Using Signals in QGraphicsItem

JonB

@adrien-lsh
Since you seem to have provided a nice, standalone, complete example I can copy and paste --- unlike so many other people --- I will give it a play now....

adrien-lsh

@JonB
Thanks! Let me know if you need any more information.

JonB

@JonB
Your code above, copied and pasted, works fine for me. Ubuntu 24.04, Qt 6.4.2, PySide6. Or at least I get an open window with a line in it. Could you be very specific about what you do to make it "crash"?

adrien-lsh

@JonB
Try to move the line.

JonB

@adrien-lsh
Yeah, that's easier said than done! It's so thin, and I can't tell when I have actually grabbed it or not! Anyway I have reproed now, it does a SIGSEGV from

0x00007ffff711fc14 in PySide::SignalManager::emitSignal(QObject*, char const*, _object*) ()
   from /home/jon/.venv/pyside6/lib/python3.12/site-packages/PySide6/libpyside6.abi3.so.6.7
(gdb) bt
#0  0x00007ffff711fc14 in PySide::SignalManager::emitSignal(QObject*, char const*, _object*) ()
    at /home/jon/.venv/pyside6/lib/python3.12/site-packages/PySide6/libpyside6.abi3.so.6.7

Looking into that now...

adrien-lsh

@JonB
Yeah I should've set a bigger width ^^. Thank you.

JonB

@adrien-lsh
I think it is to do with the initialisation, requiring both inherited classes to be initialised. You were only doing so for the QGraphicsLineItem, not initialising the QObject. I find the following does work without crashing:

class CroppingLine(QObject, QGraphicsLineItem):
    moved = Signal(object, QPointF)

    def __init__(self, x1: float, y1: float, x2: float, y2: float):
        #super(QGraphicsLineItem, self).__init__(x1, y1, x2, y2)
        QObject.__init__(self)
        QGraphicsLineItem.__init__(self, x1, y1, x2, y2)

There may be other ways of doing this with super() and/or not swapping the inheritance order, I just know this works fine.

adrien-lsh

@JonB
Of course ! It never initialized the QObject so signals would never work.
Thank you for your help !

SGaist

Hi,

It's Python3, remove the class from your super calls. This should allow Python to do proper initialisation of the chain of classes (though sometimes it might fail for complex cases).

[Edit: we are in such a complex case]

JonB

@SGaist
But the OP started with just

super().__init__(x1, y1, x2, y2)

and, for whatever reason, that segments (later, on signal emit) because the QObject part has not been initialized, apparently. That is why I tried splitting it into two separate explicit initializations, which makes it work. So what exactly are you proposing?

SGaist

@JonB I misread the code !

We are in the territory where your solution (using __init__) is the way to go. Multiple inheritance in Python is quite tricky...

QObject expects an optional parent parameter which is not given here but it's not expecting the parameters for the QGraphicsLineItem constructor hence the failure. You can try to use the kwargs trick however it won't work either as it's not part of any of the method signature and thus can't work either.

Nlight

This post is deleted!