Connecting Qt functionality with Qml

  • Hi ,

    Am writing play function to play music in Qt Cpp file but my UI is in Qml if i wanted to move that music slider according to my current playing song how can i link qt functionality with that slider image on my UI

  • So my first basic solution is this:
    in C++:

    // Update playtime information for UI
    // repeat this every tick
    engine->rootContext()->setContextProperty("playTime", secondsPlayed);
    // set songLength property when you change a song
    engine->rootContext()->setContextProperty("songLength", secondsSongLength);

    in QML you have then say a value property for your slider:

    Slider {
       value: playTime/songLength

    QML binding will ensure that value will be recalculated each time the properties playTime/songLength are changed

  • Moderators

    instead of consecutively writing context properties i would suggest a cleaner way. (i am not quite sure if property binding is really working for context properties?)

    Create a custom (QObject derived) controller class and correctly define your properties. On each update notification just set the property.

    class MyController : public QObject
           Q_PROPERTY( qreal progress READ progress WRITE setProgress NOTIFY progressChanged )
         void progressChanged( qreal progress );
           qreal progress() const { return mProgress; }
           void setProgress(qreal progress)
                  if( mProgress != progress )
                      mProgress = progress;
                      emit progressChanged(progress);
          qreal mProgress;
    // register type to QML
    qmlRegisterType<MyController>();  //tell QML the available properties, methods, etc.; Also this type wont be createable within QML
    engine->rootContext()->setContextProperty( "MyControllerContextProperty", new MyController(engine) );
    MyItem {
          id: root
          Connections {
                  target: MyControllerContextProperty
                  onProgress: root.doSomething()
          // or also possible
          ProgressBar {
                 value: MyControllerContextProperty.progress   //gets automatically updated when you update the property in C++

    This technique works with every QObject derived class. Read on here.

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