Category: controversial

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.

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
        }
    }
}