QML - Tutorial 030. Registering a QML Type as a Singleton object

QML, singleton

In addition to C ++ classes in QML, Singleton can also use QML types, which are separate QML files.

This is also used by qmlRegisterSingletonType , but in order for this type to work as a singleton of the object, it is necessary to register the pragma Singleton in the QML file itself.

Let's make a small application that will also output several messages, while messages will be picked up through the Singleton QML Type method.

Project structure

• SingletonQML.pro - project profile
• main.cpp - File with main function
• main.qml - Main QML file
• Util.qml - Singleton QML

The project profile will be created by default and will not be changed.

main.cpp

As the Singleton object use the file Util.qml. To make it a Singleton, it must be registered with qmlRegisterSingletonType, and also specify the pragma Singleton directive in it. First, we will register it.

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQml> // We connect to use the qmlRegisterSingletonType function
 
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
 
    // We register the qml file by specifying its path.
    qmlRegisterSingletonType(QUrl("qrc:/Util.qml"), "Util", 1, 0, "Util");
 
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
 
    return app.exec();
}

Util.qml

In this file, be sure to specify the directive pragma Singleton.

pragma Singleton // We indicate that this QML Type is a singleton
import QtQuick 2.0
 
Item {
    function getMessage(messageNumber)
    {
        switch(messageNumber)
        {
        case 1:
            return "First Message"
        case 2:
            return "Second Message"
        case 3:
            return "Third Message"
        default:
            return "Nothing found"
        }
    }
}

main.qml

And now we use the singleton object to output text using this function.

import QtQuick 2.5
import QtQuick.Window 2.2
import QtQuick.Controls 1.4
 
import Util 1.0 // Thus, the import of QML singleton
 
 
Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Singleton Class")
 
    ListView {
        anchors.fill: parent
        delegate: Item {
            height: 48
            width: parent.width
            Text {
                anchors.fill: parent
                text: model.text
 
                verticalAlignment: Text.AlignVCenter
                horizontalAlignment: Text.AlignHCenter
            }
        }
 
        model: listModel
    }
 
    ListModel {
        id: listModel
 
        Component.onCompleted: {
            // The appeal to the singleton method looks like a call to the static method
            listModel.append({'text': Util.getMessage(1)})
            listModel.append({'text': Util.getMessage(2)})
            listModel.append({'text': Util.getMessage(3)})
            listModel.append({'text': Util.getMessage(4)})
        }
    }
}

Conclusion

In this way, you can use singleton QML objects to emulate the use of static methods, and this is the right approach for working with similar functionality.

Download archive with project