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Return value from QObject in another thread via signals/slots

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  • SGaistS Offline
    SGaistS Offline
    SGaist
    Lifetime Qt Champion
    wrote on last edited by
    #2

    Hi,

    Do you mean get the result in the returned value of the function call ?

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    Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

    S 1 Reply Last reply
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    • S St.Stanislav

      Hello!

      I have faced with the following situation: I have ProcessingObject in the one thread and its interface ProcessingObjectInterface in another. ProcessingObject has some inner structure and process data. I want to send data from ProcessingObjectInterface to ProcessingObject, process it there and receive result in ProcessingObjectInterface.
      Sending data is simple, just using signas/slots mechanism, but what is the simplest and correct way to receive result in sender?

      jsulmJ Offline
      jsulmJ Offline
      jsulm
      Lifetime Qt Champion
      wrote on last edited by
      #3

      @St-Stanislav said in Return value from QObject in another thread via signals/slots:

      but what is the simplest and correct way to receive result in sender?

      In the same way you send data?
      ProcessingObject would emit a signal when it has finished its work and send the result as parameter in a signal.

      https://forum.qt.io/topic/113070/qt-code-of-conduct

      S 1 Reply Last reply
      1
      • SGaistS SGaist

        Hi,

        Do you mean get the result in the returned value of the function call ?

        S Offline
        S Offline
        St.Stanislav
        wrote on last edited by St.Stanislav
        #4

        @SGaist Yes, exactly. So in example I suppose that ProcessingObject just multiplies argument by 2. So I want to send signal

        void processingRequested(float value = 2);
        

        To slot

        float multiply(float value) {
            return value * 2;
        }
        

        and receive last function's returning value.

        1 Reply Last reply
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        • jsulmJ jsulm

          @St-Stanislav said in Return value from QObject in another thread via signals/slots:

          but what is the simplest and correct way to receive result in sender?

          In the same way you send data?
          ProcessingObject would emit a signal when it has finished its work and send the result as parameter in a signal.

          S Offline
          S Offline
          St.Stanislav
          wrote on last edited by
          #5

          @jsulm Yes, I thought about this approach, but what if I want to process something twice? In example above I want to process numbers 2 and 4. In this case ProcessingObject will emit the same signal twice but with the different arguments. So I need to add some queue of values that I'm processing. Then I need some identifiers for prevent mixing up results. It's not very complicated, but I hope that more simple built-in approach is existed.

          JonBJ 1 Reply Last reply
          0
          • S St.Stanislav

            @jsulm Yes, I thought about this approach, but what if I want to process something twice? In example above I want to process numbers 2 and 4. In this case ProcessingObject will emit the same signal twice but with the different arguments. So I need to add some queue of values that I'm processing. Then I need some identifiers for prevent mixing up results. It's not very complicated, but I hope that more simple built-in approach is existed.

            JonBJ Offline
            JonBJ Offline
            JonB
            wrote on last edited by
            #6

            @St-Stanislav
            Hang on! IMHO at least, slots should not return a value. The assumption you are making is that there will only be one slot attached to the signal, and that is unwarranted/not guaranteed at all. Strange things happen when multiple slots are attached and you rely on the return result.

            I agree it can be a "shame" that you cannot use signals/slots to get a return value, but that's not what they are for. Not to mention, it completely breaks if the connection is queued, for whatever reason.

            I wouldn't advise it, but a better way (to allow for multiple slots) would be to pass a reference parameter for the "result".

            S 1 Reply Last reply
            1
            • JonBJ JonB

              @St-Stanislav
              Hang on! IMHO at least, slots should not return a value. The assumption you are making is that there will only be one slot attached to the signal, and that is unwarranted/not guaranteed at all. Strange things happen when multiple slots are attached and you rely on the return result.

              I agree it can be a "shame" that you cannot use signals/slots to get a return value, but that's not what they are for. Not to mention, it completely breaks if the connection is queued, for whatever reason.

              I wouldn't advise it, but a better way (to allow for multiple slots) would be to pass a reference parameter for the "result".

              S Offline
              S Offline
              St.Stanislav
              wrote on last edited by
              #7

              @JonB Thank you for reply! Yes, I understand signas/slots nature and its purpose, so that's why I try to find correct workaround :) So if I understand correctly, you assume something like this:

              signals:
              void processingRequested(float& result, float value);
              
              slots:
              void multiply(float &result, float value) {
                  result = value * 2;
              }
              

              Am I right? So how sender will know, when result will be changed by receiver?

              1 Reply Last reply
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              • SGaistS Offline
                SGaistS Offline
                SGaist
                Lifetime Qt Champion
                wrote on last edited by
                #8

                Can you describe what exactly you are trying to achieve ?

                Understanding what you want to do will help us with the design.

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

                S 1 Reply Last reply
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                • SGaistS SGaist

                  Can you describe what exactly you are trying to achieve ?

                  Understanding what you want to do will help us with the design.

                  S Offline
                  S Offline
                  St.Stanislav
                  wrote on last edited by St.Stanislav
                  #9

                  @SGaist So I have program that displays data from external device. I have two threads (with appropriate objects): one is for data processing and second is for interface and displayed data holding. If in processing object one parameter is changed, I need to replot data from the displaying object based on that parameter. This parameter is something like calibration for X-axis and object has needed methods. So in my suggest I need to send X-coordinate from data in the displaying object to the processing object, compute new result there and apply it in displaying object with replacing the previous one. I hope I explained it clearly enough, anyway I've tried :)

                  Hm, so maybe I can emit signal that parameter was changed, send it to displaying object and apply it there. I need implement calibration method in displaying object as well, so it will be implemented twice. Don't know, is it good way, but it should be as workaround.

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                  • SGaistS Offline
                    SGaistS Offline
                    SGaist
                    Lifetime Qt Champion
                    wrote on last edited by
                    #10

                    The propagation of the change through signal and slots is the correct way to do it.

                    Why would you need to implement things twice ? You should have one object responsible for these mathematical operations rather than duplicate them in several places.

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

                    S 1 Reply Last reply
                    0
                    • SGaistS SGaist

                      The propagation of the change through signal and slots is the correct way to do it.

                      Why would you need to implement things twice ? You should have one object responsible for these mathematical operations rather than duplicate them in several places.

                      S Offline
                      S Offline
                      St.Stanislav
                      wrote on last edited by
                      #11

                      @SGaist Yes, and math operations are provided by Processing object: I read raw data from serial port, send raw array to Processing object and Processing Object resend them further to display:
                      SerialPort => Processing (math) => Display
                      And everything is done with signals/slots: Serial port emits data, Processing object catch this data, process it and emits processed data, Display catch it and display. But sometimes I need to change already displayed data with the same math processing:
                      Display => Processing (math) => Display
                      So that's why I'm not sure, how to do it correctly :)

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                      • SGaistS Offline
                        SGaistS Offline
                        SGaist
                        Lifetime Qt Champion
                        wrote on last edited by
                        #12

                        Do you have an example of such a case ?

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

                        S 1 Reply Last reply
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                        • SGaistS SGaist

                          Do you have an example of such a case ?

                          S Offline
                          S Offline
                          St.Stanislav
                          wrote on last edited by
                          #13

                          @SGaist Sorry for my late reply. What kind of examples do you mean? Which code I use for processing and emitting or just an abstract description of what emit and catch in slots?

                          1 Reply Last reply
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                          • SGaistS Offline
                            SGaistS Offline
                            SGaist
                            Lifetime Qt Champion
                            wrote on last edited by
                            #14

                            An example of what could trigger that double evaluation that is giving you trouble.

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

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