How to get a valid object instance of a QQuickItem on C++ side

[Nelson_Piquet]

Alright. I have searched a lot but haven't got a good solution yet. I am new to Qt. I have a class which is a QQuickItem like so,

class MyQuickItemClass : public QQuickItem
{
    Q_OBJECT
    SetInfo(SomeCppClass object)
};

I do a qmlRegisterType in my main.cpp to register it on the qml side like this,

qmlRegisterType< MyQuickItemClass >("MyQuickItemClass", 1, 0, "MyQuickItemClass");

All fine till here. But -> I want to set an object instance & some properties in MyQuickItemClass which some C++ logic in it as well & then pass the MyQuickItemClass object to qml. Or, get a valid instance of MyQuickItemClass from Qml. How can I get a vlid instance MyQuickItemClass object instance from QML on C++ side in main.cpp ?

I tried doing the following learning from the link here. But this technique creates two separate objects of MyQuickItemClass. One from QML, & one from c++ side. Hence does not work for me.

Following is how I am trying to do this after lot of searching.

int main(int argc, char *argv[]) 
{
  qmlRegisterType< MyQuickItemClass >("MyQuickItemClass", 1, 0, "MyQuickItemClass");
  QQmlApplicationEngine engine;
  SomeCppClass someCppClassObject;
  someCppClassObject.updateSomething();

  MyQuickItemClass myquickItemObject;
  myquickItemObject.SetInfo(someCppClassObject);
  engine.rootContext()->setContextProperty("myquickItemObject", &myquickItemObject);

  engine.load(QUrl(QStringLiteral("qrc:/qml/main.qml")));
  return app.exec();
}

But, doing the above gets the constructor of MyQuickItemClass called twice. Once from cpp side when I created an object, and once from qml side. Verified this by placing a breakpoint in the constructor of MyQuickItemClass as well. As a result, someCppClassObject that I had set is null inside MyQuickItemClass when program runs. Because qml has made the final call to MyQuickItemClass to instantiate, thusly ignoring the MyQuickItemClass object that I created in main.cpp.

Here is my qml´ code for MyQuickItemClass`:

import QtQuick 2.5
import MyQuickItemClass 1.0

ParentContainerItem {
  id: parentItem
  color: "black"

  MyQuickItemClass {
      id: myQuickItemID
      visible: true
      objectName: "myQuickItem"

      property bool someProperty1: false
      property bool someProperty2: true

      anchors.top: parent.top
      anchors.horizontalCenter: parent.horizontalCenter
  }

  //Other qml components
}

And this is the C++ class whose object needs to be set into MyQuickItemClass.

SomeCppClass {
  //Pure C++ class. No Qt
}

Please note that I need to keep MyQuickItemClass derived from QQuickItem. Please suggest...

[p3c0]

@Nelson_Piquet Use findChild to find that instantiated QML type from C++ using objectName and cast it to QQuickItem

[Nelson_Piquet]

@p3c0 Thanks for your attention on this. Can you pls show some sample code ?

[p3c0]

@Nelson_Piquet Follow this link:
http://doc.qt.io/qt-5/qtqml-cppintegration-interactqmlfromcpp.html#accessing-loaded-qml-objects-by-object-name

[Nelson_Piquet]

@p3c0 I saw the code in the link you provided.

QObject *rect = object->findChild<QObject*>("rect");
if (rect)
     rect->setProperty("color", "red");

Can I pass a pure C++ class object instance into setProperty ?

[p3c0]

@Nelson_Piquet I guess. Maybe you can try QVariant::fromValue but first Does this solve your original question? I.e getting access of QML object from C++?

[Nelson_Piquet]

@p3c0 I read & tried to understand this technique of findChild. Not sure how I can change my main.cpp to include this technique of findchild. Doesnt it require to change the way I am loading my main.qml ? Below is the full code demonstration of how I should do this with findchild. Right ?

QQuickView view;
view.setSource(QUrl::fromLocalFile("main.qml"));
view.show();
QObject *object = view.rootObject();
QObject *rect = object->findChild<QObject*>("MyQuickItemClass");
if (rect)
    rect->setProperty("color", "red");

But Can I set a C++ class object into SetProperty ?

I looked at some other possibilities: Below is something I have cooked up but YET TO SOLVE THE PROBLEM !!!!

qmlRegisterType< MyQuickItemClass >("MyQuickItemClass", 1, 0, "MyQuickItemClass");
QQmlApplicationEngine engine;
HelloCpp cppObjectOnqmlSide;
qmlRegisterType<HelloCpp>("HelloCppClass", 1, 0, "HelloCppObj");
engine.rootContext()->setContextProperty("theCppObject", &cppObjectOnqmlSide);
engine.load(QUrl(QStringLiteral("qrc:/qml/main.qml")));
return app.exec();

Now on the QML side, below is what I am doing

import HelloCppClass 1.0

HelloCppObj {
        id: theCppObject
}

MyQuickItemClass {
  id: myQuickItemID
  visible: true
  objectName: "myQuickItem"

  property bool someProperty1: false
  property bool someProperty2: true

  anchors.top: parent.top
  anchors.horizontalCenter: parent.horizontalCenter

 Component.onCompleted: {
     print("MyQuickItemClass Component.onCompleted called")
     theCppObject.printMessage("someMessage") //Can I pass MyQuickItemClass object into printMessage ?
  }

}

My question is : Can I pass MyQuickItemClass object into printMessage ? How should I change printMessage to do that ? If I can, then this should solve my problem

[p3c0]

@Nelson_Piquet Your original question as seen in the title of this post:

How to get a valid object instance of a QQuickItem on C++ side

Yes. Using findChild was the answer.

Now coming to your second question:

Can I pass MyQuickItemClass object into printMessage ? How should I change printMessage to do that ? If I can, then this should solve my problem

Yes. Modify your printMessage function to accept MyQuickItemClass class instance as a paramater. Then when you call the function from QML just pass this to it:

Component.onCompleted: {
     print("MyQuickItemClass Component.onCompleted called")
     theCppObject.printMessage(this) 
}

Or if its from outside just pass MyQuickItemClass id i.e myQuickItemID

[Nelson_Piquet]

@p3c0 Sorry about swaying from the original question. Thanks a lot for all the information. That helps.