[Interest] Adding a C++ wrapper class renders my QML custom type unusable

[Mitch Curtis]

Can you provide a small, reproducible example?

From: Interest [mailto:interest-bounces+mitch.curtis=qt.io at qt-project.org] On Behalf Of Rob Allan
Sent: Wednesday, 31 August 2016 8:09 AM
To: interest at qt-project.org
Subject: [Interest] Adding a C++ wrapper class renders my QML custom type unusable


I have a custom QML type, CustomButton, defined in CustomButton.qml. Initially this was a fairly simple type, with a 'clicked' signal, and a few JavaScript functions that set up the button content. I was able to use this custom button from other QML files, and from C++ code (using QMetaObject::invokeMethod() to invoke its methods), and it all worked pretty well.

As this button became more complex, I realised that I really needed a C++ wrapper class (or backing class?) to deal with some of the additional complexity. I've added C++ wrapper classes to other QML types before, with varying degrees of success, and I think I understand the basic steps involved. I did the following:

  1.  Created a minimal wrapper in CustomButton.h and CustomButton.cpp (I made this as minimal as possible to begin with so as not to affect the existing behavior - I thought!):

#include <QQuickItem>



class CustomButton : public QQuickItem

{

    Q_OBJECT



public:

    CustomButton();

};

  1.  Added the necessary qmlRegisterType() call to my application startup code:

qmlRegisterType<CustomButton>("com.glob.myApp", 1, 0, "CustomButton");

  1.  Added an import statement to my CustomButton.qml file:

import com.glob.myApp 1.0

  1.  Changed the root item in CustomButton.qml from 'Item' to 'CustomButton'.

So far all appeared to be OK - the project compiled, and CustomButton.qml appeared to be error-free in the QML editor. But then things started going downhill.

I noticed that, in another QML file that used CustomButton, it could no longer 'see' the signals on my type - a reference to 'onClicked' was red-underlined, and hovering over it showed 'Invalid property name'. At runtime, it failed to create the referencing QML object due to this error. It looks as if the introduction of the C++ wrapper class has 'hidden' the signals defined in the original CustomButton.qml file.

I found I could get past this error by deleting the signal definitions from CustomButton.qml, and instead adding them to CustomButton.h:

signals:

    void clicked(const QString text, int eventCode);

    etc...

That allowed it to build and run. I'm not sure whether this was correct and the signals would now have worked, because I then struck another problem - my existing C++ code could no longer invoke the JavaScript functions defined in CustomButton.qml. For example, when I tried to invoke a method 'setContent' (which previously worked just fine) I now got this runtime error:

QMetaObject::invokeMethod: No such method CustomButton::setContent(QVariant,QVariant)

Again, it appears that adding a C++ wrapper class has 'hidden' the function definitions in the QML file, that were previously available to the rest of the system.

OK, I thought, maybe I have to define these functions on the C++ class in some way. I tried declaring an INVOKABLE function in the .h file that matched my JavaScript function, and that failed with a link error - the compiler wanted me to define an implementation for this function in my CPP file - but I don't want to implement it in my CPP file, as the implementation exists in the QML file! Just to see where it got me, I tried adding an implementation to the CPP file, and inside this function, attempted to "invoke" the JavaScript function. That failed, presumably because I had now introduced a kind of circularity and was probably attempting to invoke the same handler that I was already in!

I also tried declaring these functions as 'slots' on the C++ class, but fared no better - the compiler still wanted a CPP implementation, and I wasn't sure if I was really meant to add one, and if I did, how I would then invoke the JavaScript function from it.

In short - adding a C++ wrapper class (with almost nothing in it) has broken a previously functional QML type implementation, by apparently 'hiding' signals and functions that exist in the QML.

Can anyone advise what I'm doing wrong here? Or suggest some relevant documentation that might give me the answers I need?

Thanks,
Rob
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