Using Signals in QGraphicsItem
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@JonB
Yes that's what is happening. I am using PySide6 and here is an example:from PySide6.QtCore import QObject, QPointF, Signal from PySide6.QtGui import QPen, Qt from PySide6.QtWidgets import QApplication, QGraphicsItem, QGraphicsView, QGraphicsScene, QGraphicsLineItem class GraphicsView(QGraphicsView): def __init__(self, parent=None): super().__init__(parent) scene = QGraphicsScene() self.line = CroppingLine(0, 0, 100, 100) scene.addItem(self.line) self.setScene(scene) class CroppingLine(QGraphicsLineItem, QObject): moved = Signal(object, QPointF) def __init__(self, x1: float, y1: float, x2: float, y2: float): super().__init__(x1, y1, x2, y2) self.setPen(QPen(Qt.GlobalColor.blue, 3)) self.setFlags(QGraphicsItem.GraphicsItemFlag.ItemIsMovable) def mouseMoveEvent(self, event): super().mouseMoveEvent(event) self.moved.emit(self, self.scenePos()) if __name__ == '__main__': import sys app = QApplication(sys.argv) window = GraphicsView() window.show() sys.exit(app.exec())About the swap of the superclasses, I dont think it would work because the
super().__init__()would not callQGraphicsLineIteminit function butQObject's.wrote on 25 Feb 2025, 09:12 last edited by JonB@adrien-lsh
Before you make me try this, I did ask you putQObjectfirst, that is at least required using C++/moc so I'd like you to test it. As I said, you should look up howsuper()works in Python, I think you can call something likesuper(QGraphicsLineItem).__init__()(actually it might besuper(QGraphicsLineItem, self).__init__()?) if you need to, but you are the Python programmer! -
@adrien-lsh
Before you make me try this, I did ask you putQObjectfirst, that is at least required using C++/moc so I'd like you to test it. As I said, you should look up howsuper()works in Python, I think you can call something likesuper(QGraphicsLineItem).__init__()(actually it might besuper(QGraphicsLineItem, self).__init__()?) if you need to, but you are the Python programmer!wrote on 25 Feb 2025, 09:23 last edited by@JonB
Nope it does not work withsuper(QGraphicsLineItem, self).__init__(...)because Python is trying to call the super class ofQGraphicsLineItemwhich isQGraphicsItemand my class does not directly inherit from it so it doesnt work.I did achieve the inheritance swap using
super(QObject, self).__init__(...)tho and my line displays correctly but it still crashes :/ -
@JonB
Nope it does not work withsuper(QGraphicsLineItem, self).__init__(...)because Python is trying to call the super class ofQGraphicsLineItemwhich isQGraphicsItemand my class does not directly inherit from it so it doesnt work.I did achieve the inheritance swap using
super(QObject, self).__init__(...)tho and my line displays correctly but it still crashes :/wrote on 25 Feb 2025, 09:24 last edited by@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
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....wrote on 25 Feb 2025, 09:31 last edited by@JonB
Thanks! Let me know if you need any more information. -
@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.... -
@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"?wrote on 25 Feb 2025, 09:32 last edited by@JonB
Try to move the line. -
@JonB
Try to move the line.wrote on 25 Feb 2025, 09:35 last edited by@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 aSIGSEGVfrom0x00007ffff711fc14 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.6.7Looking into that now...
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@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 aSIGSEGVfrom0x00007ffff711fc14 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.6.7Looking into that now...
wrote on 25 Feb 2025, 09:36 last edited by@JonB
Yeah I should've set a bigger width ^^. Thank you. -
@JonB
Yeah I should've set a bigger width ^^. Thank you.wrote on 25 Feb 2025, 09:50 last edited by 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 theQGraphicsLineItem, not initialising theQObject. 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. -
A adrien-lsh has marked this topic as solved on 25 Feb 2025, 10:03
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@adrien-lsh
I think it is to do with the initialisation, requiring both inherited classes to be initialised. You were only doing so for theQGraphicsLineItem, not initialising theQObject. 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.wrote on 25 Feb 2025, 10:03 last edited by@JonB
Of course ! It never initialized theQObjectso signals would never work.
Thank you for your help ! -
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]
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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]
wrote on 25 Feb 2025, 10:27 last edited by JonB@SGaist
But the OP started with justsuper().__init__(x1, y1, x2, y2)and, for whatever reason, that segments (later, on signal emit) because the
QObjectpart 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
But the OP started with justsuper().__init__(x1, y1, x2, y2)and, for whatever reason, that segments (later, on signal emit) because the
QObjectpart 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?@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.
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wrote on 25 Feb 2025, 20:02 last edited by NlightThis post is deleted!
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