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How to use Windows Message Loop and Qt Application Exec?

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  • A arablau

    Hi Everyone,

    I'm new to Qt and am using it with the Canon SDK for a camera application.

    There problem is that the Canon SDK requires the same thread that spawns the SDK to also run the windows message loop:

    MSG msg = { };
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    

    However, the thread that initializes the SDK initializes it in my GUI initialization and runs the Qt Application exec() function.

    Is there a way I can handle windows messages from this thread?

    I want to keep the Camera class with the SDK initialize as part of my class that extends QMainWindow but I'm not sure how to get the messages handled properly.

    Thanks all!

    Y Offline
    Y Offline
    YJCN
    wrote on last edited by
    #3

    @arablau I meet the same problem. Could you please tell me how to solve it?

    aha_1980A 1 Reply Last reply
    0
    • Y YJCN

      @arablau I meet the same problem. Could you please tell me how to solve it?

      aha_1980A Offline
      aha_1980A Offline
      aha_1980
      Lifetime Qt Champion
      wrote on last edited by
      #4

      Hi @YJCN,

      have you read the (still existing, wow!) links @SGaist recommended?

      You might be interested by QWidget's "winEvent":http://qt-project.org/doc/qt-4.8/qwidget.html#winEvent or QCoreApplication's "winEventFilter":http://qt-project.org/doc/qt-4.8/qcoreapplication.html#winEventFilter

      Regards

      Qt has to stay free or it will die.

      Y 1 Reply Last reply
      3
      • aha_1980A aha_1980

        Hi @YJCN,

        have you read the (still existing, wow!) links @SGaist recommended?

        You might be interested by QWidget's "winEvent":http://qt-project.org/doc/qt-4.8/qwidget.html#winEvent or QCoreApplication's "winEventFilter":http://qt-project.org/doc/qt-4.8/qcoreapplication.html#winEventFilter

        Regards

        Y Offline
        Y Offline
        YJCN
        wrote on last edited by
        #5

        @aha_1980 Thanks for your reply. I have read the links. In Qt5, the winEvent has been changed to nativeEvent. This is my code.But it doesn't work.

        
        bool MainWindow::nativeEvent(const QByteArray &eventType, void *message, long *result)
        {
            //Q_UNUSED(eventType);
            MSG* msg = reinterpret_cast<MSG*>(message);
            cout<<"in nativeEvent:"<<msg->message<<endl;
            if(msg->message==275)
            {
                //GetMessage(msg, NULL, NULL, NULL);
                TranslateMessage(msg);
                cout << "TranslateMessage" << endl;
                DispatchMessage(msg); // Ends up calling objectEventFnc
                cout << "DispatchMessage" << endl;
                return true;
            }
                return false;
        }
        
        jsulmJ 1 Reply Last reply
        0
        • Y YJCN

          @aha_1980 Thanks for your reply. I have read the links. In Qt5, the winEvent has been changed to nativeEvent. This is my code.But it doesn't work.

          
          bool MainWindow::nativeEvent(const QByteArray &eventType, void *message, long *result)
          {
              //Q_UNUSED(eventType);
              MSG* msg = reinterpret_cast<MSG*>(message);
              cout<<"in nativeEvent:"<<msg->message<<endl;
              if(msg->message==275)
              {
                  //GetMessage(msg, NULL, NULL, NULL);
                  TranslateMessage(msg);
                  cout << "TranslateMessage" << endl;
                  DispatchMessage(msg); // Ends up calling objectEventFnc
                  cout << "DispatchMessage" << endl;
                  return true;
              }
                  return false;
          }
          
          jsulmJ Online
          jsulmJ Online
          jsulm
          Lifetime Qt Champion
          wrote on last edited by jsulm
          #6

          @YJCN said in How to use Windows Message Loop and Qt Application Exec?:

          But it doesn't work

          What exactly doesn't work?
          "Note: Events are only delivered to this event handler if the widget is has a native Window handle." from http://doc.qt.io/qt-5/qwidget.html#nativeEvent

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

          Y 1 Reply Last reply
          2
          • jsulmJ jsulm

            @YJCN said in How to use Windows Message Loop and Qt Application Exec?:

            But it doesn't work

            What exactly doesn't work?
            "Note: Events are only delivered to this event handler if the widget is has a native Window handle." from http://doc.qt.io/qt-5/qwidget.html#nativeEvent

            Y Offline
            Y Offline
            YJCN
            wrote on last edited by
            #7

            @jsulm I use the Canon EDSDK to control the EOS 800D. I want to download image from camera. after taking picture. In EDSDK, the download is handled by callback function. I test in command line programm ,and there must be this code after sending a take picture command. Otherwise, the programm can't enter the callback funtion.

            MSG msg = { };
            while (GetMessage(&msg, NULL, 0, 0))
            {
                TranslateMessage(&msg);
                DispatchMessage(&msg);
            }
            

            There is the reason I find in Google.

            If you're using the EDSDK on Windows, you have to have a Windows message loop in your main thread, otherwise callbacks won't happen. (This is because the EDSDK uses the obsolete COM STA threading model instead of real threads.)
            

            https://social.microsoft.com/Forums/en-US/a431209c-be99-4394-88b3-59879cb790a4/objecteventhandler-callback-nevercalled?forum=Offtopic
            https://stackoverflow.com/questions/16839640/edsdk-callbacks-not-working/18624865

            mrjjM 1 Reply Last reply
            0
            • Y YJCN

              @jsulm I use the Canon EDSDK to control the EOS 800D. I want to download image from camera. after taking picture. In EDSDK, the download is handled by callback function. I test in command line programm ,and there must be this code after sending a take picture command. Otherwise, the programm can't enter the callback funtion.

              MSG msg = { };
              while (GetMessage(&msg, NULL, 0, 0))
              {
                  TranslateMessage(&msg);
                  DispatchMessage(&msg);
              }
              

              There is the reason I find in Google.

              If you're using the EDSDK on Windows, you have to have a Windows message loop in your main thread, otherwise callbacks won't happen. (This is because the EDSDK uses the obsolete COM STA threading model instead of real threads.)
              

              https://social.microsoft.com/Forums/en-US/a431209c-be99-4394-88b3-59879cb790a4/objecteventhandler-callback-nevercalled?forum=Offtopic
              https://stackoverflow.com/questions/16839640/edsdk-callbacks-not-working/18624865

              mrjjM Offline
              mrjjM Offline
              mrjj
              Lifetime Qt Champion
              wrote on last edited by
              #8

              @YJCN
              Hi
              in case that nativeEvent simply cannot work with decade-old COM crap (1998)
              there is always the options of using a plain c++ app that will handle the downloading / save to file and
              you simply call it with QProcess from Qt GUI app.

              Y 1 Reply Last reply
              3
              • mrjjM mrjj

                @YJCN
                Hi
                in case that nativeEvent simply cannot work with decade-old COM crap (1998)
                there is always the options of using a plain c++ app that will handle the downloading / save to file and
                you simply call it with QProcess from Qt GUI app.

                Y Offline
                Y Offline
                YJCN
                wrote on last edited by
                #9

                @mrjj The Canon 's demo is using MFC. Why MFC could use?

                jsulmJ mrjjM 2 Replies Last reply
                0
                • Y YJCN

                  @mrjj The Canon 's demo is using MFC. Why MFC could use?

                  jsulmJ Online
                  jsulmJ Online
                  jsulm
                  Lifetime Qt Champion
                  wrote on last edited by
                  #10

                  @YJCN said in How to use Windows Message Loop and Qt Application Exec?:

                  Why MFC could use?

                  Because it is Microsofts own GUI framework using the Windows event loop...

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

                  1 Reply Last reply
                  2
                  • Y YJCN

                    @mrjj The Canon 's demo is using MFC. Why MFC could use?

                    mrjjM Offline
                    mrjjM Offline
                    mrjj
                    Lifetime Qt Champion
                    wrote on last edited by
                    #11

                    @YJCN
                    MFC is very old too :)

                    Anyway i wonder with that line

                    if(msg->message==275)

                    means ? where did u get this magic number from ?

                    Y 1 Reply Last reply
                    0
                    • mrjjM mrjj

                      @YJCN
                      MFC is very old too :)

                      Anyway i wonder with that line

                      if(msg->message==275)

                      means ? where did u get this magic number from ?

                      Y Offline
                      Y Offline
                      YJCN
                      wrote on last edited by
                      #12

                      @mrjj
                      This my CMD programm code

                      int main (){       
                              EdsCameraRef camera;
                      	EdsUInt32 tv;
                              init_camera(camera);
                      	//start_liveview(camera);
                      	//TakePicture(camera);
                      	
                      	take_photo(camera,1,1000);
                      	//TakePhoto();
                      	//dispose(camera);
                      	cout << "before message" << endl;
                      	MSG msg;
                      	while (GetMessage(&msg, NULL, NULL, NULL))
                      	{
                      		cout << "in while" << " msg.message:"<<msg.message<<endl;
                      		TranslateMessage(&msg);
                      		cout << "TranslateMessage" << endl;
                      		DispatchMessage(&msg); // Ends up calling objectEventFnc
                      		cout << "DispatchMessage" << endl;
                      		break;
                      	}
                      	cout << "after message" << endl;
                              EdsCloseSession(camera);
                      	cout << "EdsCloseSession(camera);";
                      	EdsTerminateSDK();
                      	system("pause");
                              return 0;
                      

                      This is output
                      0_1537429196448_3c672381-54c0-4382-832e-91e484ab0433-image.png

                      1 Reply Last reply
                      0
                      • mrjjM Offline
                        mrjjM Offline
                        mrjj
                        Lifetime Qt Champion
                        wrote on last edited by mrjj
                        #13

                        HI
                        ahh, so 275 is the ID for what SDK is doing.
                        So you call
                        init_camera(camera); in main window also ?
                        (and take_photo(camera,1,1000);
                        and camera is a member variable in main window so it doesn't run out of scope?

                        but you never see 275 ? in natvieEvent?

                        Y 1 Reply Last reply
                        0
                        • mrjjM mrjj

                          HI
                          ahh, so 275 is the ID for what SDK is doing.
                          So you call
                          init_camera(camera); in main window also ?
                          (and take_photo(camera,1,1000);
                          and camera is a member variable in main window so it doesn't run out of scope?

                          but you never see 275 ? in natvieEvent?

                          Y Offline
                          Y Offline
                          YJCN
                          wrote on last edited by
                          #14

                          @mrjj Yes. I put the code(init_camera(camera) and take_photo(camera,1,1000) ) in Button's slot function, after taking picture, the programm can't enter callback function(handleObjectEvent) and there is no 275 in natvieEvent.

                          mrjjM 1 Reply Last reply
                          0
                          • Y YJCN

                            @mrjj Yes. I put the code(init_camera(camera) and take_photo(camera,1,1000) ) in Button's slot function, after taking picture, the programm can't enter callback function(handleObjectEvent) and there is no 275 in natvieEvent.

                            mrjjM Offline
                            mrjjM Offline
                            mrjj
                            Lifetime Qt Champion
                            wrote on last edited by
                            #15

                            Hi
                            and just to be 100% sure
                            camera is NOT a local variable in Button's slot function, but in the .h file as a true member of
                            main maindiw (in .h file) ?
                            (im checking its not a death by scope issue)

                            If unsure what i ask, please show full Button's slot function. :)

                            Y 1 Reply Last reply
                            0
                            • mrjjM mrjj

                              Hi
                              and just to be 100% sure
                              camera is NOT a local variable in Button's slot function, but in the .h file as a true member of
                              main maindiw (in .h file) ?
                              (im checking its not a death by scope issue)

                              If unsure what i ask, please show full Button's slot function. :)

                              Y Offline
                              Y Offline
                              YJCN
                              wrote on last edited by
                              #16

                              @mrjj This is Button's slot function code

                              void MainWindow::on_pushButton_2_clicked()
                              {
                                  EdsCameraRef camera;
                                  init_camera(camera);
                                  //start_liveview(camera);
                                  //TakePicture(camera);
                                  take_photo(camera,1,1000);
                                  //TakePhoto();
                                  //dispose(camera);
                                  cout << "before message" << endl;
                                  MSG msg;
                                  while (::GetMessage(&msg, NULL, NULL, NULL))
                                  {
                                      TranslateMessage(&msg);
                                      DispatchMessage(&msg); // Ends up calling objectEventFnc
                                      break;
                                  }
                                  cout << "after message" << endl;
                                  EdsCloseSession(camera);
                                  cout << "EdsCloseSession(camera);";
                                  EdsTerminateSDK();
                              }
                              
                              
                              jsulmJ 1 Reply Last reply
                              0
                              • Y YJCN

                                @mrjj This is Button's slot function code

                                void MainWindow::on_pushButton_2_clicked()
                                {
                                    EdsCameraRef camera;
                                    init_camera(camera);
                                    //start_liveview(camera);
                                    //TakePicture(camera);
                                    take_photo(camera,1,1000);
                                    //TakePhoto();
                                    //dispose(camera);
                                    cout << "before message" << endl;
                                    MSG msg;
                                    while (::GetMessage(&msg, NULL, NULL, NULL))
                                    {
                                        TranslateMessage(&msg);
                                        DispatchMessage(&msg); // Ends up calling objectEventFnc
                                        break;
                                    }
                                    cout << "after message" << endl;
                                    EdsCloseSession(camera);
                                    cout << "EdsCloseSession(camera);";
                                    EdsTerminateSDK();
                                }
                                
                                
                                jsulmJ Online
                                jsulmJ Online
                                jsulm
                                Lifetime Qt Champion
                                wrote on last edited by jsulm
                                #17

                                @YJCN said in How to use Windows Message Loop and Qt Application Exec?:

                                while (::GetMessage(&msg, NULL, NULL, NULL))
                                {
                                TranslateMessage(&msg);
                                DispatchMessage(&msg); // Ends up calling objectEventFnc
                                break;
                                }

                                You execute this loop only once (because of the break).
                                Shouldn't you call break after you got the message you need?

                                @mrjj camera is local :-) but I don't thin this is the issue here.

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

                                mrjjM JonBJ 2 Replies Last reply
                                4
                                • jsulmJ jsulm

                                  @YJCN said in How to use Windows Message Loop and Qt Application Exec?:

                                  while (::GetMessage(&msg, NULL, NULL, NULL))
                                  {
                                  TranslateMessage(&msg);
                                  DispatchMessage(&msg); // Ends up calling objectEventFnc
                                  break;
                                  }

                                  You execute this loop only once (because of the break).
                                  Shouldn't you call break after you got the message you need?

                                  @mrjj camera is local :-) but I don't thin this is the issue here.

                                  mrjjM Offline
                                  mrjjM Offline
                                  mrjj
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #18

                                  @jsulm
                                  I agree, it should not run out of scope if it works like we think :)
                                  But you are right, maybe it just get first MSG as then it breaks.

                                  1 Reply Last reply
                                  0
                                  • jsulmJ jsulm

                                    @YJCN said in How to use Windows Message Loop and Qt Application Exec?:

                                    while (::GetMessage(&msg, NULL, NULL, NULL))
                                    {
                                    TranslateMessage(&msg);
                                    DispatchMessage(&msg); // Ends up calling objectEventFnc
                                    break;
                                    }

                                    You execute this loop only once (because of the break).
                                    Shouldn't you call break after you got the message you need?

                                    @mrjj camera is local :-) but I don't thin this is the issue here.

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

                                    @YJCN

                                    @jsulm said in How to use Windows Message Loop and Qt Application Exec?:

                                    Shouldn't you call break after you got the message you need?

                                    Indeed, the loop picks up the first message and that's it. What makes you think that message is the right/only one which needs to be processed this way? It could be any old message....

                                    Y 1 Reply Last reply
                                    1
                                    • JonBJ JonB

                                      @YJCN

                                      @jsulm said in How to use Windows Message Loop and Qt Application Exec?:

                                      Shouldn't you call break after you got the message you need?

                                      Indeed, the loop picks up the first message and that's it. What makes you think that message is the right/only one which needs to be processed this way? It could be any old message....

                                      Y Offline
                                      Y Offline
                                      YJCN
                                      wrote on last edited by
                                      #20

                                      @jsulm @JonB @mrjj Yes, you are right !!! I comment the "break" . It works well!! THANK YOU!☺☺☺☺☺☺☺☺☺☺

                                      JonBJ 1 Reply Last reply
                                      3
                                      • Y YJCN

                                        @jsulm @JonB @mrjj Yes, you are right !!! I comment the "break" . It works well!! THANK YOU!☺☺☺☺☺☺☺☺☺☺

                                        JonBJ Offline
                                        JonBJ Offline
                                        JonB
                                        wrote on last edited by
                                        #21

                                        @YJCN
                                        You have to do more than just comment out the break, if that's what you're saying. If that's all you do the ::GetMessage() loop will never exit, and your code will never return from on_pushButton_2_clicked(), e.g. you cannot press it again to take another photo. Presumably you need to find out whatever is the final message your camera issues on taking a photo and exit the loop once that has been received.

                                        Y 1 Reply Last reply
                                        2
                                        • JonBJ JonB

                                          @YJCN
                                          You have to do more than just comment out the break, if that's what you're saying. If that's all you do the ::GetMessage() loop will never exit, and your code will never return from on_pushButton_2_clicked(), e.g. you cannot press it again to take another photo. Presumably you need to find out whatever is the final message your camera issues on taking a photo and exit the loop once that has been received.

                                          Y Offline
                                          Y Offline
                                          YJCN
                                          wrote on last edited by
                                          #22

                                          @JonB Yes. I just use a five times for loop to solve it . ^_^

                                          JonBJ 1 Reply Last reply
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