[SOLVED] Problem using QML object in C++ method !

[Manfred]

Hi everybody!

I need your help with something I thought trivial but that is driving me nuts

I have got a QList list of C++ ClassX objects that are instantiated in my main C++ file.
The ClassX is registered with the QML engine an I can access the list and its item just fine in QML.

Now here comes the problem:
I've a C++ method defined as invokable that takes a pointer to a ClassX object.
When I call the method from QML, passing one ClassX object from the list (which is perfectly ok in QML, I can access its attributes) but the argument on the C++ side is NULL.

Any ideas?
thanks in advance!
best regards
Manfred

[p3c0]

@Manfred How do you pass the ClassX object from QML side ? Have you instantiated it ?

[Manfred]

@p3c0 Hi thanks for your reply!
Yes, I instantiate the ClassX object on the C++ side and add it to a QList which is created in C++ as well
QList<ClassX*> m_XList; // data model exposed to qml

the list is then exported via a property like this:
Q_PROPERTY(QQmlListProperty<ClassX> xList READ getXList NOTIFY xListChanged)

Using a qml ListView I can access the C++ model just fine but when I call my C++ method from the same QML file, passing one item of that ListView, the pointer argument on the C++ side is null!
here is the C++ method declaration:
Q_INVOKABLE void doSomething( ClassX *x);

BTW is there a way to mark text as source on this forum?
thanks in advance!
cheers
Manfred

[p3c0]

@Manfred As I said earlier you will need to instantiate it from QML side or somehow have access to that particular object. From the docs:

If a C++ method has a parameter with a QObject* type, the parameter value can be passed from QML using an object id or a JavaScript var value that references the object.

So from QML side you can either do something like this:

import ClassX 1.0 //assuming registered already
ClassX { //instantiate
  id: classx
}

doSomething(classx) //access using id

Or since you have ListView get that particular instance may be by creating a new Q_INVOKABLE method which will return the required instance and upon return hold it in a var on QML side and pass it to the above function.

BTW is there a way to mark text as source on this forum?

Do you mean highlighting source code ?

[Manfred]

@p3c0 Hi and thanks again for your reply!
Unfortunately I cannot instantiate the object in QML as it is part of the data model which is instantiated and managed by C++.
Well I think I'll change my Q_Invokable so that it takes an attribute of the object such as a string and then search it in C++ inside the list.

But I still do not understand why the pointer of the object gets null on C++ although it is valid on the QML side.

Concerning the source code, yes I meant highlight as you did it for the source examples in your reply.
How did you do that?

Thanks in advance and have a nice day!
cheers
Manfred

[p3c0]

@Manfred

But I still do not understand why the pointer of the object gets null on C++ although it is valid on the QML side.

How did you pass it from QML ? Can you post the relevant code ?
Also did you try the second method that I mentioned earlier.

Concerning the source code, yes I meant highlight as you did it for the source examples in your reply.
How did you do that?

Surround code with ``` (3 backticks) for blocks and ` (single) for one line. It is also posted at the bottom of this post.

[Manfred]

@p3c0 hI

thank you for your reply, sorry I didn't see the markdown stuff.. 8-)

Here is a much simplified version of my code. Please bear in mind that the object instance which is created on the C++ side is correctly exported to QML and that I can perfectly access any attributes of this same instance from QML.
But when I pass this object back to the C++ method, the pointer to it is NULL from inside the method.

I hope I'm clear enough ;-)

-------------------- C++ HEADER ------------------------
class App : public QObject
{
    Q_OBJECT
    Q_PROPERTY( SomeClass* classInstance READ get_classInstance NOTIFY classInstanceChanged)
public:
    Q_INVOKABLE void doSomething( SomeClass *classInstance );
	
    explicit App(QObject *parent = 0);
    virtual ~App();
    SomeClass *get_classInstance();
    QQmlListProperty<SomeClass> getSomeClassList();
protected:
    QList<SomeClass*> m_SomeClassList; // data model exposed to qml
    SomeClass* m_classInstance;

signals:
    void classInstanceChanged();

public slots:
};
-------------------- QML  ------------------------
import QtQuick 2.4
import com.mylibrary 1.0

Rectangle {

    property SomeClass classInstance: null

    Component.onCompleted: {
        classInstance = theApp.classInstance
    }

    MouseArea {
        anchors.fill: parent
        onClicked: {
            theApp.doSomething( classInstance );            
        }
    }
}

[Manfred]

Hi again,
really no ideas what's going on?

[p3c0]

@Manfred Sorry for late reply. To make use of pointers in Q_PROPERTY you have to register it too I suppose. Check this post.

[Manfred]

@p3c0 Hi thanks for your reply!

I read trough that post and double checked my code and I had everything ok.
While playing around, trying this and that it suddenly started working!
I think I had a problem where the object on the qml side was not yet initialized when calling the C++ method.
I now make sure this is the case in Component.onCompleted and all is fine!

Thanks again for all your input
have a nice day
Manfred