Detaching Q_Property before QML Component has been destroying.

Requ

I got the Qml property which value is base on value passed from backend via reading Q_Property:

property bool isFullScreen: System.Fullscreen

Everythis worked fine, until I was needed to upgrade project to 5.14.2. Now when I'm closing application I got error:

qrc:/qml/main.qml:13: TypeError: Cannot read property 'Fullscreen' of null

No I'm trying to figure out how can if thats happen. I was trying to use Component.onDestruction , but it is emited after backend has been detached from qml front.

Also I was trying this:

property bool isFullScreen: 
{ 
 if (Component.status === Component.Ready)
      return System.Fullscreen 
 else
    return false
}

But the Component is binding only to false

Any idea?

sierdzio

Your backend gets destroyed before QML does. The solution is usually to rearrange how you initialise things in your main.cpp. Make sure backend outlives the engine and you won't see these errors (btw. they are completely harmless).

Requ

Hi @sierdzio I was thinking about it but not sure how to do that.

Currently I just have the Connection between QApplication signal and my backend slot to clean befor quiting the App.

I was trying to add

QObject::connect(&app, &QApplication::aboutToQuit, [&engine, &system] {  engine.quit(); system.disconnect(); )};

But it doesn't change anything. I guess there should be a method to stop engine but can't find in docs https://doc.qt.io/qt-5/qqmlapplicationengine.html

Am I doing something wrong or maybe you know how the enginge should be stopped?

sierdzio

You can parent your QObjects to QML engine and Qt will clean it up for you.

QQmlEngine engine;
So something like: auto system = new System(&engine);
engine.rootContext()->setContextProperty("System",  system);

Requ

@sierdzio Hmm.. that's how I connected them and somehow from 5.14.x this isn't working correctly.

QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("theSystem", integrator->system());

sierdzio

Yeah, but where is that integrator created? What manages it's lifetime?