从 C++ 更新 QML 对象的正确方法？

Question

I've found this How to modify a QML Text from C++ but I've hear that it's not thread-safe to update QML objects lik this from C++

What is the proper way of doing so?

I think the simplest example (a text widget) is enough for me to understand.

Answer 1

Note: I have not pointed out that this code is not thread-safe, I have pointed out that your code in your previous question is not thread-safe since you modify the GUI from a different thread than the one it belongs to.

I have pointed out that the code of this answer is dangerous and not recommended because the life cycle of the QML elements are not managed by the developer and the QML engine could eliminate them without notifying us, therefore I recommend creating QObject to obtain or send information between C++ and QML.

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

class Helper: public QObject{
    Q_OBJECT
    Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged)
    QString m_text;
public:
    using QObject::QObject;
    QString text() const{
        return m_text;
    }
public slots:
    void setText(QString text){
        if (m_text == text)
            return;
        m_text = text;
        emit textChanged(m_text);
    }
signals:
    void textChanged(QString text);
};

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    Helper helper;
    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("helper", &helper);

    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    helper.setText("Change you text here...");

    return app.exec();
}
#include "main.moc"

main.qml

Text {
    id: text1
    color: "red"
    
    font.pixelSize: 12
}

or

Text {
    id: text1
    color: "red"
    text: "This text should change..."
    font.pixelSize: 12
}