Initialize QML-signal before Component.onCompleted

Question

I have the following problem: I wrote a qml-GUI and an interface class to communicate with some C++ code by connecting signals on the qml-side with slots on the c++ side. The event or button based trigger work fine, but I need one signal that has to be triggerd directly at startup. I tried it by using Component.onCompleted from my ApplicationWindow. Howevery,

the output "setInitDrone() called" is generated but getInitDrone() is never reached. The QT documentation says: "The order of running the onCompleted handlers is undefined."

Can I make sure that the signal has already been initialized when I'm trying to send it, or is there any other method instead of using Component.onCompleted?
Thanks for any help!

main.qml:

ApplicationWindow{   
  id: appWindow
  visible: true
  minimumHeight: 800
  minimumWidth: 700
  visibility: Window.Maximized

  signal setInitDrone()

  Component.onCompleted: {
    setInitDrone()
    print("setInitDrone() called")
  }
}

qml_cpp_interface.cpp:

void Qml_cpp_interface::getInitDrone(){
  qDebug() << "Cpp initDrone received";
  flightserver.init();
}

groundstation.cpp:

QGuiApplication app(argc, argv);

QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

//Connect with C++ code
QObject *item = engine.rootObjects().first();
Qml_cpp_interface qml_cpp_interface;

QObject::connect(item, SIGNAL(setInitDrone()), &qml_cpp_interface,SLOT(getInitDrone()));

return app.exec();

dtech dtech · Accepted Answer · 2017-07-03T21:04:40

You are doing it wrong, instead of accessing QML stuff from C++, you should be accessing C++ from QML.

Expose Qml_cpp_interface to QML as it would make sense exposing a core logic interface. Since you are doing initialization, you don't even need a signal, since presumably, you are only going to initialize once, which is what it means to "initialize".

Then you can simply call the initialization via

Component.onCompleted: Qml_cpp_interface.getInitDrone()

Also, I don't see any valid reason to initialize from QML, I mean you can directly initialize from C++, even implicitly from Qml_cpp_interface's constructor. So by the time your GUI loads, you are already initialized.

Initialize QML-signal before Component.onCompleted

1 Answers