I have multiple qml Files that get pushed via StackView. How do I connect them to C++

Question

My Projects contains 6 qml Files: The main.qml opens a new ApplicationWindow and declares the toolbar. It also initalizes StackView with the initalItem homescreen.qml. On the Home Screen I have different buttons which open different qml Files, via stack.push("URL"). Besides the main.qml all Files start with Item{}. I've been able to connect signals from the main.qml and the home.qml. But I've been unable to access Objects that are deeper in the stack. I don't know if I hvae to change my .cpp code to access the other objects, or if I should change the Initalization of StackView, so that all Files are loaded and accessible at the beginning. Here is the code, broke down to the very basics:

• main.qml

  ApplicationWindow {
        Rectangle{
                  id: homeButton
                  objectName: "homeButton"
                  signal qmlSignal(string msg)
                  MouseArea {
                       onClicked:  {stack.push({item:"qrc:/home.qml}); homeButton.qmlSignal("Hello")}
                  }
        }
        StackView{
             initalItem: "qrc:/home.qml"
        }
  

  }

• secondframe.qml // A randomw qml File that comes after the Home Screen

  Item {
        Rectangle{
                  id: test
                  objectName: "test"
                  signal qmlSignal(string msg)
                  MouseArea {
                       onClicked:  {stack.push({item:"qrc:/thirdframe.qml}); test.qmlSignal("Hello")}
                  }
        }
  }
  

• main.cpp

  QApplication app (argc, argv);
  QQmlEngine enigne;
  QQmlComponent component(&engine, QUrl(QStringLiteral("qrl:/main.qml")));
  QObject *object = componet.create();
  QQmlComponent newcomponent(&engine, QUrl(QStringLiteral("qrl:/secondframe.qml")));
  QObject *newobject = newcomponet.create();
  
  MyClass myClass
  QObject *home = object->findChild<QObject*>("homeButton");    // I'm able to connect to every Object in the main.qml or home.qml
  QObject::connect(home,SIGNAL(qmlSignal(Qstring)), &myClass, SLOT(cppSlot(QString)));
  QObject *test = newobject->findChild<QObject*>("test");       // Can't connect to the Objects in secondframe.qml
  QObject::connect(test,SIGNAL(qmlSignal(Qstring)), &myClass, SLOT(cppSlot(QString)));
  

Answer 1

A way better approach than to reach into the QML tree and pull out objects that might or might not be there is to provide C++ based API to QML.

1. Create a QObject based class that has the methods QML needs to be able to call as slots or Q_INVOKABLE

   class MyAPI : public QObject
   {
       Q_OBJECT
   public slots:
       void cppSlot(const QString &text);
   };
   

2. Create an instance of that and expose it to QML

   MyAPI myApi;
   QQmlEngine engine;
   engine.rootContext()->setContextProperty("_cppApi", &myApi);
   

3. Use in QML as if "_cppApi" is an object id

   MouseArea {
        onClicked:  {stack.push({item:"qrc:/thirdframe.qml}); _cppApi.cppSlot("Hello")}
   }
   

Answer 2

After pushing qmls on the stack, objects will be accessible. So when you emit some signal, when object will be on the stack, solution can look like this:

MyClass myClass
QObject *home = object->findChild<QObject*>("homeButton");    // I'm able to connect to every Object in the main.qml or home.qml
QObject::connect(home,SIGNAL(qmlSignal(Qstring)), &myClass, SLOT(cppSlot(QString)));

connect( myClass, MyClass::emitWhenQMLIsAlreadyOnTheStack, this, [this](){
    QObject *test = newobject->findChild<QObject*>("test");       // Now you should be able to connect
    QObject::connect(test,SIGNAL(qmlSignal(Qstring)), &myClass, SLOT(cppSlot(QString));
});