Determine the owner of the QML object attached to C++

I have a C++ class that I exposed to QML the usual way, and I'm tying to control QML. The problem is that it cannot be instantiated without setting. This is exactly the same that how LayoutUpdateHandler works, but I just can't understand how this class manages about who controls it is attached to.

The LayoutUpdateHandler constructor takes a pointer to a control as an argument. There is a constructor without arguments, but the docs say that if you use it, there is no way to define the target control later, do the actually useless LayoutUpdateHandler. Here is a typical use of QML:

Container {
   attachedObjects: [
      LayoutUpdateHandler {
         onLayoutFrameChanged: {
            // ---Do some stuff
         }
      }
   ]
}

Don't forget that LayoutUpdateHandler MUST have the target of control set when it is instantiated, it can be done later. Somehow the code above works, and the class instantiated with the parent container as the argument to the constructor object. Now I need to do the same thing with my own class, but I can't understand how to move on the instantiation of the parent control. No matter what I try the argument never happened and only the parameterless constructor is never called.

Anyone have an idea how it works and how I can get the QML to instantiate my class with the parent control passed as argument?

You're not the only dev who has tried to do.  I did it too and have been successful. You are on the right track.

Your class must inherit from BaseObject and override its classBegin() function.  This function will be called after your class has been instantiated, and her mother has been defined.

For example, I'm doing something like this:

class MyClass : public BaseObject
{
public:
    explicit MyClass();
    explicit MyClass(Control *target)
    virtual ~MyClass();
    virtual void classBegin();
private:
    void setTargetControl(Control *target);
};

MyClass::MyClass() : BaseObject(0)
{
}

MyClass::MyClass(Control *target) : BaseObject(target)
{
    setTargetControl(target);
}

MyClass::~MyClass()
{
}

MyClass::classBegin()
{
    BaseObject::classBegin();
    if(parent()) {
        Control *target = qobject_cast(parent());
        setTargetControl(target);
    }
}

MyClass::setTargetControl(Control *target)
{
    // Do your actual initialization here, which requres the target control.
    // In this case, the target control is the control that has this as part of
    // its attached objects block in QML-land
}

Tags: BlackBerry Developers

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