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QML engine rootObjects still empty when Component.onCompleted is invoked

  • Why is the app engine's rootObject list still empty even when Component.onCompleted is invoked?

    I want to call a C++ singleton backend method from qml when the engine has completely loaded the qml, without any user interaction. The backend method needs to access a specific qml item, so the method takes an objectName as input parameter, then invokes rootObject->findChild(objectName) to get the item's object. The qml looks like this:

    Window {
      Surface3D {
         Surface3DSeries {
           objectName = "myseries"
      // Invoke BackEnd C++ method when Window is complete
      Component.onCompleted: {

    BackEnd.foo() looks like this:

    bool BackEnd::foo() {
      char *seriesName = seriesNameString.toLatin1().data();
      if (g_appEngine->rootObjects().isEmpty()) {
          // rootObjects are empty - how to find seriesName object???
          qDebug("rootObjects are empty");
          return false;
        // Get root window
      QObject *root = qobject_cast<QQuickWindow*>(g_appEngine->rootObjects().value(0));
     QSurface3DSeries *surface3DSeries = 
           root->findChild<QSurface3DSeries *>(seriesName);
      if (!surface3DSeries) {
          qCritical() << "Couldn't find QSurface3DSeries " << seriesNameString << " object";
          return false;

    So how can I invoke BackEnd.foo() from qml and find the object specified by objectName?


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