如何在 C++ 运行时更改 QML 对象的属性？

Question

I want to change the text of a QML Object during runtime.

I tried it as following, but the text just stays empty.

This is the BackEnd class:

class BackEnd : public QObject {
    Q_OBJECT
    Q_PROPERTY(QString userFieldText READ userFieldText WRITE setUserFieldText)
public:
    explicit BackEnd(QObject *parent = nullptr);

    QString userFieldText();
    void setUserFieldText(QString &username);
private:
    QString _userFieldText;
};

In the qml file, I include window.backend, create a new BackEnd instance and try to access the values like

BackEnd {
 id: backend
}

Text {
 ...
 text: backend.userFieldText
}

I register the class like that.

qmlRegisterType<BackEnd>("window.backend", 0, 1, "BackEnd");

In a seperate thread where I would like to change the objects I create an instance of the BackEnd class and call the setter function.

BackEnd backend;
QString user("set by backend");
backend.setUserFieldText(user);

Compilation works, it runs but does not change anything. I already tried putting it in a timer in the QML code and updating it every second but nothing seems to work.

Answer 1

You have the following errors:

• As you point you have created an instance of Backend in one thread, and another instance in QML, so the modification of the state of one instance does not modify the other instance. In these cases where you want to have an object in C++ and QML it is better to create a context-property with setContextProperty() .

• QML only accepts objects that live in the main thread so the Backend object can not be created in another thread, one possibility is that you create another object that lives in the secondary thread and transmits the data to the main thread by signals, another possibility is to use QThread that accepts the creation of signals and connect it to the Backend object.

• The properties that you want to do binding in QML must be notifiable through a signal.

Considering the above, an example is the following:

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QThread>
class BackEnd : public QObject {
    Q_OBJECT
    Q_PROPERTY(QString userFieldText READ userFieldText WRITE setUserFieldText NOTIFY userFieldTextChanged)
public:
    explicit BackEnd(QObject *parent = nullptr):
        QObject(parent){}
    QString userFieldText() const {
        return _userFieldText;
    }
    void setUserFieldText(const QString &username){
        if(userFieldText() == username) return;
        _userFieldText = username;
        emit userFieldTextChanged();
    }
signals:
    void userFieldTextChanged();
private:
    QString _userFieldText;
};
class WorkerThread: public QThread
{
    Q_OBJECT
public:
    using QThread::QThread;
    ~WorkerThread() override {
        requestInterruption();
        quit();
        wait();
    }
signals:
    void textChanged(const QString &);
protected:
    void run() override{
        while (!isInterruptionRequested()) {
            emit textChanged(QString("set by backend: %1 ").arg(counter));
            QThread::msleep(100);
            counter++;
        }
    }
private:
    int counter = 0;
};
int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);
    BackEnd backend;
    WorkerThread thread;
    QObject::connect(&thread, &WorkerThread::textChanged, &backend, &BackEnd::setUserFieldText);
    thread.start();
    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("backend", &backend);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
    return app.exec();
}
#include "main.moc"

main.qml

import QtQuick 2.9
import QtQuick.Window 2.2

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
    Text {
        anchors.centerIn: parent
        text: backend.userFieldText
    }
}