How to change the properties of qml objects in qt qml in the created class?

Question

Qt

there is a main.qml file

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Controls 2.4

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
    Rectangle{
        x: 100
        y: 300
        width: 100;
        height: 100;
        MouseArea{
            anchors.fill: parent
        }
    }
}

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("backend", &backend);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
    return app.exec();
}

and let mainwindow.h

#include "QObject"
#include "QString"

class Backend: public QObject{
    Q_OBJECT
    Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged)
public:
    void change();
};

how to bind the change function in the mainwindow.cpp file to clicking on the rectangle and at the same time change its color?

Answer 1

1. In real code, do not use absolute positioning, except in rare cases
2. You now have a MouseArea of ​​zero size, you need to use anchors.fill: parent
3. In the onClicked: backend.a handler, you did complete nonsense, firstly, the backend does not such a property, and secondly, you did not change it in any way, and did not even display it on the screen.
4. In order for the C++ class methods to become available in QML , they must be in the public slots
section . In this case, you could add a color property to the backend , and bind it to the Rectangle , like color: backend.color . Well and inonClicked will already call backend.change() , which will update the property.
In general, read articles , a book , other people's sources , and documentation so that such elementary questions do not arise.