Signal-Slot connection after destruction of qml objects? - c++

Question

What happens with the connection of signals from c++ objects to qml objects slots after destruction of qml objects?
Item {
function qmlFunction() {
cppObject.cppObjectFunction()
}
Component.onCompleted: {
cppObject.someSignal.connect(qmlFunction);
}
Component.onDestruction: {
cppObject.someSignal.disconnect(qmlFunction);
}
}
Before I wrote Component.onDestruction with disconnect() the program displayed an error message:
qrc:/qml/xxxxxxxx.qml:77: TypeError: Cannot call method 'cppObjectFunction' of undefined
Is the disconnection of signals and slots not performed automatically?
Object cppObject "always" exist and passed to qml this way:
main.cpp
QQmlApplicationEngine engine;
Model* model = new Model::instance(&engine);
engine.setObjectOwnership(model, QQmlEngine::CppOwnership);
engine.rootContext()->setContextProperty("cppObject", model);
engine.load(QUrl(QStringLiteral("qrc:/qml/main.qml")));
QML item is loaded by the Loader and can be reloaded several times during the program. Naturally, the error occurs after the QML item has been reloaded and cppObject triggers a signal someSignal.
on windows: Qt 5.6.2 <- my program log error to debug console
on linux: Qt 5.9.2 <- my program crashes
main.qml
import QtQuick 2.3
import QtQuick.Controls 1.2
import QtQuick.Layouts 1.1
import QtQuick.Dialogs 1.2
import QtQuick.Controls.Styles 1.3
import QtQuick.Window 2.2
File with error
import QtQuick 2.3
import QtQuick.Layouts 1.1
import QtQuick.Controls 1.3

Answer 1

Easy solution is to :
istantiate the cppObject directly from QML to ensure that both objects are properly cleaned up by adding to your main.cpp file before you load the QML file
qmlRegisterType<Model>("com.your.app", 1, 0, "CppModel");
and then in your QML file include:
import com.your.app 1.0
....
CppModel {
id: cppObject
onSomeSignal: { cppObject.cppObjectFunction(); }
}
This way will make the extra connections and the extra qmlFunction() both unnecessary.
and maybe this too:
If cppObjectFunction() is a public slot from cppObject that is defined on the C++ side, you can try adding Q_INVOKABLE to the front of it in order to add it to the metaobject system.
Q_INVOKABLE void cppObjectFunction() { /* your code */ }