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How to determine if signal is already connected?

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  • J.HilkJ J.Hilk

    @SPlatten said in How to determine if signal is already connected?:

    its the constructor of this widget that connects the signals.

    well, if you do your connects in the constructor of the widget, than you shouldn't come into the situation, where the valueChanged signal is not connected, right ?

    SPlattenS Offline
    SPlattenS Offline
    SPlatten
    wrote on last edited by
    #10

    @J-Hilk , perhaps I haven't made myself clear, I have two widgets that share spinners and a slider, in the constructor of the same two widgets I want to check if the spinners and slider have already had a signal connected, if the answer is no then I want to go ahead with the connection.

    Kind Regards,
    Sy

    Pl45m4P 1 Reply Last reply
    0
    • SPlattenS SPlatten

      @J-Hilk , perhaps I haven't made myself clear, I have two widgets that share spinners and a slider, in the constructor of the same two widgets I want to check if the spinners and slider have already had a signal connected, if the answer is no then I want to go ahead with the connection.

      Pl45m4P Offline
      Pl45m4P Offline
      Pl45m4
      wrote on last edited by
      #11

      @SPlatten said in How to determine if signal is already connected?:

      I have two widgets that share spinners and a slider, in the constructor of the same two widgets I want to check if the spinners and slider have already had a signal connected, if the answer is no then I want to go ahead with the connection.

      But why? You wrote the code. If you haven't connected it already, it's probably not connected :) And if you did, it should stay connected unless you disconnect it manually somewhere, somehow.

      Don't quite unterstand the issue you have here :)


      If debugging is the process of removing software bugs, then programming must be the process of putting them in.

      ~E. W. Dijkstra

      SPlattenS 1 Reply Last reply
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      • Pl45m4P Pl45m4

        @SPlatten said in How to determine if signal is already connected?:

        I have two widgets that share spinners and a slider, in the constructor of the same two widgets I want to check if the spinners and slider have already had a signal connected, if the answer is no then I want to go ahead with the connection.

        But why? You wrote the code. If you haven't connected it already, it's probably not connected :) And if you did, it should stay connected unless you disconnect it manually somewhere, somehow.

        Don't quite unterstand the issue you have here :)

        SPlattenS Offline
        SPlattenS Offline
        SPlatten
        wrote on last edited by
        #12

        @Pl45m4 , the issue is I have two instances of the same widget, there are three common widgets for setting attributes on instances of the two, in the constructor of the widget I want to automatically check the shared widgets and then only connect if they haven't already been connected.

        The question I am asking is can I detected using the Qt library calls if a signal has already been connected to a widget.

        Kind Regards,
        Sy

        Pl45m4P 1 Reply Last reply
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        • SPlattenS SPlatten

          @Pl45m4 , the issue is I have two instances of the same widget, there are three common widgets for setting attributes on instances of the two, in the constructor of the widget I want to automatically check the shared widgets and then only connect if they haven't already been connected.

          The question I am asking is can I detected using the Qt library calls if a signal has already been connected to a widget.

          Pl45m4P Offline
          Pl45m4P Offline
          Pl45m4
          wrote on last edited by
          #13

          @SPlatten

          Besides calling connect again and check its output, there is

          int QObject::receivers(const char *signal)

          • https://doc.qt.io/qt-5/qobject.html#receivers

          Dont know if it's recommended to use...

          Warning: This function violates the object-oriented principle of modularity. However, it might be useful when you need to perform expensive initialization only if something is connected to a signal.

          I guess not, but maybe it helps :)


          If debugging is the process of removing software bugs, then programming must be the process of putting them in.

          ~E. W. Dijkstra

          SPlattenS 1 Reply Last reply
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          • Pl45m4P Pl45m4

            @SPlatten

            Besides calling connect again and check its output, there is

            int QObject::receivers(const char *signal)

            • https://doc.qt.io/qt-5/qobject.html#receivers

            Dont know if it's recommended to use...

            Warning: This function violates the object-oriented principle of modularity. However, it might be useful when you need to perform expensive initialization only if something is connected to a signal.

            I guess not, but maybe it helps :)

            SPlattenS Offline
            SPlattenS Offline
            SPlatten
            wrote on last edited by
            #14

            @Pl45m4 , doesn't calling connect just create another connection? Thats where I am now.

            Kind Regards,
            Sy

            J.HilkJ 1 Reply Last reply
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            • SPlattenS SPlatten

              @Pl45m4 , doesn't calling connect just create another connection? Thats where I am now.

              J.HilkJ Offline
              J.HilkJ Offline
              J.Hilk
              Moderators
              wrote on last edited by
              #15

              @SPlatten
              @Pl45m4 is on the right track here,

              you can/should subclass your spinner and create a function,

              bool isValueChanged(){
                 static const QMetaMethod valueChangedSignal = QMetaMethod::fromSignal(&MyObject::valueChanged);
              return isSignalConnected(valueChangedSignal);
              }
              

              https://doc.qt.io/qt-5/qobject.html#isSignalConnected


              Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


              Q: What's that?
              A: It's blue light.
              Q: What does it do?
              A: It turns blue.

              SPlattenS Pl45m4P 2 Replies Last reply
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              • J.HilkJ J.Hilk

                @SPlatten
                @Pl45m4 is on the right track here,

                you can/should subclass your spinner and create a function,

                bool isValueChanged(){
                   static const QMetaMethod valueChangedSignal = QMetaMethod::fromSignal(&MyObject::valueChanged);
                return isSignalConnected(valueChangedSignal);
                }
                

                https://doc.qt.io/qt-5/qobject.html#isSignalConnected

                SPlattenS Offline
                SPlattenS Offline
                SPlatten
                wrote on last edited by
                #16

                @J-Hilk , thank you, penny has dropped, I see what I have to do now.

                Kind Regards,
                Sy

                1 Reply Last reply
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                • J.HilkJ J.Hilk

                  @SPlatten
                  @Pl45m4 is on the right track here,

                  you can/should subclass your spinner and create a function,

                  bool isValueChanged(){
                     static const QMetaMethod valueChangedSignal = QMetaMethod::fromSignal(&MyObject::valueChanged);
                  return isSignalConnected(valueChangedSignal);
                  }
                  

                  https://doc.qt.io/qt-5/qobject.html#isSignalConnected

                  Pl45m4P Offline
                  Pl45m4P Offline
                  Pl45m4
                  wrote on last edited by Pl45m4
                  #17

                  @J-Hilk

                  lol, found receivers() and sender() but not isSignalConnected().

                  Sometimes it's hard to find something, if you don't know what you are looking for :D
                  (function names)


                  If debugging is the process of removing software bugs, then programming must be the process of putting them in.

                  ~E. W. Dijkstra

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                  • A Offline
                    A Offline
                    aleksejs
                    wrote on last edited by aleksejs
                    #18

                    I'm working with Qt not so long, but I see how cumbersome is signal-slot mechanism in comparison to Borland Event-Handler mechanism already.
                    In Borland you write

                    button->OnClick = buttonClickHandler;
                    

                    or

                    if( !button->OnClick )
                        button->OnClick = buttonClickHandler;
                    

                    Until now I haven't seen better Event mechanism than Borland's.

                    JonBJ 1 Reply Last reply
                    0
                    • A aleksejs

                      I'm working with Qt not so long, but I see how cumbersome is signal-slot mechanism in comparison to Borland Event-Handler mechanism already.
                      In Borland you write

                      button->OnClick = buttonClickHandler;
                      

                      or

                      if( !button->OnClick )
                          button->OnClick = buttonClickHandler;
                      

                      Until now I haven't seen better Event mechanism than Borland's.

                      JonBJ Online
                      JonBJ Online
                      JonB
                      wrote on last edited by
                      #19

                      @aleksejs
                      How did that work/what did you specify when buttonClickHandler is a method in a class?

                      Christian EhrlicherC 1 Reply Last reply
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                      • JonBJ JonB

                        @aleksejs
                        How did that work/what did you specify when buttonClickHandler is a method in a class?

                        Christian EhrlicherC Offline
                        Christian EhrlicherC Offline
                        Christian Ehrlicher
                        Lifetime Qt Champion
                        wrote on last edited by Christian Ehrlicher
                        #20

                        ... Or if two distinct functions should be called where none is aware of the other?

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