Category: Informatics and programming

In the XAML world it’s very common to use the MVVM pattern. I will explain how to use the technique in a similar way with Qt and QML.

The idea is to not have too much code in the view component. Instead we have declarative bindings and move most if not all of our view code to a so called ViewModel. The ViewModel will sit in between the actual model and the view. The ViewModel typically has one to one properties for everything that the view displays. Manipulating the properties of the ViewModel alters the view through bindings. You typically don’t alter the view directly.

In our example we have two list-models, two texts and one button: available-items, accepted-items, available-count, accepted-count and a button. Pressing the button moves stuff from available to accepted. Should be a simple example.

First the ViewModel.h file. The class will have a property for ~ everything the view displays:

#ifndef VIEWMODEL_H
#define VIEWMODEL_H

#include <QAbstractListModel>
#include <QObject>

class ViewModel : public QObject
{
	Q_OBJECT

	Q_PROPERTY(QAbstractListModel* availableItems READ availableItems NOTIFY availableItemsChanged )
	Q_PROPERTY(QAbstractListModel* acceptedItems READ acceptedItems NOTIFY acceptedItemsChanged )
	Q_PROPERTY(int available READ available NOTIFY availableChanged )
	Q_PROPERTY(int accepted READ accepted NOTIFY acceptedChanged )
public:

	ViewModel( QObject *parent = 0 );
	~ViewModel() { }

	QAbstractListModel* availableItems()
		{ return m_availableItems; }

	QAbstractListModel* acceptedItems()
		{ return m_acceptedItems; }

	int available ()
		{ return m_availableItems->rowCount(); }

	int accepted ()
		{ return m_acceptedItems->rowCount(); }

	Q_INVOKABLE void onButtonClicked( int availableRow );

signals:
	void availableItemsChanged();
	void acceptedItemsChanged();
	void availableChanged();
	void acceptedChanged();

private:
	QAbstractListModel* m_availableItems;
	QAbstractListModel* m_acceptedItems;
};

#endif

The ViewModel.cpp implementation of the ViewModel. This is of course a simple example. The idea is that ViewModels can be quite complicated while the view.qml remains simple:

#include <QStringListModel>

#include "ViewModel.h"

ViewModel::ViewModel( QObject *parent ) : QObject ( parent )
{
	QStringList available;
	QStringList accepted;

	available << "Two" << "Three" << "Four" << "Five";
	accepted << "One";

	m_availableItems = new QStringListModel( available, this );
	emit availableItemsChanged();

	m_acceptedItems = new QStringListModel( accepted, this );
	emit acceptedItemsChanged();
}

void ViewModel::onButtonClicked(int availableRow)
{
	QModelIndex availableIndex = m_availableItems->index( availableRow, 0, QModelIndex() );
	QVariant availableItem = m_availableItems->data( availableIndex, Qt::DisplayRole );

	int acceptedRow = m_acceptedItems->rowCount();

	m_acceptedItems->insertRows( acceptedRow, 1 );

	QModelIndex acceptedIndex = m_acceptedItems->index( acceptedRow, 0, QModelIndex() );
	m_acceptedItems->setData( acceptedIndex, availableItem );
	emit acceptedChanged();

	m_availableItems->removeRows ( availableRow, 1, QModelIndex() );
	emit availableChanged();
}

The view.qml. We’ll try to have as few JavaScript code as possible; the idea is that coding itself is done in the ViewModel. The view should only be view code (styling, UI, animations, etc). The import url and version are defined by the use of qmlRegisterType in the main.cpp file, lower:

import QtQuick 2.0
import QtQuick.Controls 1.2

import be.codeminded.ViewModelExample 1.0

Rectangle {
    id: root
    width: 640; height: 320

	property var viewModel: ViewModel { }

	Rectangle {
		id: left
		anchors.left: parent.left
		anchors.top: parent.top
		anchors.bottom: button.top
		width: parent.width / 2
		ListView {
		    id: leftView
			anchors.left: parent.left
			anchors.right: parent.right
			anchors.top: parent.top
			anchors.bottom: leftText.top

			delegate: rowDelegate
		        model: viewModel.availableItems
		}
		Text {
			id: leftText
			anchors.left: parent.left
			anchors.right: parent.right
			anchors.bottom: parent.bottom
			height: 20
			text: viewModel.available
		}
	}

	Rectangle {
		id: right
		anchors.left: left.right
		anchors.right: parent.right
		anchors.top: parent.top
		anchors.bottom: button.top
		ListView {
		    id: rightView
			anchors.left: parent.left
			anchors.right: parent.right
			anchors.top: parent.top
			anchors.bottom: rightText.top

			delegate: rowDelegate
		        model: viewModel.acceptedItems
		}
		Text {
			id: rightText
			anchors.left: parent.left
			anchors.right: parent.right
			anchors.bottom: parent.bottom
			height: 20
			text: viewModel.accepted
		}
	}

	Component {
		id: rowDelegate
		Rectangle {
			width: parent.width
			height: 20
			color: ListView.view.currentIndex == index ? "red" : "white"
			Text { text: 'Name:' + display }
			MouseArea {
				anchors.fill: parent
				onClicked: parent.ListView.view.currentIndex = index
			}
		}
	}

	Button {
		id: button
		anchors.left: parent.left
		anchors.right: parent.right
		anchors.bottom: parent.bottom
		height: 20
	        text: "Accept item"
		onClicked: viewModel.onButtonClicked( leftView.currentIndex );
	}
}

A main.cpp example. The qmlRegisterType defines the url to import in the view.qml file:

#include <QGuiApplication>
#include <QQuickView>
#include <QtQml>
#include <QAbstractListModel>

#include "ViewModel.h"

int main(int argc, char *argv[])
{
	QGuiApplication app(argc, argv);
	QQuickView view;
	qRegisterMetaType<QAbstractListModel*>("QAbstractListModel*");
	qmlRegisterType<ViewModel>("be.codeminded.ViewModelExample", 1, 0, "ViewModel");
	view.setSource(QUrl("qrc:/view.qml"));
	view.show();
	return app.exec();
}

A project.pro file. Obviously should you use cmake nowadays. But oh well:

TEMPLATE += app
QT += quick
SOURCES += ViewModel.cpp main.cpp
HEADERS += ViewModel.h
RESOURCES += project.qrc

And a project.qrc file:

<!DOCTYPE RCC>
<RCC version="1.0">
<qresource prefix="/">
    <file>view.qml</file>
</qresource>
</RCC>

My colleague Henk Van Der Laak made a interesting tool that checks your code against the QML coding conventions. It uses the internal parser’s abstract syntax tree of Qt 5.6 and a visitor design.

It has a command line, but being developers ourselves we want an API too of course. Then we can integrate it in our development environments without having to use popen!

So this is how to use that API:

// Parse the code
QQmlJS::Engine engine;
QQmlJS::Lexer lexer(&engine);
QQmlJS::Parser parser(&engine);

QFileInfo info(a_filename);
bool isJavaScript = info.suffix().toLower() == QLatin1String("js");
lexer.setCode(code,  1, !isJavaScript);
bool success = isJavaScript ? parser.parseProgram() : parser.parse();
if (success) {
    // Check the code
    QQmlJS::AST::UiProgram *program = parser.ast();
    CheckingVisitor checkingVisitor(a_filename);
    program->accept(&checkingVisitor);
    foreach (const QString &warning, checkingVisitor.getWarnings()) {
        qWarning() << qPrintable(warning);
    }
}

As we all know has Qt types like QPointer, QSharedPointer and we know about its object trees. So when do we use what?

Let’s first go back to school, and remember the difference between composition and aggregation. Most of you probably remember drawings like this?

It thought us when to use composition, and when to use aggregation:

• Use composition when the user can’t exist without the dependency. For example a Human can’t exist without a Head unless it ceases to be a human. You could also model Arm, Hand, Finger and Leg as aggregates but it might not make sense in your model (for a patient in a hospital perhaps it does?)

• Use aggregate when the user can exist without the dependency: A car without a passenger is still a car in most models.

This model in the picture will for example tell us that a car’s passenger must have ten fingers.

But what does this have to do with QPointer, QSharedPointer and Qt’s object trees?

First situation is a shared composition. Both Owner1 and Owner2 can’t survive without Shared (composition, filled up diamonds). For this situation you would typically use a QSharedPointer<Shared> at Owner1 and Owner2:

If there is no other owner, then it’s probably better to just use Qt’s object trees and setParent() instead. Note that for example QML’s GC is not very well aware of QSharedPointer, but does seem to understand Qt’s object trees.

Second situation are shared users. User1 and User2 can stay alive when Shared goes away (aggregation, empty diamonds). In this situation you typically use a QPointer<Shared> at User1 and at User2. You want to be aware when Shared goes away. QPointer<Shared>’s isNull() will become true after that happened.

Third situation is a mixed one. In this case you could at Owner use a QSharedPointer<Shared> or a parented raw QObject pointer (using setParent()), but a QPointer<Shared> at User. When Owner goes away and its destructor (due to the parenting) deletes Shared, User can check for it using the previously mentioned isNull check.

Finally if you have a typical object tree, then use QObject’s infrastructure for this.