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Using Signals in QGraphicsItem

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  • A Offline
    A Offline
    adrien-lsh
    wrote on 25 Feb 2025, 08:13 last edited by
    #7

    Ok I'm having another problem now :/ The program crashes when I try to move the lines (without any information in the console)

    class CroppingLine(QGraphicsLineItem, QObject):
        moved = Signal(object, QPointF)
    
        def __init__(self, x1: float, y1: float, x2: float, y2: float, is_vertical: bool):
            super().__init__(x1, y1, x2, y2)
    
            self.is_vertical = is_vertical
            self.setPen(QPen(Qt.GlobalColor.black, 2))
            self.setCursor(Qt.CursorShape.SizeVerCursor if is_vertical
                           else Qt.CursorShape.SizeHorCursor)
            self.setFlags(QGraphicsItem.GraphicsItemFlag.ItemIsMovable)
    
        def mouseMoveEvent(self, event):
            super().mouseMoveEvent(event)
            self.moved.emit(self, self.scenePos())
    

    Is there something I did wrong ?

    J 1 Reply Last reply 25 Feb 2025, 08:17
    0
    • A adrien-lsh
      25 Feb 2025, 08:13

      Ok I'm having another problem now :/ The program crashes when I try to move the lines (without any information in the console)

      class CroppingLine(QGraphicsLineItem, QObject):
          moved = Signal(object, QPointF)
      
          def __init__(self, x1: float, y1: float, x2: float, y2: float, is_vertical: bool):
              super().__init__(x1, y1, x2, y2)
      
              self.is_vertical = is_vertical
              self.setPen(QPen(Qt.GlobalColor.black, 2))
              self.setCursor(Qt.CursorShape.SizeVerCursor if is_vertical
                             else Qt.CursorShape.SizeHorCursor)
              self.setFlags(QGraphicsItem.GraphicsItemFlag.ItemIsMovable)
      
          def mouseMoveEvent(self, event):
              super().mouseMoveEvent(event)
              self.moved.emit(self, self.scenePos())
      

      Is there something I did wrong ?

      J Offline
      J Offline
      JonB
      wrote on 25 Feb 2025, 08:17 last edited by
      #8

      @adrien-lsh
      What happens if you do not attach a slot to the signal?
      What happens if you do not emit the signal?
      What happens if you define and emit a signal without any parameters?

      1 Reply Last reply
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      • A Offline
        A Offline
        adrien-lsh
        wrote on 25 Feb 2025, 08:23 last edited by adrien-lsh
        #9

        @JonB
        I tried all of that already, when the signal is emitted (even without or with different parameters) it crashes: the line does not move, it waits a bit, then the window closes.
        It crashes on the Signal.emit().

        btw sorry I cannot respond faster I have the 10m new guy cooldown

        J 1 Reply Last reply 25 Feb 2025, 08:38
        1
        • A adrien-lsh
          25 Feb 2025, 08:23

          @JonB
          I tried all of that already, when the signal is emitted (even without or with different parameters) it crashes: the line does not move, it waits a bit, then the window closes.
          It crashes on the Signal.emit().

          btw sorry I cannot respond faster I have the 10m new guy cooldown

          J Offline
          J Offline
          JonB
          wrote on 25 Feb 2025, 08:38 last edited by
          #10

          @adrien-lsh
          So you are claiming if you inherit from QObject, define a signal and emit it without a slot attached it crashes?

          Try swapping the inheritance order to make QObject the first class inherited from (i.e. (QObject, QGraphicsLineItem). This would be required for moc from C++, I don't know if it might be required from Python. I am (then) also slightly unsure about plain super() calls from Python, I don't know if you need to qualify that to indicate which of the two super classes to use.

          You have not said what version of Qt you are using and whether PySide or PyQt. If you produce a minimal, standalone example which I can copy and paste I will test here.

          1 Reply Last reply
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          • A Offline
            A Offline
            adrien-lsh
            wrote on 25 Feb 2025, 09:04 last edited by adrien-lsh
            #11

            @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 call QGraphicsLineItem init function but QObject's.

            J 1 Reply Last reply 25 Feb 2025, 09:12
            0
            • A adrien-lsh has marked this topic as unsolved on 25 Feb 2025, 09:09
            • A adrien-lsh
              25 Feb 2025, 09:04

              @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 call QGraphicsLineItem init function but QObject's.

              J Offline
              J Offline
              JonB
              wrote on 25 Feb 2025, 09:12 last edited by JonB
              #12

              @adrien-lsh
              Before you make me try this, I did ask you put QObject first, that is at least required using C++/moc so I'd like you to test it. As I said, you should look up how super() works in Python, I think you can call something like super(QGraphicsLineItem).__init__() (actually it might be super(QGraphicsLineItem, self).__init__()?) if you need to, but you are the Python programmer!

              A 1 Reply Last reply 25 Feb 2025, 09:23
              0
              • J JonB
                25 Feb 2025, 09:12

                @adrien-lsh
                Before you make me try this, I did ask you put QObject first, that is at least required using C++/moc so I'd like you to test it. As I said, you should look up how super() works in Python, I think you can call something like super(QGraphicsLineItem).__init__() (actually it might be super(QGraphicsLineItem, self).__init__()?) if you need to, but you are the Python programmer!

                A Offline
                A Offline
                adrien-lsh
                wrote on 25 Feb 2025, 09:23 last edited by
                #13

                @JonB
                Nope it does not work with super(QGraphicsLineItem, self).__init__(...) because Python is trying to call the super class of QGraphicsLineItem which is QGraphicsItem and 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 :/

                J 1 Reply Last reply 25 Feb 2025, 09:24
                0
                • A adrien-lsh
                  25 Feb 2025, 09:23

                  @JonB
                  Nope it does not work with super(QGraphicsLineItem, self).__init__(...) because Python is trying to call the super class of QGraphicsLineItem which is QGraphicsItem and 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 :/

                  J Offline
                  J Offline
                  JonB
                  wrote on 25 Feb 2025, 09:24 last edited by
                  #14

                  @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....

                  A J 2 Replies Last reply 25 Feb 2025, 09:31
                  0
                  • J JonB
                    25 Feb 2025, 09:24

                    @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....

                    A Offline
                    A Offline
                    adrien-lsh
                    wrote on 25 Feb 2025, 09:31 last edited by
                    #15

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

                    1 Reply Last reply
                    0
                    • J JonB
                      25 Feb 2025, 09:24

                      @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....

                      J Offline
                      J Offline
                      JonB
                      wrote on 25 Feb 2025, 09:31 last edited by
                      #16

                      @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"?

                      A 1 Reply Last reply 25 Feb 2025, 09:32
                      0
                      • J JonB
                        25 Feb 2025, 09:31

                        @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"?

                        A Offline
                        A Offline
                        adrien-lsh
                        wrote on 25 Feb 2025, 09:32 last edited by
                        #17

                        @JonB
                        Try to move the line.

                        J 1 Reply Last reply 25 Feb 2025, 09:35
                        0
                        • A adrien-lsh
                          25 Feb 2025, 09:32

                          @JonB
                          Try to move the line.

                          J Offline
                          J Offline
                          JonB
                          wrote on 25 Feb 2025, 09:35 last edited by
                          #18

                          @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...

                          A 1 Reply Last reply 25 Feb 2025, 09:36
                          0
                          • J JonB
                            25 Feb 2025, 09:35

                            @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...

                            A Offline
                            A Offline
                            adrien-lsh
                            wrote on 25 Feb 2025, 09:36 last edited by
                            #19

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

                            J 1 Reply Last reply 25 Feb 2025, 09:50
                            0
                            • A adrien-lsh
                              25 Feb 2025, 09:36

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

                              J Offline
                              J Offline
                              JonB
                              wrote on 25 Feb 2025, 09:50 last edited by JonB
                              #20

                              @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.

                              A 1 Reply Last reply 25 Feb 2025, 10:03
                              1
                              • A adrien-lsh has marked this topic as solved on 25 Feb 2025, 10:03
                              • J JonB
                                25 Feb 2025, 09:50

                                @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.

                                A Offline
                                A Offline
                                adrien-lsh
                                wrote on 25 Feb 2025, 10:03 last edited by
                                #21

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

                                1 Reply Last reply
                                0
                                • S Offline
                                  S Offline
                                  SGaist
                                  Lifetime Qt Champion
                                  wrote on 25 Feb 2025, 10:23 last edited by SGaist
                                  #22

                                  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]

                                  Interested in AI ? www.idiap.ch
                                  Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                                  J 1 Reply Last reply 25 Feb 2025, 10:27
                                  0
                                  • S SGaist
                                    25 Feb 2025, 10:23

                                    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]

                                    J Offline
                                    J Offline
                                    JonB
                                    wrote on 25 Feb 2025, 10:27 last edited by JonB
                                    #23

                                    @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?

                                    S 1 Reply Last reply 25 Feb 2025, 11:18
                                    0
                                    • J JonB
                                      25 Feb 2025, 10:27

                                      @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?

                                      S Offline
                                      S Offline
                                      SGaist
                                      Lifetime Qt Champion
                                      wrote on 25 Feb 2025, 11:18 last edited by SGaist
                                      #24

                                      @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.

                                      Interested in AI ? www.idiap.ch
                                      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                                      1 Reply Last reply
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                                        Nlight
                                        wrote on 25 Feb 2025, 20:02 last edited by Nlight
                                        #25
                                        This post is deleted!
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