Q_PROPERTY Produces TypeError When Setting QQuickWidget ContextProperty

daviddev

Hello everyone. I am setting a Q_OBJECT class with a QString Q_PROPERTY in a QQuickWidget. The application loads and displays the test text perfectly fine. However it always produces the error

TypeError: Cannot read property 'message' of null

Debugging shows the type error is produced in the Application Output in MainWindow's constructor where

ui->qmlQuickWidget->rootContext()->setContextProperty("testQML", &testQML);

and the QML Debugger Console from w.show() in main.

Is there a way of setting the QML file that will remove this error from occurring or a better implementation in the .qml file itself?

I am running on QT 6.9.3 on Windows 10. All the relevant code is below. Thank you.

//TestQML.qml
import QtQuick 2.15

Text{
    text: testQML.message
}

//MainWindow.cpp
#include "MainWindow.hpp"
#include "TestQML.hpp"
#include "./ui_mainwindow.h"
#include <QQmlContext>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow){
    ui->setupUi(this);
    TestQML testQML;
    testQML.setMessage(QString("test"));
    ui->qmlQuickWidget->rootContext()->setContextProperty("testQML", &testQML);
    ui->qmlQuickWidget->setSource(QUrl::fromLocalFile("../../TestQML.qml"));


}

MainWindow::~MainWindow()
{
    delete ui;
}

//TestQML.cpp
#include "TestQML.hpp"

TestQML::TestQML(QObject *parent)
    : QObject(parent){
    myMessage = "Test Message";
}

QString TestQML::message() const {
    return myMessage;
}

void TestQML::setMessage(const QString &newMessage){
    if (myMessage != newMessage) {
        myMessage = newMessage;
        emit messageChanged();
    }
}

//TestQML.hpp
#ifndef TESTQML_HPP
#define TESTQML_HPP

#include <QObject>
#include <QString>

class TestQML : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString message READ message WRITE setMessage NOTIFY messageChanged)
public:
    explicit TestQML(QObject *parent = nullptr);
    QString  message() const;
    void setMessage(const QString &newMessage);
signals:
    void messageChanged();

private:
    QString myMessage;
};

#endif // TESTQML_HPP

JKSH

@daviddev The main issue is that your TestQML object is a local variable -- it gets destroyed as soon as the MainWindow constructor returns. So, a quick fix is to allocate your TestQML object on the heap (auto testQML = new TestQML(this);).

However, that is not the recommended fix. We recommend making your TestQML object a QML singleton, using the QML_ELEMENT and QML_SINGLETON macros (see https://doc.qt.io/qt-6/qtqml-cppintegration-overview.html )

daviddev

I greatly appreciate this. Would there be a project that has a QML_SINGLETON implementation with a QQuickWidget you know about? The closest source I have discovered is this forum post however it uses a QQMLApplicationEngine connection while I am trying to use QQuickWidget. The closest Example QT provides is the QQuickWidgetVersusWindow_opengl which I am getting lost in.
https://forum.qt.io/topic/133764/unable-to-access-q_property-for-qml_singleton-object?_=1761145429336

Also, when should I use the pragma Singleton in the .qml file vs the QML_ELEMENT and QML_SINGLETON?

daviddev

I have resolved the TypeError error with both a Singleton approach as well as the heap approach. I am trying to post the heap solution below, but it keeps getting marked as spam, as well as the Qml implenetation for the Singleton. I have unfortunately had to use a pointer in the MainWindow header file for the QQuickWidget and set it in the Window's constructor.

Would somebody know how to setup the Singleton version that will update the QQuickWidget? I am able to setup the Singleton version, however I have not been able to update it for the life of me.

JKSH

@daviddev said in Q_PROPERTY Produces TypeError When Setting QQuickWidget ContextProperty:

when should I use the pragma Singleton in the .qml file vs the QML_ELEMENT and QML_SINGLETON?

Use the pragma if you implement your singleton in QML
Use QML_ELEMENT if you implement your singleton in C++

Would somebody know how to setup the Singleton version that will update the QQuickWidget? I am able to setup the Singleton version, however I have not been able to update it for the life of me.

The QML engine instantiates the singleton, so you don't do that manually.

You can retrieve a pointer to your singleton by calling QQmlEngine::singletonInstance<>(): https://doc.qt.io/qt-6/qqmlengine.html#singletonInstance-1

// Replace "MyModule" with your actual module's name
auto instance = ui->qmlQuickWidget->engine()->singletonInstance<TestQML*>("MyModule", "TestQML");
instance->updateSomething();

Note: To avoid confusion, I recommend that you give your C++ class and your *.qml file different names

daviddev

Thank you @JKSH this really helped.