they do it?

First of all we will make a status object, to put the status of the asynchronous operations in (asyncundoable.h).

class AbstractAsyncStatus: public QObject
{
    Q_OBJECT

    Q_PROPERTY(bool success READ success CONSTANT)
    Q_PROPERTY(int extra READ extra CONSTANT)
public:
    AbstractAsyncStatus(QObject *parent):QObject (parent) {}
    virtual bool success() = 0;
    virtual int extra() = 0;
};

We will be passing it around as a QSharedPointer, so that lifetime management becomes easy. But typing that out is going to give us long APIs. So let’s make a typedef for that (asyncundoable.h).

typedef QSharedPointer<AbstractAsyncStatus> AsyncStatusPointer;

Now let’s make ourselves an undo command that allows us to wait for asynchronous undo and asynchronous redo. We’re combining QUndoCommand and QFutureInterface here (asyncundoable.h).

class AbstractAsyncUndoable: public QUndoCommand
{
public:
    AbstractAsyncUndoable( QUndoCommand *parent = nullptr )
        : QUndoCommand ( parent )
        , m_undoFuture ( new QFutureInterface<AsyncStatusPointer>() )
        , m_redoFuture ( new QFutureInterface<AsyncStatusPointer>() ) {}
    QFuture<AsyncStatusPointer> undoFuture()
        { return m_undoFuture->future(); }
    QFuture<AsyncStatusPointer> redoFuture()
        { return m_redoFuture->future(); }

protected:
    QScopedPointer<QFutureInterface<AsyncStatusPointer> > m_undoFuture;
    QScopedPointer<QFutureInterface<AsyncStatusPointer> > m_redoFuture;

};

Okay, let’s implement these with an example operation. First the concrete status object (asyncexample1command.h).

class AsyncExample1Status: public AbstractAsyncStatus
{
    Q_OBJECT
    Q_PROPERTY(bool example1 READ example1 CONSTANT)
public:
    AsyncExample1Status ( bool success, int extra, bool example1,
                          QObject *parent = nullptr )
        : AbstractAsyncStatus(parent)
        , m_example1 ( example1 )
        , m_success ( success )
        , m_extra ( extra ) {}
    bool example1() { return m_example1; }
    bool success() Q_DECL_OVERRIDE { return m_success; }
    int extra() Q_DECL_OVERRIDE { return m_extra; }
private:
    bool m_example1 = false;
    bool m_success = false;
    int m_extra = -1;
};

Let’s make a QUndoCommand that uses a timer to simulate asynchronous behavior. We could also use QtConcurrent’s run function to use a QThreadPool and QRunnable instances that also implement QFutureInterface, of course. Seasoned Qt developers know what I mean. For the sake of example, I wanted to illustrate that QFuture can also be used for asynchronous things that aren’t threads. We’ll use the lambda because QUndoCommand isn’t a QObject, so no easy slots. That’s the only reason (asyncexample1command.h).

class AsyncExample1Command: public AbstractAsyncUndoable
{
public:
    AsyncExample1Command(bool example1, QUndoCommand *parent = nullptr)
        : AbstractAsyncUndoable ( parent ), m_example1(example1) {}
    void undo() Q_DECL_OVERRIDE {
        m_undoFuture->reportStarted();
        QTimer *timer = new QTimer();
        timer->setSingleShot(true);
        QObject::connect(timer, &QTimer::timeout, [=]() {
            QSharedPointer<AbstractAsyncStatus> result;
            result.reset(new AsyncExample1Status ( true, 1, m_example1 ));
            m_undoFuture->reportFinished(&result);
            timer->deleteLater();
        } );
        timer->start(1000);
    }
    void redo() Q_DECL_OVERRIDE {
        m_redoFuture->reportStarted();
        QTimer *timer = new QTimer();
        timer->setSingleShot(true);
        QObject::connect(timer, &QTimer::timeout, [=]() {
            QSharedPointer<AbstractAsyncStatus> result;
            result.reset(new AsyncExample1Status ( true, 2, m_example1 ));
            m_redoFuture->reportFinished(&result);
            timer->deleteLater();
        } );
        timer->start(1000);
    }
private:
    QTimer m_timer;
    bool m_example1;
};

Let’s now define something we get from the strategy design pattern; a editor behavior. Implementations provide an editor all its editing behaviors (abtracteditorbehavior.h).

class AbstractEditorBehavior : public QObject
{
    Q_OBJECT
public:
    AbstractEditorBehavior( QObject *parent) : QObject (parent) {}

    virtual QFuture<AsyncStatusPointer> performExample1( bool example1 ) = 0;
    virtual QFuture<AsyncStatusPointer> performUndo() = 0;
    virtual QFuture<AsyncStatusPointer> performRedo() = 0;
    virtual bool canRedo() = 0;
    virtual bool canUndo() = 0;
};

So far so good, so let’s make an implementation that has a QUndoStack and that therefor is undoable (undoableeditorbehavior.h).

class UndoableEditorBehavior: public AbstractEditorBehavior
{
public:
    UndoableEditorBehavior(QObject *parent = nullptr)
        : AbstractEditorBehavior (parent)
        , m_undoStack ( new QUndoStack ){}

    QFuture<AsyncStatusPointer> performExample1( bool example1 ) Q_DECL_OVERRIDE {
        AsyncExample1Command *command = new AsyncExample1Command ( example1 );
        m_undoStack->push(command);
        return command->redoFuture();
    }
    QFuture<AsyncStatusPointer> performUndo() {
        const AbstractAsyncUndoable *undoable =
            dynamic_cast<const AbstractAsyncUndoable *>(
                    m_undoStack->command( m_undoStack->index() - 1));
        m_undoStack->undo();
        return const_cast<AbstractAsyncUndoable*>(undoable)->undoFuture();
    }
    QFuture<AsyncStatusPointer> performRedo() {
        const AbstractAsyncUndoable *undoable =
            dynamic_cast<const AbstractAsyncUndoable *>(
                    m_undoStack->command( m_undoStack->index() ));
        m_undoStack->redo();
        return const_cast<AbstractAsyncUndoable*>(undoable)->redoFuture();
    }
    bool canRedo() Q_DECL_OVERRIDE { return m_undoStack->canRedo(); }
    bool canUndo() Q_DECL_OVERRIDE { return m_undoStack->canUndo(); }
private:
    QScopedPointer<QUndoStack> m_undoStack;
};

Now we only need an editor, right (editor.h)?

class Editor: public QObject
{
    Q_OBJECT
    Q_PROPERTY(AbstractEditorBehavior* editorBehavior READ editorBehavior CONSTANT)
public:
    Editor(QObject *parent=nullptr) : QObject(parent)
        , m_editorBehavior ( new UndoableEditorBehavior ) { }
    AbstractEditorBehavior* editorBehavior() { return m_editorBehavior.data(); }
    Q_INVOKABLE void example1Async(bool example1) {
        QFutureWatcher<AsyncStatusPointer> *watcher = new QFutureWatcher<AsyncStatusPointer>(this);
        connect(watcher, &QFutureWatcher<AsyncStatusPointer>::finished,
                this, &Editor::onExample1Finished);
        watcher->setFuture ( m_editorBehavior->performExample1(example1) );
    }
    Q_INVOKABLE void undoAsync() {
        if (m_editorBehavior->canUndo()) {
            QFutureWatcher<AsyncStatusPointer> *watcher = new QFutureWatcher<AsyncStatusPointer>(this);
            connect(watcher, &QFutureWatcher<AsyncStatusPointer>::finished,
                    this, &Editor::onUndoFinished);
            watcher->setFuture ( m_editorBehavior->performUndo() );
        }
    }
    Q_INVOKABLE void redoAsync() {
        if (m_editorBehavior->canRedo()) {
            QFutureWatcher<AsyncStatusPointer> *watcher = new QFutureWatcher<AsyncStatusPointer>(this);
            connect(watcher, &QFutureWatcher<AsyncStatusPointer>::finished,
                    this, &Editor::onRedoFinished);
            watcher->setFuture ( m_editorBehavior->performRedo() );
        }
    }
signals:
    void example1Finished( AsyncExample1Status *status );
    void undoFinished( AbstractAsyncStatus *status );
    void redoFinished( AbstractAsyncStatus *status );
private slots:
    void onExample1Finished() {
        QFutureWatcher<AsyncStatusPointer> *watcher =
                dynamic_cast<QFutureWatcher<AsyncStatusPointer>*> (sender());
        emit example1Finished( watcher->result().objectCast<AsyncExample1Status>().data() );
        watcher->deleteLater();
    }
    void onUndoFinished() {
        QFutureWatcher<AsyncStatusPointer> *watcher =
                dynamic_cast<QFutureWatcher<AsyncStatusPointer>*> (sender());
        emit undoFinished( watcher->result().objectCast<AbstractAsyncStatus>().data() );
        watcher->deleteLater();
    }
    void onRedoFinished() {
        QFutureWatcher<AsyncStatusPointer> *watcher =
                dynamic_cast<QFutureWatcher<AsyncStatusPointer>*> (sender());
        emit redoFinished( watcher->result().objectCast<AbstractAsyncStatus>().data() );
        watcher->deleteLater();
    }
private:
    QScopedPointer<AbstractEditorBehavior> m_editorBehavior;
};

Okay, let’s register this up to make it known in QML and make ourselves a main function (main.cpp).

#include <QtQml>
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <editor.h>
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    qmlRegisterType<Editor>("be.codeminded.asyncundo", 1, 0, "Editor");
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    return app.exec();
}

Now, let’s make ourselves a simple QML UI to use this with (main.qml).

import QtQuick 2.3
import QtQuick.Window 2.2
import QtQuick.Controls 1.2
import be.codeminded.asyncundo 1.0
Window {
    visible: true
    width: 360
    height: 360
    Editor {
        id: editor
        onUndoFinished: text.text = "undo"
        onRedoFinished: text.text = "redo"
        onExample1Finished: text.text = "whoohoo " + status.example1
    }
    Text {
        id: text
        text: qsTr("Hello World")
        anchors.centerIn: parent
    }
    Action {
        shortcut: "Ctrl+z"
        onTriggered: editor.undoAsync()
    }
    Action {
        shortcut: "Ctrl+y"
        onTriggered: editor.redoAsync()
    }
    Button  {
        onClicked: editor.example1Async(99);
    }
}

You can find the sources of this complete example at github. Enjoy!

Asynchronous undoable and redoable APIs

Combining QFuture with QUndoCommand made a lot of sense for us. The undo and the redo methods of the QUndoCommand can also be asynchronous, of course. We wanted to use QFuture without involving threads, because our asynchronosity is done through a process and IPC, and not a thread. It’s the design mistake of QtConcurrent‘s run method, in my opinion. That meant using QFutureInterface instead (which is undocumented, but luckily public – so it’ll remain with us until at least Qt’s 6.y.z releases).

So how do we make a QUndoCommand that has a undo, and that has a redo method that returns a asynchronous QFuture<ResultType>?

We just did that, today. I’m very satisfied with the resulting API and design. It might have helped if QUndoStack would be a QUndoStack<T> and QUndoCommand would have been a QUndoCommand<T> with undo and redo’s return type being T. Just an idea for the Qt 6.y.z developers.

Making something that is ‘undoable editable’ with Qt

Among the problems we’ll face is that we want asynchronous APIs that are undoable and that we want to switch to read only, undoable editing, non-undoable editing and that QML doesn’t really work well with QFuture. At least not yet. We want an interface that is easy to talk with from QML. Yet we want to switch between complicated behaviors.

We will also want synchronous mode and asynchronous mode. Because I just invented that requirement out of thin air.

Ok, first the “design”. We see a lot of behaviors, for something that can do something. The behaviors will perform for that something, the actions it can do. That is the strategy design pattern, then. It’s the one about ducks and wing fly behavior and rocket propelled fly behavior and the ostrich that has a can’t fly behavior. For undo and redo, we have the command pattern. We have this neat thing in Qt for that. We’ll use it. We don’t reinvent the wheel. Reinventing the wheel is stupid.

Let’s create the duck. I mean, the thing-editor as I will use “Thing” for the thing that is being edited. We want copy (sync is sufficient), paste (must be aysnc), and edit (must be async). We could also have insert and delete, but those APIs would be just like edit. Paste is usually similar to insert, of course. Except that it can be a combined delete and insert when overwriting content. The command pattern allows you to make such combinations. Not the purpose of this example, though.

Enough explanation. Let’s start! The ThingEditor, is like the flying Duck in strategy. This is going to be more or less the API that we will present to the QML world. It could be your ViewModel, for example (ie. you could let your ThingViewModel subclass ThingEditor).

class ThingEditor : public QObject
{
    Q_OBJECT

    Q_PROPERTY ( ThingEditingBehavior* editingBehavior READ editingBehavior
                 WRITE setEditingBehavior NOTIFY editingBehaviorChanged )
    Q_PROPERTY ( Thing* thing READ thing WRITE setThing NOTIFY thingChanged )

public:
    explicit ThingEditor( QSharedPointer<Thing> &a_thing,
            ThingEditingBehavior *a_editBehavior,
            QObject *a_parent = nullptr );

    explicit ThingEditor( QObject *a_parent = nullptr );

    Thing* thing() const { return m_thing.data(); }
    virtual void setThing( QSharedPointer<Thing> &a_thing );
    virtual void setThing( Thing *a_thing );

    ThingEditingBehavior* editingBehavior() const { return m_editingBehavior.data(); }
    virtual void setEditingBehavior ( ThingEditingBehavior *a_editingBehavior );

    Q_INVOKABLE virtual void copyCurrentToClipboard ( );
    Q_INVOKABLE virtual void editCurrentAsync( const QString &a_value );
    Q_INVOKABLE virtual void pasteCurrentFromClipboardAsync( );

signals:
    void editingBehaviorChanged ();
    void thingChanged();
    void editCurrentFinished( EditCurrentCommand *a_command );
    void pasteCurrentFromClipboardFinished( EditCurrentCommand *a_command );

private slots:
    void onEditCurrentFinished();
    void onPasteCurrentFromClipboardFinished();

private:
    QScopedPointer<ThingEditingBehavior> m_editingBehavior;
    QSharedPointer<Thing> m_thing;
    QList<QFutureWatcher<EditCurrentCommand*> *> m_editCurrentFutureWatchers;
    QList<QFutureWatcher<EditCurrentCommand*> *> m_pasteCurrentFromClipboardFutureWatchers;
};

For the implementation of this class, I’ll only provide the non-obvious pieces. I’m sure you can do that setThing, setEditingBehavior and the constructor yourself. I’m also providing it only once, and also only for the EditCurrentCommand. The one about paste is going to be exactly the same.

void ThingEditor::copyCurrentToClipboard ( )
{
    m_editingBehavior->copyCurrentToClipboard( );
}

void ThingEditor::onEditCurrentFinished( )
{
    QFutureWatcher<EditCurrentCommand*> *resultWatcher
            = static_cast<QFutureWatcher<EditCurrentCommand*>*> ( sender() );
    emit editCurrentFinished ( resultWatcher->result() );
    if (m_editCurrentFutureWatchers.contains( resultWatcher )) {
        m_editCurrentFutureWatchers.removeAll( resultWatcher );
    }
    delete resultWatcher;
}

void ThingEditor::editCurrentAsync( const QString &a_value )
{
    QFutureWatcher<EditCurrentCommand*> *resultWatcher
            = new QFutureWatcher<EditCurrentCommand*>();
    connect ( resultWatcher, &QFutureWatcher<EditCurrentCommand*>::finished,
              this, &ThingEditor::onEditCurrentFinished, Qt::QueuedConnection );
    resultWatcher->setFuture ( m_editingBehavior->editCurrentAsync( a_value ) );
    m_editCurrentFutureWatchers.append ( resultWatcher );
}

For QUndo we’ll need a QUndoCommand. For each undoable action we indeed need to make such a command. You could add more state and pass it to the constructor. It’s common, for example, to pass Thing, or the ThingEditor or the behavior (this is why I used QSharedPointer for those: as long as your command lives in the stack, you’ll need it to hold a reference to that state).

class EditCurrentCommand: public QUndoCommand
{
public:
    explicit EditCurrentCommand( const QString &a_value,
                                 QUndoCommand *a_parent = nullptr )
        : QUndoCommand ( a_parent )
        , m_value ( a_value ) { }
    void redo() Q_DECL_OVERRIDE {
       // Perform action goes here
    }
    void undo() Q_DECL_OVERRIDE {
      // Undo what got performed goes here
    }
private:
    const QString &m_value;
};

You can (and probably should) also make this one abstract (and/or a so called pure interface), as you’ll usually want many implementations of this one (one for every kind of editing behavior). Note that it leaks the QUndoCommand instances unless you handle them (ie. storing them in a QUndoStack). That in itself is a good reason to keep it abstract.

class ThingEditingBehavior : public QObject
{
    Q_OBJECT

    Q_PROPERTY ( ThingEditor* editor READ editor WRITE setEditor NOTIFY editorChanged )
    Q_PROPERTY ( Thing* thing READ thing NOTIFY thingChanged )

public:
    explicit ThingEditingBehavior( ThingEditor *a_editor,
                                   QObject *a_parent = nullptr )
        : QObject ( a_parent )
        , m_editor ( a_editor ) { }

    explicit ThingEditingBehavior( QObject *a_parent = nullptr )
        : QObject ( a_parent ) { }

    ThingEditor* editor() const { return m_editor.data(); }
    virtual void setEditor( ThingEditor *a_editor );
    Thing* thing() const;

    virtual void copyCurrentToClipboard ( );
    virtual QFuture<EditCurrentCommand*> editCurrentAsync( const QString &a_value, bool a_exec = true );
    virtual QFuture<EditCurrentCommand*> pasteCurrentFromClipboardAsync( bool a_exec = true );

protected:
    virtual EditCurrentCommand* editCurrentSync( const QString &a_value, bool a_exec = true );
    virtual EditCurrentCommand* pasteCurrentFromClipboardSync( bool a_exec = true );

signals:
    void editorChanged();
    void thingChanged();

private:
    QPointer<ThingEditor> m_editor;
    bool m_synchronous = true;
};

That setEditor, the constructor, etc: these are too obvious to write here. Here are the non-obvious ones:

void ThingEditingBehavior::copyToClipboard ( )
{
}

EditCurrentCommand* ThingEditingBehavior::editCurrentSync( const QString &a_value, bool a_exec )
{
    EditCurrentCommand *ret = new EditCurrentCommand ( a_value );
    if ( a_exec )
        ret->redo();
    return ret;
}

QFuture<EditCurrentCommand*> ThingEditingBehavior::editCurrentAsync( const QString &a_value, bool a_exec )
{
    QFuture<EditCurrentCommand*> resultFuture =
            QtConcurrent::run( QThreadPool::globalInstance(), this,
                               &ThingEditingBehavior::editCurrentSync,
                               a_value, a_exec );
    if (m_synchronous)
        resultFuture.waitForFinished();
    return resultFuture;
}

And now we can make the whole thing undoable by making a undoable editing behavior. I’ll leave a non-undoable editing behavior as an exercise to the reader (ie. just perform redo() on the QUndoCommand, don’t store it in the QUndoStack and immediately delete or cmd->deleteLater() the instance).

Note that if m_synchronous is false, that (all access to) m_undoStack, and the undo and redo methods of your QUndoCommands, must be (made) thread-safe. The thread-safety is not the purpose of this example, though.

class UndoableThingEditingBehavior : public ThingEditingBehavior
{
    Q_OBJECT
public:
    explicit UndoableThingEditingBehavior( ThingEditor *a_editor,
                                           QObject *a_parent = nullptr );
protected:
    EditCellCommand* editCurrentSync( const QString &a_value, bool a_exec = true ) Q_DECL_OVERRIDE;
    EditCurrentCommand* pasteCurrentFromClipboardSync( bool a_exec = true ) Q_DECL_OVERRIDE;
private:
    QScopedPointer<QUndoStack> m_undoStack;
};

EditCellCommand* UndoableThingEditingBehavior::editCurrentSync( const QString &a_value, bool a_exec )
{
    Q_UNUSED(a_exec)
    EditCellCommand *undoable = ThingEditingBehavior::editCurrentSync(  a_value, false );
    m_undoStack->push( undoable );
    return undoable;
}

EditCellCommand* UndoableThingEditingBehavior::pasteCurrentFromClipboardSync( bool a_exec )
{
    Q_UNUSED(a_exec)
    EditCellCommand *undoable = ThingEditingBehavior::pasteCurrentFromClipboardSync( false );
    m_undoStack->push( undoable );
    return undoable;
}

Duck typing

Imagine you have a duck. Imagine you have a wall. Now imagine you throw the duck with a lot of force against a wall. Duck typing means that the duck hitting the wall quacks like a duck would.

ps. Replace wall with API and duck with ugly stupid script written by an idiot. You can leave quacks.

How to expose a QList<MyListClass> in a ViewModel to QML

MyPlugin/MyPlugin.cpp:

#include <ViewModels/MyListClass.h>
#include <ViewModels/DisplayViewModel.h>

qmlRegisterUncreatableType<MyListClass>( a_uri, 1, 0, "MyListClass",
         "Use access via DisplayViewModel instead");
qmlRegisterType<DisplayViewModel>( a_uri, 1, 0, "DisplayViewModel");

Utils/MyQMLListUtils.h

#define MY_DECLARE_QML_LIST(type, name, owner, prop) \
QQmlListProperty<type> name(){ \
   return QQmlListProperty<type>( \
               this, 0,&owner::count ## type ## For ## name ## List, \
               &owner::at ## type ## For ## name ## List); \
} \
static int count ## type ## For ## name ## List(QQmlListProperty<type>*property){ \
   owner *m = qobject_cast<owner *>(property->object); \
   return m->prop.size(); \
} \
static type *at ## type ## For ## name ## List( \
        QQmlListProperty<type>*property, int index){ \
   owner *m = qobject_cast<owner *>(property->object); \
   return m->prop[index]; \
}

ViewModels/DisplayViewModel.h

#ifndef DISPLAYVIEWMODEL_H
#define DISPLAYVIEWMODEL_H

#include <QObject>
#include <QtQml>
#include <ViewModels/MyListClass.h>
#include <Utils/MyQMLListUtils.h>

class DisplayViewModel : public QObject
{
    Q_OBJECT

    Q_PROPERTY(constQString title READ title WRITE setTitle NOTIFY titleChanged )
    Q_PROPERTY(constQList<MyListClass*> objects READ objects
                                          NOTIFY objectsChanged ) 
    Q_PROPERTY( QQmlListProperty<MyListClass> objectList READ objectList
                                              NOTIFY objectsChanged )
public:
    explicit DisplayViewModel( QObject *a_parent = nullptr );
    explicit DisplayViewModel( const QString &a_title,
                               QList<MyListClass*> a_objects,
                               QObject *a_parent = nullptr );
    const QString title()
        { return m_title; }
    void setTitle( const QString &a_title ); 
    const QList<MyListClass*> objects ()
        { return m_objects; } 
    Q_INVOKABLE void appendObject( MyListClass *a_object);
    Q_INVOKABLE void deleteObject( MyListClass *a_object);
    Q_INVOKABLE void reset( );

protected:
    MY_DECLARE_QML_LIST(MyListClass, objectList, DisplayViewModel, m_objects)

signals:
    void titleChanged();
    void objectsChanged();

private:
    QString m_title;
    QList<MyListObject*> m_objects;
};

#endif// DISPLAYVIEWMODEL_H

DisplayViewModel.cpp

#include "DisplayViewModel.h"

DisplayViewModel::DisplayViewModel( const QString &a_title,
                                    QList<MyListClass*> a_objects,
                                    QObject *a_parent )
    : QObject ( a_parent )
    , m_title ( a_title )
    , m_objects ( a_objects )
{
    foreach (MyListClass* mobject, m_objects) {
        mobject->setParent (this);
    }
}

void DisplayViewModel::setTitle (const QString &a_title )
{
    if ( m_title != a_title ) {
        m_title = a_title;
        emit titleChanged();
    }
}

void DisplayViewModel::reset( )
{
    foreach ( MyListClass *mobject, m_objects ) {
        mobject->deleteLater();
    }
    m_objects.clear();
    emit objectsChanged();
}

void DisplayViewModel::appendObject( MyListClass *a_object )
{
    a_object->setParent( this );
    m_objects.append( a_object );
    emit objectsChanged();
}

void DisplayViewModel::deleteObject( MyListClass *a_object )
{
    if (m_objects.contains( a_object )) {
        m_objects.removeOne( a_object );
        a_object->deleteLater();
        emit objectsChanged();
    }
}

Tester.cpp

#include <ViewModels/DisplayViewModel.h>
#include <ViewModels/MyListClass.h>

QList<MyListClass*> objectList;
for( int i = 0; i < 100 ; ++i ) {
    objectList.append ( new MyListClass (i) );
}
DisplayViewModel *viewModel = new DisplayViewModel (objectList);
viewModel->appendObject ( new MyListClass (101) );

Display.qml

import QtQuick 2.5
import MyPlugin 1.0

Repeater { 
    property DisplayViewModel viewModel: DisplayViewModel { } 
    model: viewModel.objectList
    delegate: Item {
        property MyListClass object: modelData
        Text {
            text: object.property
        }
    }
}

Scrum is (best done) like a soccer team

As a freelancer I saw many companies, many situations and worked with many Project Managers, Product Owners, Scrum Masters and god knows what names the HR department came up with.

What is most important, in my experience, is that the people leading the team try to focus the people in the group on as few common goals as possible during one sprint. The more stories or goals the team has to finish, the more individualism and the fewer things will get done (with done being defined by your definition of done).

Differently put you should try to make your team work like how a soccer team plays. You try to make three, four or five goals per sprint. But you do this as a team.

Example: When a story isn’t finished at the end of the sprint; it’s the team’s fault. Not the fault of the one guy that worked on it. The team has to find a solution. If it’s always the same guy being lazy, that’s not the project’s or the team’s results. You or HR deals with that guy later. Doing so is outside of Scrum’s scope. It’s not for your retrospective discussion. But do sanction the team for not delivering.

Another example is not to put too much stories on the task board and yet to keep stories as small and/or well defined as possible. Too much stories or too large stories will result in every individual picking a different story (or subtask of the larger story) to be responsible for. At the end of the sprint none of these will be really finished. And the developers in the team won’t care about the other features being developed during the sprint. So they will dislike having to review other people’s features. They’ll have difficulty finding a reviewer. They won’t communicate with each other. They’ll become careless and dispassionate.

Your planning caused that. That makes you a bad team player. The coach is part of the team.

Luxe crisis in Qt land

Het is weer eens crisis. Het is altijd crisis. Deze keer is het crisis omdat ik niet weet welke “C++ met Qt” recruiter eerst te beantwoorden. Zelden was het zo erg als nu.

Ik besloot een paar weken geleden eens even niet op Linked-In te antwoorden. Sindsdien heb ik na een ruime schatting ongeveer tien Belgische recruiters die een Qt/C++ ontwikkelaar zoeken en een veelvoud daarvan in Nederland (onder meer voor het bedrijf waar ik nu voor werk). En euh .. voor Duitsland zou ik een database moeten aanleggen om het te kunnen volgen. England negeer en blokkeer ik al een tijdje.

Toegegeven zijn heel wat recruiters voor hetzelfde bedrijf op zoek. Er moet toch iets lucratief aan zijn opdat er zoveel tegelijk los gaan?

Het probleem van C++ met Qt is volgens mij dat er te weinig zielen het kunnen. Dit houdt de explosie van het gebruik er van meer tegen dan goed is, denk ik. Voornamelijk in embedded en industriële toepassingen loopt het goed. Vandaar misschien dat het nogal druk in Duitsland is? Internet of crap dingen ook wel wat. Maar het zijn toch vooral webapp developers die men daar zoekt. Verbazingwekkend is er ook veel pro-Qt belangstelling van alles wat met mediaplayers te maken heeft?

Wij met C++/Qt ervaring zouden beter wat meer lessen en cursussen geven aan de jongeren. We kunnen deze markt toch niet alleen aan.

Just saying.

Iemand nog ideeën?

Misschien moet er toch eens iets komen om met de project managers te gaan overleggen? Ik heb al eens een soort van gilde voorgesteld he. Eind van het jaar is het altijd hetzelfde. Plots hebben ze zicht op de goedkeuring van één of ander budget van hun CEO. En dan zoeken ze allemaal te samen, liefst voor eergisteren, naar twintig programmeurs. Een beetje zoals wilde beesten.

Eigenlijk is het al niet meer enkel het eind van het jaar, maar valt het meestal met de kwartaalovergangen samen.

Truly huge files and the problem of continuous virtual address space

As we all know does mmap, or even worse on Windows CreateFileMapping, need contiguous virtual address space for a given mapping size. That can become a problem when you want to load a file of a gigabyte with mmap.

The solution is of course to mmap the big file using multiple mappings. For example like adapting yesterday’s demo this way:

void FileModel::setFileName(const QString &fileName)
{
    ...
    if (m_file->open(QIODevice::ReadOnly)) {
        if (m_file->size() > MAX_MAP_SIZE) {
            m_mapSize = MAX_MAP_SIZE;
            m_file_maps.resize(1 + m_file->size() / MAX_MAP_SIZE, nullptr);
        } else {
            m_mapSize = static_cast(m_file->size());
            m_file_maps.resize(1, nullptr);
        }
        ...
    } else {
        m_index->open(QFile::ReadOnly);
        m_rowCount = m_index->size() / 4;
    }
    m_file_maps[0] = m_file->map(0, m_mapSize, QFileDevice::NoOptions);
    qDebug() << "Done loading " << m_rowCount << " lines";
    map_index = m_index->map(0, m_index->size(), QFileDevice::NoOptions);

    beginResetModel();
    endResetModel();
    emit fileNameChanged();
}

And in the data() function:

QVariant FileModel::data( const QModelIndex& index, int role ) const
{
    QVariant ret;
    ...
    quint32 mapIndex = pos_i / MAX_MAP_SIZE;
    quint32 map_pos_i = pos_i % MAX_MAP_SIZE;
    quint32 map_end_i = end_i % MAX_MAP_SIZE;
    uchar* map_file = m_file_maps[mapIndex];
    if (map_file == nullptr)
        map_file = m_file_maps[mapIndex] = m_file->map(mapIndex * m_mapSize, m_mapSize, QFileDevice::NoOptions);
    position = m_file_maps[mapIndex] + map_pos_i;
    if (position) {
            const int length = static_cast(end_i - pos_i);
            char *buffer = (char*) alloca(length+1);
            if (map_end_i >= map_pos_i)
                strncpy (buffer, (char*) position, length);
            else {
                const uchar *position2 = m_file_maps[mapIndex+1];
                if (position2 == nullptr) {
                    position2 = m_file_maps[mapIndex+1] = m_file->map((mapIndex+1) *
                         m_mapSize, m_mapSize, QFileDevice::NoOptions);
                }
                strncpy (buffer, (char*) position, MAX_MAP_SIZE - map_pos_i);
                strncpy (buffer + (MAX_MAP_SIZE - map_pos_i), (char*) position2, map_end_i);
            }
            buffer[length] = 0;
            ret = QVariant(QString(buffer));
        }
    }
    return ret;
}

You could also not use mmap for the very big source text file and use m_file.seek(map_pos_i) and m_file.read(buffer, length). The most important mapping is of course the index one, as the reading of the individual lines can also be done fast enough with normal read() calls (as long as you don’t have to do it for each and every line of the very big file and as long as you know in a O(1) way where the QAbstractListModel’s index.row()’s data is).

But you already knew that. Right?

Loading truly truly huge text files with a QAbstractListModel

Sometimes people want to do crazy stuff like loading a gigabyte sized plain text file into a Qt view that can handle QAbstractListModel. Like for example a QML ListView. You know, the kind of files you generate with this commando:

base64 /dev/urandom | head -c 100000000 > /tmp/file.txt

But, how do they do it?

FileModel.h

So we will make a custom QAbstractListModel. Its private member fields I will explain later:

#ifndef FILEMODEL_H
#define FILEMODEL_H

#include <QObject>
#include <QVariant>
#include <QAbstractListModel>
#include <QFile>

class FileModel: public QAbstractListModel {
    Q_OBJECT

    Q_PROPERTY(QString fileName READ fileName WRITE setFileName NOTIFY fileNameChanged )
public:
    explicit FileModel( QObject* a_parent = nullptr );
    virtual ~FileModel();

    int columnCount(const QModelIndex &parent) const;
    int rowCount( const QModelIndex& parent =  QModelIndex() ) const Q_DECL_OVERRIDE;
    QVariant data( const QModelIndex& index, int role = Qt::DisplayRole ) const  Q_DECL_OVERRIDE;
    QVariant headerData( int section, Qt::Orientation orientation,
                         int role = Qt::DisplayRole ) const  Q_DECL_OVERRIDE;
    void setFileName(const QString &fileName);
    QString fileName () const
        { return m_file->fileName(); }
signals:
    void fileNameChanged();
private:
    QFile *m_file, *m_index;
    uchar *map_file;
    uchar *map_index;
    int m_rowCount;
    void clear();
};

#endif// FILEMODEL_H

FileModel.cpp

We will basically scan the very big source text file for newline characters. We’ll write the offsets of those to a file suffixed with “.mmap”. We’ll use that new file as a sort of “partition table” for the very big source text file, in the data() function of QAbstractListModel. But instead of sectors and files, it points to newlines.

The reason why the scanner itself isn’t using the mmap’s address space is because apparently reading blocks of 4kb is faster than reading each and every byte from the mmap in search of \n characters. Or at least on my hardware it was.

You should probably do the scanning in small qEventLoop iterations (make sure to use nonblocking reads, then) or in a thread, as your very big source text file can be on a unreliable or slow I/O device. Plus it’s very big, else you wouldn’t be doing this (please promise me to just read the entire text file in memory unless it’s hundreds of megabytes in size: don’t micro optimize your silly homework notepad.exe clone).

Note that this is demo code with a lot of bugs like not checking for \r and god knows what memory leaks and stuff was remaining when it suddenly worked. I leave it to the reader to improve this. An example is that you should check for validity of the “.mmap” file: your very big source text file might have changed since the newline partition table was made.

Knowing that I’ll soon find this all over the place without any of its bugs fixed, here it comes ..

#include "FileModel.h"

#include <QDebug>

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <pthread.h>
#include <unistd.h>

FileModel::FileModel( QObject* a_parent )
    : QAbstractListModel( a_parent )
    , m_file (nullptr)
    , m_index(nullptr)
    , m_rowCount ( 0 ) { }

FileModel::~FileModel() { clear(); }

void FileModel::clear()
{
    if (m_file) {
        if (m_file->isOpen() && map_file != nullptr)
            m_file->unmap(map_file);
        delete m_file;
    }
    if (m_index) {
        if (m_index->isOpen() && map_index != nullptr)
            m_index->unmap(map_index);
        delete m_index;
    }
}

void FileModel::setFileName(const QString &fileName)
{
   clear();
   m_rowCount = 0;
   m_file = new QFile(fileName);
   int cur = 0;
   m_index = new QFile(m_file->fileName() + ".mmap");
   if (m_file->open(QIODevice::ReadOnly)) {
       if (!m_index->exists()) {
           char rbuffer[4096];
           m_index->open(QIODevice::WriteOnly);
           char nulbuffer[4];
           int idxnul = 0;
           memset( nulbuffer +0, idxnul >> 24 & 0xff, 1 );
           memset( nulbuffer +1, idxnul >> 16 & 0xff, 1 );
           memset( nulbuffer +2, idxnul >>  8 & 0xff, 1 );
           memset( nulbuffer +3, idxnul >>  0 & 0xff, 1 );
           m_index->write( nulbuffer, sizeof(quint32));
           qDebug() << "Indexing to" << m_index->fileName();
           while (!m_file->atEnd()) {
               int in = m_file->read(rbuffer, 4096);
               if (in == -1)
                   break;
               char *newline = (char*) 1;
               char *last = rbuffer;
               while (newline != 0) {
                   newline = strchr ( last, '\n');
                   if (newline != 0) {
                     char buffer[4];
                     int idx = cur + (newline - rbuffer);
                     memset( buffer +0, idx >> 24 & 0xff, 1 );
                     memset( buffer +1, idx >> 16 & 0xff, 1 );
                     memset( buffer +2, idx >>  8 & 0xff, 1 );
                     memset( buffer +3, idx >>  0 & 0xff, 1 );
                     m_index->write( buffer, sizeof(quint32));
                     m_rowCount++;
                     last = newline + 1;
                  }
               }
               cur += in;
           }
           m_index->close();
           m_index->open(QFile::ReadOnly);
           qDebug() << "done";
       } else {
           m_index->open(QFile::ReadOnly);
           m_rowCount = m_index->size() / 4;
       }
       map_file= m_file->map(0, m_file->size(), QFileDevice::NoOptions);
       qDebug() << "Done loading " << m_rowCount << " lines";
       map_index = m_index->map(0, m_index->size(), QFileDevice::NoOptions);
   }
   beginResetModel();
   endResetModel();
   emit fileNameChanged();
}

static quint32
read_uint32 (const quint8 *data)
{
    return data[0] << 24 |
           data[1] << 16 |
           data[2] << 8 |
           data[3];
}

int FileModel::rowCount( const QModelIndex& parent ) const
{
    Q_UNUSED( parent );
    return m_rowCount;
}

int FileModel::columnCount(const QModelIndex &parent) const
{
    Q_UNUSED( parent );
    return 1;
}

QVariant FileModel::data( const QModelIndex& index, int role ) const
{
    if( !index.isValid() )
        return QVariant();
    if (role == Qt::DisplayRole) {
        QVariant ret;
        quint32 pos_i = read_uint32(map_index + ( 4 * index.row() ) );
        quint32 end_i;
        if ( index.row() == m_rowCount-1 )
            end_i = m_file->size();
        else
            end_i = read_uint32(map_index + ( 4 * (index.row()+1) ) );
        uchar *position;
        position = map_file +  pos_i;
        uchar *end = map_file + end_i;
        int length = end - position;
        char *buffer = (char*) alloca(length +1);
        memset (buffer, 0, length+1);
        strncpy (buffer, (char*) position, length);
        ret = QVariant(QString(buffer));
        return ret;
    }
    return QVariant();
}

QVariant FileModel::headerData( int section, Qt::Orientation orientation, int role ) const
{
    Q_UNUSED(section);
    Q_UNUSED(orientation);
    if (role != Qt::DisplayRole)
           return QVariant();
    return QString("header");
}

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQml>// qmlRegisterType

#include "FileModel.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    qmlRegisterType<FileModel>( "FileModel", 1, 0, "FileModel" );
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    return app.exec();
}

main.qml

import QtQuick 2.3
import QtQuick.Window 2.2
import FileModel 1.0

Window {
    visible: true

    FileModel { id: fileModel }
    ListView {
        id: list
        anchors.fill: parent
        delegate: Text { text: display }
        MouseArea {
            anchors.fill: parent
            onClicked: {
                list.model = fileModel
                fileModel.fileName = "/tmp/file.txt"
            }
        }
    }
}

profile.pro

TEMPLATE = app
QT += qml quick
CONFIG += c++11
SOURCES += main.cpp \
    FileModel.cpp
RESOURCES += qml.qrc
HEADERS += \
    FileModel.h

qml.qrc

<RCC>
    <qresource prefix="/">
        <file>main.qml</file>
    </qresource>
</RCC>

Composition and aggregation with QObject

Consider these rather simple relationships between classes

Continuing on this subject, here are some code examples.

Class1 & Class2: Composition
An instance of Class1 can not exist without an instance of Class2.

Example of composition is typically a Bicycle and its Wheels, Saddle and a HandleBar: without these the Bicycle is no longer a Bicycle but just a Frame.

It can no longer function as a Bicycle. Example of when you need to stop thinking about composition versus aggregation is whenever you say: without the other thing can’t in our software the first thing work.

Note that you must consider this in the context of Class1. You use aggregation or composition based on how Class2 exists in relation to Class1.

Class1 with QScopedPointer:

#ifndef CLASS1_H
#define CLASS1_H

#include <QObject>
#include <QScopedPointer>
#include <Class2.h>

class Class1: public QObject
{
    Q_PROPERTY( Class2* class2 READ class2 WRITE setClass2 NOTIFY class2Changed)
public:
    Class1( QObject *a_parent = nullptr )
        : QObject ( a_parent) {
        // Don't use QObject parenting on top here
        m_class2.reset (new Class2() );
    }
    Class2* class2() {
        return m_class2.data();
    }
    void setClass2 ( Class2 *a_class2 ) {
        Q_ASSERT (a_class2 != nullptr); // Composition can't set a nullptr!
        if ( m_class2.data() != a_class2 ) {
            m_class2.reset( a_class2 );
            emit class2Changed()
        }
    }
signals:
    void class2Changed();
private:
    QScopedPointer<Class2> m_class2;
};

#endif// CLASS1_H

Class1 with QObject parenting:

#ifndef CLASS1_H
#define CLASS1_H

#include <QObject>
#include <Class2.h>

class Class1: public QObject
{
    Q_PROPERTY( Class2* class2 READ class2 WRITE setClass2 NOTIFY class2Changed)
public:
    Class1( QObject *a_parent = nullptr )
        : QObject ( a_parent )
        , m_class2 ( nullptr ) {
        // Make sure to use QObject parenting here
        m_class2 = new Class2( this );
    }
    Class2* class2() {
        return m_class2;
    }
    void setClass2 ( Class2 *a_class2 ) {
         Q_ASSERT (a_class2 != nullptr); // Composition can't set a nullptr!
         if ( m_class2 != a_class2 ) {
             // Make sure to use QObject parenting here
             a_class2->setParent ( this );
             delete m_class2; // Composition can never be nullptr
             m_class2 = a_class2;
             emit class2Changed();
         }
    }
signals:
    void class2Changed();
private:
    Class2 *m_class2;
};

#endif// CLASS1_H

Class1 with RAII:

#ifndef CLASS1_H
#define CLASS1_H

#include <QObject>
#include <QScopedPointer>

#include <Class2.h>

class Class1: public QObject
{
    Q_PROPERTY( Class2* class2 READ class2 CONSTANT)
public:
    Class1( QObject *a_parent = nullptr )
        : QObject ( a_parent ) { }
    Class2* class2()
        { return &m_class2; }
private:
    Class2 m_class2;
};
#endif// CLASS1_H

Class3 & Class4: Aggregation

An instance of Class3 can exist without an instance of Class4. Example of composition is typically a Bicycle and its driver or passenger: without the Driver or Passenger it is still a Bicycle. It can function as a Bicycle.

Example of when you need to stop thinking about composition versus aggregation is whenever you say: without the other thing can in our software the first thing work.

Class3:

#ifndef CLASS3_H
#define CLASS3_H

#include <QObject>

#include <QPointer>
#include <Class4.h>

class Class3: public QObject
{
    Q_PROPERTY( Class4* class4 READ class4 WRITE setClass4 NOTIFY class4Changed)
public:
    Class3( QObject *a_parent = nullptr );
    Class4* class4() {
        return m_class4.data();
    }
    void setClass4 (Class4 *a_class4) {
         if ( m_class4 != a_class4 ) {
             m_class4 = a_class4;
             emit class4Changed();
         }
    }
signals:
    void class4Changed();
private:
    QPointer<Class4> m_class4;
};
#endif// CLASS3_H

Class5, Class6 & Class7: Shared composition
An instance of Class5 and-or an instance of Class6 can not exist without a instance of Class7 shared by Class5 and Class6. When one of Class5 or Class6 can and one can not exist without the shared instance, use QWeakPointer at that place.

Class5:

#ifndef CLASS5_H
#define CLASS5_H

#include <QObject>
#include <QSharedPointer>

#include <Class7.h>

class Class5: public QObject
{
    Q_PROPERTY( Class7* class7 READ class7 CONSTANT)
public:
    Class5( QObject *a_parent = nullptr, Class7 *a_class7 );
        : QObject ( a_parent )
        , m_class7 ( a_class7 ) { }
    Class7* class7()
        { return m_class7.data(); }
private:
    QSharedPointer<Class7> m_class7;
};

Class6:

#ifndef CLASS6_H
#define CLASS6_H

#include <QObject>
#include <QSharedPointer>

#include <Class7.h>

class Class6: public QObject
{
    Q_PROPERTY( Class7* class7 READ class7 CONSTANT)
public:
    Class6( QObject *a_parent = nullptr, Class7 *a_class7 )
        : QObject ( a_parent )
        , m_class7 ( a_class7 ) { }
    Class7* class7()
        { return m_class7.data(); }
private:
    QSharedPointer<Class7> m_class7;
};
#endif// CLASS6_H

Interfaces with QObject

FlyBehavior:

#ifndef FLYBEHAVIOR_H
#define FLYBEHAVIOR_H
#include <QObject>
// Don't inherit QObject here (you'll break multiple-implements)
class FlyBehavior {
    public:
        Q_INVOKABLE virtual void fly() = 0;
};
Q_DECLARE_INTERFACE(FlyBehavior , "be.codeminded.Flying.FlyBehavior /1.0") 
#endif// FLYBEHAVIOR_H

FlyWithWings:

#ifndef FLY_WITH_WINGS_H
#define FLY_WITH_WINGS_H
#include <QObject>  
#include <Flying/FlyBehavior.h>
// Do inherit QObject here (this is a concrete class)
class FlyWithWings: public QObject, public FlyBehavior
{
    Q_OBJECT
    Q_INTERFACES( FlyBehavior )
public:
    explicit FlyWithWings( QObject *a_parent = nullptr ): QObject ( *a_parent ) {}
    ~FlyWithWings() {}

    virtual void fly() Q_DECL_OVERRIDE;
}
#endif// FLY_WITH_WINGS_H