QGestureRecognizer.registerRecognizer() crashes using PySide2

[nutrx]

@jeremy_k yes absolutely, I didn't pay attention to that method, because it still fails if I ignore otherwise, or everything from the beginning

    def recognize(self, state: QGesture, watched, event: QEvent) -> QGestureRecognizer.Result:
        return QGestureRecognizer.Ignore

or in the example above:

    def recognize(self, state: QGesture, watched, event: QEvent) -> QGestureRecognizer.Result:
        if event.type() == QEvent.TouchBegin:
            touch_event: QTouchEvent = event
            if len(touch_event.touchPoints()) == 1:
                state.press = touch_event.touchPoints()[0]
                state.pos = state.press
                return QGestureRecognizer.MayBeGesture
        elif event.type() == QEvent.TouchUpdate:
            touch_event: QTouchEvent = event
            state.last = state.pos
            state.pos = touch_event.touchPoints()[0]
            return QGestureRecognizer.TriggerGesture
        return QGestureRecognizer.Ignore

Shouldn't that be sufficient?

[jeremy_k]

@nutrx said in QGestureRecognizer.registerRecognizer() crashes using PySide2:

@jeremy_k yes absolutely, I didn't pay attention to that method, because it still fails if I ignore otherwise, or everything from the beginning

    def recognize(self, state: QGesture, watched, event: QEvent) -> QGestureRecognizer.Result:
        return QGestureRecognizer.Ignore

...

Shouldn't that be sufficient?

I'm not sure what you mean here. With the addition of the default Ignore, the application displays a window and doesn't do anything noteworthy in my environment. Are you still experiencing a crash?

[nutrx]

@jeremy_k yes, exactly. I just tried to get it to run, so I could move on from there. But it still crashes the same as before. Sorry, I don't have PyQt set up rn

[jeremy_k]

PanGestureRecognizer doesn't have an __init__ to call QGestureRecognizer.__init__. I vaguely recall reports of misbehavior using PyQt if a Qt object's constructor isn't invoked. This may have been limited to QObjects. It might not apply to PySide. Either way, it's an inexpensive experiment to run. QGestureRecognizer's C++ constructor is empty, but there may be some bookkeeping on the Python side.

[nutrx]

@jeremy_k you mean, I should try just calling QGestureRecognizer's __init__ in PanGestureRecognizer.__init__? I would've guessed that's ambiguous, or does instanciation lead to a direct call of the C++ parent constructor instead of the parent's __init__ in Python? However, I just tried, still no luck :/

[jeremy_k]

class PanGestureRecognizer(QGestureRecognizer):
    def __init__(self):
        super().__init__()

This doesn't seem to be necessary with pure python classes. The wording of object.__init__ led me to believe that it was, but a test suggests that the base class __init__ is called if no derived __init__ is defined.

[JonB]

@jeremy_k
Well, in Python just like in, say, C++, if you do not define your own __init__() (constructor in C++) the language will always cause the base class's __init__()/constructor. If you do define your own, you must then explicitly call the base class __init__()/constructor, in either language. in Python.

[EDIT As @jeremy_k pointed out below, I was wrong to say this applies in all C++ cases. Let's leave it as: in Python, if you define any derived __init__() you must (should) explicitly call the base __init__() from there; but if you do not define a derived __init__() the base one will still be called.]

I vaguely recall reports of misbehavior using PyQt if a Qt object's constructor isn't invoked

I don't know if somehow PySide2/PyQt requires you to explicitly define your own __init__() and call the base, unless you have some reference for that?

[jeremy_k]

@JonB said in QGestureRecognizer.registerRecognizer() crashes using PySide2:

@jeremy_k
Well, in Python just like in, say, C++, if you do not define your own __init__() (constructor in C++) the language will always cause the base class's __init__()/constructor. If you do define your own, you must then explicitly call the base class __init__()/constructor, in either language.

That's inaccurate for C++ base class default constructors. The compiler will insert a call to the base constructor if the programmer does not. Explicit invocation is only required if there are arguments to pass.

[JonB]

@jeremy_k
You are quite correct, and thank you for pointing this out. Partly I was unconsciously thinking of constructors with parameters, and partly I make slips as I swap between C#/C++/Python/JavaScript! I will correct my previous post, so as not to mislead.

[SGaist]

You may have found a bug in the PySide2 implementation. Did you check if you have the same behaviour with PySide6 ?

[nutrx]

@SGaist yes, same with PySide6

[SGaist]

Then you should go to the bug report system and see if there's already something there. If not, please consider opening a new issue providing your minimal runnable script that shows that behaviour.

[nutrx]

Yeah, just found a bugreport reporting this exact issue. It's open since 2015..........
I am really confused. I mean ... isn't this making the whole custom gesture recognizer system completely unsuable?

[SGaist]

It's an issue on Python and it has likely not been used that much with that language. You can add a comment on the issue to remind the current assigned person that this is still not solved.