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  4. How to catch a signal emitted from cpp class in Qml ?

How to catch a signal emitted from cpp class in Qml ?

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  • Naveen_DN Offline
    Naveen_DN Offline
    Naveen_D
    wrote on last edited by
    #1

    Hi all,

    I am using s signal slot communication for cpp and qml interaction. but when i emit a signal from cpp class i am not able to catch that in Qml...can anyone guide me in this matter... the code is as follows...

    thanks.

    my .h code

    #ifndef HANDLETEXTFIELD_H
    #define HANDLETEXTFIELD_H
    
    #include <QObject>
    #include <QDebug>
    
    class HandleTextField : public QObject
    {
        Q_OBJECT
    public:
        explicit HandleTextField(QObject *parent = 0);
    
    signals:
        void setTextField();
    
    public slots:
        Q_INVOKABLE void handleSubmitTextField();
    
    };
    
    #endif // HANDLETEXTFIELD_H
    
    

    my .cpp code

    #include "handletextfield.h"
    
    HandleTextField::HandleTextField(QObject *parent) :
        QObject(parent)
    {
    }
    
    void HandleTextField::handleSubmitTextField()
    {
        qDebug() << "c++: HandleTextField::handleSubmitTextField:" << endl;
        emit setTextField();
    }
    

    my main.cpp

    #include <QGuiApplication>
    #include <QQmlApplicationEngine>
    #include <QQuickWindow>
    #include <QQmlContext>
    
    #include "handletextfield.h"
    
    int main(int argc, char *argv[])
    {
        QGuiApplication app(argc, argv);
        qmlRegisterType<HandleTextField>("com.handletextfield",1,0,"HandleTextField");
    
        QQmlApplicationEngine engine;
        engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    
        HandleTextField handleTextField;
    
        QObject *topLevel = engine.rootObjects().value(0);
        QQuickWindow *window = qobject_cast<QQuickWindow *>(topLevel);
    
    //    QObject::connect(window, SIGNAL(submitTextField(QString)),
    //                         &handleTextField, SLOT(handleSubmitTextField(QString)));
    
        QObject::connect(&handleTextField, SIGNAL(setTextField()),
                             window, SLOT(setTextField()));
    
        return app.exec();
    }
    
    

    my qml.main code

    import QtQuick 2.3
    import QtQuick.Window 2.2
    import QtQuick.Controls 1.2
    import com.handletextfield 1.0
    
    Window {
        visible: true
        width: 360
        height: 360
    
        HandleTextField {
            id: testsignal
        }
    
        MouseArea {
            anchors.fill: parent
            onClicked: {
                testsignal.handleSubmitTextField()
            }
        }
        function setTextField(){
            console.log("setTextField signal: catched ")
        }
    }
    
    

    Naveen_D

    1 Reply Last reply
    0
    • sneubertS Offline
      sneubertS Offline
      sneubert
      wrote on last edited by
      #2

      QQuick creates signal handlers for your cpp classes like:

      HandleTextField {
              id: testsignal
              onSetTextField: { //your code}
          }
      

      http://doc.qt.io/qt-5/qtqml-cppintegration-exposecppattributes.html#exposing-signals

      1 Reply Last reply
      2
      • J Offline
        J Offline
        Jagh
        wrote on last edited by
        #3

        You have 2 different objects of class HandleTextField in your code(one created in main(), the other in Qml), and you connect/send a signal from different ones. For your code to work, you have to connect the HandleTextField that you created in your Qml code, and you dont need the object in main() anymore, you don't do anything with it anyway.

        In Qmal this would look like:

        HandleTextField {
           id: testsignal
           onSetTextField: {
              console.log("signal called")
           }
        }
        
        Naveen_DN 2 Replies Last reply
        2
        • J Jagh

          You have 2 different objects of class HandleTextField in your code(one created in main(), the other in Qml), and you connect/send a signal from different ones. For your code to work, you have to connect the HandleTextField that you created in your Qml code, and you dont need the object in main() anymore, you don't do anything with it anyway.

          In Qmal this would look like:

          HandleTextField {
             id: testsignal
             onSetTextField: {
                console.log("signal called")
             }
          }
          
          Naveen_DN Offline
          Naveen_DN Offline
          Naveen_D
          wrote on last edited by
          #4

          @Jagh but if i dont create a object of handletextfield in main.cpp how can i connect the signal and slot ?

          Naveen_D

          J 1 Reply Last reply
          0
          • J Jagh

            You have 2 different objects of class HandleTextField in your code(one created in main(), the other in Qml), and you connect/send a signal from different ones. For your code to work, you have to connect the HandleTextField that you created in your Qml code, and you dont need the object in main() anymore, you don't do anything with it anyway.

            In Qmal this would look like:

            HandleTextField {
               id: testsignal
               onSetTextField: {
                  console.log("signal called")
               }
            }
            
            Naveen_DN Offline
            Naveen_DN Offline
            Naveen_D
            wrote on last edited by
            #5

            @Jagh You mean to say the following part is not required in main.cpp ??

            HandleTextField handleTextField;
            
                QObject *topLevel = engine.rootObjects().value(0);
                QQuickWindow *window = qobject_cast<QQuickWindow *>(topLevel);
            
            //    QObject::connect(window, SIGNAL(submitTextField()),
            //                         &handleTextField, SLOT(handleSubmitTextField()));
            
                QObject::connect(&handleTextField, SIGNAL(setTextField()),
                                     window, SLOT(setTextField()));
            

            only i need to emit the signal and catch it in qml..in the same way as you have done ?

            Naveen_D

            1 Reply Last reply
            0
            • Naveen_DN Naveen_D

              @Jagh but if i dont create a object of handletextfield in main.cpp how can i connect the signal and slot ?

              J Offline
              J Offline
              Jagh
              wrote on last edited by
              #6

              @Naveen_D

              1. You connect objects, not classes with signal-slot mechanism. So if you connect one object HandleTextField to something, this connection will not automatically transfer to other objects of the same class.
              2. Exactly, that part is not required, and in fact, it doesn't do anything. Connections for one object in your C++ code don't transfer to unrelated objects in QML.
              3. Yep, for each signal an object emits, QML exposes a property named on<SignalName>, in your case this would be onSetTextField. You can set this property as i have done, but AFAIK it should also be possible to set it from outside, having
              testsignal.onSetTextField: {
                 console.log("signal called")
              }
              

              somewhere in your Qml should suffice

              Naveen_DN 1 Reply Last reply
              0
              • J Jagh

                @Naveen_D

                1. You connect objects, not classes with signal-slot mechanism. So if you connect one object HandleTextField to something, this connection will not automatically transfer to other objects of the same class.
                2. Exactly, that part is not required, and in fact, it doesn't do anything. Connections for one object in your C++ code don't transfer to unrelated objects in QML.
                3. Yep, for each signal an object emits, QML exposes a property named on<SignalName>, in your case this would be onSetTextField. You can set this property as i have done, but AFAIK it should also be possible to set it from outside, having
                testsignal.onSetTextField: {
                   console.log("signal called")
                }
                

                somewhere in your Qml should suffice

                Naveen_DN Offline
                Naveen_DN Offline
                Naveen_D
                wrote on last edited by Naveen_D
                #7

                @Jagh thank you for your reply...
                now i have removed that part from main.cpp...but now also when i emit a signal i am not able to catch it in Qml...
                the way i am registering that handletextfiled class, is it correct ...???

                here is the code...

                main.qml

                import QtQuick 2.3
                import QtQuick.Window 2.2
                import QtQuick.Controls 1.2
                import com.handletextfield 1.0
                
                Window {
                    visible: true
                    width: 360
                    height: 360
                
                    HandleTextField {
                        id: testsignal
                        onSetTextField: {
                            console.log("Signal catched successfully...")
                        }
                    }
                }
                

                main.cpp

                #include <QGuiApplication>
                #include <QQmlApplicationEngine>
                #include <QQuickWindow>
                #include <QQmlContext>
                
                #include "handletextfield.h"
                
                int main(int argc, char *argv[])
                {
                    QGuiApplication app(argc, argv);
                    qmlRegisterType<HandleTextField>("com.handletextfield",1,0,"HandleTextField");
                
                    QQmlApplicationEngine engine;
                    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
                
                    return app.exec();
                }
                
                

                handletextfield.cpp

                #include "handletextfield.h"
                
                HandleTextField::HandleTextField(QObject *parent) :
                    QObject(parent)
                {
                    handleSubmitTextField();
                }
                
                void HandleTextField::handleSubmitTextField()
                {
                    qDebug() << "c++: HandleTextField::handleSubmitTextField:" << endl;
                    emit setTextField();
                    qDebug()<<"Signal emitted..."<<endl;
                }
                
                

                Naveen_D

                1 Reply Last reply
                0
                • J Offline
                  J Offline
                  Jagh
                  wrote on last edited by
                  #8

                  You are sending signal way too early, before there was any chance for a connection to be established. In constructor the object hasn't been fully initialized yet, in particular, the QML-interpreter hadn't had a chance to initialize the object's properties from QML side yet.

                  Naveen_DN 1 Reply Last reply
                  0
                  • J Jagh

                    You are sending signal way too early, before there was any chance for a connection to be established. In constructor the object hasn't been fully initialized yet, in particular, the QML-interpreter hadn't had a chance to initialize the object's properties from QML side yet.

                    Naveen_DN Offline
                    Naveen_DN Offline
                    Naveen_D
                    wrote on last edited by
                    #9

                    @Jagh Okay...
                    then when should i emit a signal...so that i can catch over there...calling my function in constructor is the mistake...??

                    Naveen_D

                    1 Reply Last reply
                    0
                    • J Offline
                      J Offline
                      Jagh
                      wrote on last edited by
                      #10

                      I changed your code a bit so that handleSubmitTextField() is called in more appropriate time(actually i just added a Button and called this function in onClicked handler), and the signal handler was successfully called.

                      To when: anytime between QML object initialization (implement Component.onCompleted if you want to catch it) and object destruction is ok.

                      Naveen_DN 2 Replies Last reply
                      0
                      • J Jagh

                        I changed your code a bit so that handleSubmitTextField() is called in more appropriate time(actually i just added a Button and called this function in onClicked handler), and the signal handler was successfully called.

                        To when: anytime between QML object initialization (implement Component.onCompleted if you want to catch it) and object destruction is ok.

                        Naveen_DN Offline
                        Naveen_DN Offline
                        Naveen_D
                        wrote on last edited by
                        #11

                        @Jagh can you please post the code so that i will get a clear picture of what i need to do...thanks...

                        Naveen_D

                        1 Reply Last reply
                        0
                        • J Jagh

                          I changed your code a bit so that handleSubmitTextField() is called in more appropriate time(actually i just added a Button and called this function in onClicked handler), and the signal handler was successfully called.

                          To when: anytime between QML object initialization (implement Component.onCompleted if you want to catch it) and object destruction is ok.

                          Naveen_DN Offline
                          Naveen_DN Offline
                          Naveen_D
                          wrote on last edited by
                          #12

                          @Jagh I tried the way u told by adding a button and calling the handler...it worked thanks...\o/

                          Naveen_D

                          1 Reply Last reply
                          0
                          • J Offline
                            J Offline
                            Jagh
                            wrote on last edited by
                            #13

                            And if you really want to execute some code as soon as possible and have a guarantee that a QML object was fully initialized by the time your code is running, do it in Component.onCompleted handler of that object.

                            1 Reply Last reply
                            0
                            • Naveen_DN Offline
                              Naveen_DN Offline
                              Naveen_D
                              wrote on last edited by
                              #14

                              Ok thanks... i will try that also...:-)

                              Naveen_D

                              1 Reply Last reply
                              0
                              • Naveen_DN Offline
                                Naveen_DN Offline
                                Naveen_D
                                wrote on last edited by
                                #15

                                Hi i have doubt regarding catching a signal emitted from c++ file
                                if there are more than 2 .qml files does it catch the signal ?
                                for ex, i have one .cpp and 4 .qml such as mainform.qml,radio.qml,music.qml,setting .qml
                                so if i emit a signal and try to catch it in setting.qml does it work ??

                                Naveen_D

                                V 1 Reply Last reply
                                0
                                • Naveen_DN Naveen_D

                                  Hi i have doubt regarding catching a signal emitted from c++ file
                                  if there are more than 2 .qml files does it catch the signal ?
                                  for ex, i have one .cpp and 4 .qml such as mainform.qml,radio.qml,music.qml,setting .qml
                                  so if i emit a signal and try to catch it in setting.qml does it work ??

                                  V Offline
                                  V Offline
                                  VincentLiu
                                  wrote on last edited by
                                  #16

                                  @Naveen_D
                                  Yes it works. I have tried before.

                                  Naveen_DN 1 Reply Last reply
                                  1
                                  • V VincentLiu

                                    @Naveen_D
                                    Yes it works. I have tried before.

                                    Naveen_DN Offline
                                    Naveen_DN Offline
                                    Naveen_D
                                    wrote on last edited by
                                    #17

                                    @VincentLiu but in my case it is not working....

                                    my code is...
                                    this .cpp where i am emitting signal..

                                    if(!WordList.isEmpty())
                                                {
                                                    WordList.removeFirst();
                                                    WordList.removeLast();
                                    //                QMessageBox m_popupmsgbox;
                                    //                m_popupmsgbox.setWindowTitle("Voice Recognizer");
                                    //                Phoneme=WordList.join(" ");
                                    //                qDebug()<<"firstword"<<Phoneme<<endl;
                                    //                QSpacerItem* horizontalSpacer = new QSpacerItem(500, 100, QSizePolicy::Minimum, QSizePolicy::Expanding);
                                    //                m_popupmsgbox.setText( Phoneme);
                                    //                QGridLayout* layout = (QGridLayout*)m_popupmsgbox.layout();
                                    //                layout->addItem(horizontalSpacer, layout->rowCount(), 0, 1, layout->columnCount());
                                    //                m_popupmsgbox.exec();
                                    //                emitSignalFunc();
                                    //                m_emitSignal= new EmitSignalClass;
                                    //                m_emitSignal->emitSignalmethod();
                                                    VoiceRecognition voice;
                                                    voice.playmusicsignal();
                                                }
                                                else
                                                {
                                                    qDebug()<<"List is empty"<<endl;
                                                }
                                    

                                    my .qml is

                                    Rectangle{
                                                id:voicerecRect
                                                width: settings_main_rect.width/5
                                                height: settings_main_rect.height/4
                                                color: "transparent"
                                    
                                                VoiceRecognition {
                                                    id: voiceRecognizer
                                                    onPlaymusicsignal: {
                                                        console.log("Signal catched...")
                                                    }
                                                }
                                    
                                                Image {
                                                    id: vr_image
                                                    source: "qrc:/AppbarIcon.png"
                                                    //source: "qrc:/Voice-Recoder-icon.png"
                                                    width: parent.width-10
                                                    height: parent.height-10
                                                    smooth: true
                                                    fillMode: Image.PreserveAspectFit
                                                    anchors.centerIn: parent
                                    
                                                    Text {
                                                        id: vrtext
                                                        anchors.top: parent.bottom
                                                        anchors.horizontalCenter: vr_image.horizontalCenter
                                                        text: qsTr("Voice Recorder")
                                                        color: "white"
                                                        font.pixelSize: parent.height * (2 / 9)
                                    
                                                    }
                                                    MouseArea {
                                                        anchors.fill: parent
                                                        onClicked: {
                                                            //popup.open()
                                                            voicerecRect.color = 'green'
                                                            voiceRecognizer.vstartVoiceRecognition()
                                                        }
                                                    }
                                                }
                                            }
                                        }
                                    
                                    

                                    i have registered the class using qmlregister and i am using signal handler to catch the signal...

                                    Naveen_D

                                    V 1 Reply Last reply
                                    0
                                    • Naveen_DN Naveen_D

                                      @VincentLiu but in my case it is not working....

                                      my code is...
                                      this .cpp where i am emitting signal..

                                      if(!WordList.isEmpty())
                                                  {
                                                      WordList.removeFirst();
                                                      WordList.removeLast();
                                      //                QMessageBox m_popupmsgbox;
                                      //                m_popupmsgbox.setWindowTitle("Voice Recognizer");
                                      //                Phoneme=WordList.join(" ");
                                      //                qDebug()<<"firstword"<<Phoneme<<endl;
                                      //                QSpacerItem* horizontalSpacer = new QSpacerItem(500, 100, QSizePolicy::Minimum, QSizePolicy::Expanding);
                                      //                m_popupmsgbox.setText( Phoneme);
                                      //                QGridLayout* layout = (QGridLayout*)m_popupmsgbox.layout();
                                      //                layout->addItem(horizontalSpacer, layout->rowCount(), 0, 1, layout->columnCount());
                                      //                m_popupmsgbox.exec();
                                      //                emitSignalFunc();
                                      //                m_emitSignal= new EmitSignalClass;
                                      //                m_emitSignal->emitSignalmethod();
                                                      VoiceRecognition voice;
                                                      voice.playmusicsignal();
                                                  }
                                                  else
                                                  {
                                                      qDebug()<<"List is empty"<<endl;
                                                  }
                                      

                                      my .qml is

                                      Rectangle{
                                                  id:voicerecRect
                                                  width: settings_main_rect.width/5
                                                  height: settings_main_rect.height/4
                                                  color: "transparent"
                                      
                                                  VoiceRecognition {
                                                      id: voiceRecognizer
                                                      onPlaymusicsignal: {
                                                          console.log("Signal catched...")
                                                      }
                                                  }
                                      
                                                  Image {
                                                      id: vr_image
                                                      source: "qrc:/AppbarIcon.png"
                                                      //source: "qrc:/Voice-Recoder-icon.png"
                                                      width: parent.width-10
                                                      height: parent.height-10
                                                      smooth: true
                                                      fillMode: Image.PreserveAspectFit
                                                      anchors.centerIn: parent
                                      
                                                      Text {
                                                          id: vrtext
                                                          anchors.top: parent.bottom
                                                          anchors.horizontalCenter: vr_image.horizontalCenter
                                                          text: qsTr("Voice Recorder")
                                                          color: "white"
                                                          font.pixelSize: parent.height * (2 / 9)
                                      
                                                      }
                                                      MouseArea {
                                                          anchors.fill: parent
                                                          onClicked: {
                                                              //popup.open()
                                                              voicerecRect.color = 'green'
                                                              voiceRecognizer.vstartVoiceRecognition()
                                                          }
                                                      }
                                                  }
                                              }
                                          }
                                      
                                      

                                      i have registered the class using qmlregister and i am using signal handler to catch the signal...

                                      V Offline
                                      V Offline
                                      VincentLiu
                                      wrote on last edited by
                                      #18

                                      @Naveen_D
                                      Hi, first of all, I should say that I use different ways to do this. However, according to some similar experience on it. I guess you shouldn't use a new VoiceRecognition object in your .cpp file. I don't think distinct object derived from the same class can communicate this way. Please correct me if I am wrong. Thanks

                                      Naveen_DN 2 Replies Last reply
                                      0
                                      • V VincentLiu

                                        @Naveen_D
                                        Hi, first of all, I should say that I use different ways to do this. However, according to some similar experience on it. I guess you shouldn't use a new VoiceRecognition object in your .cpp file. I don't think distinct object derived from the same class can communicate this way. Please correct me if I am wrong. Thanks

                                        Naveen_DN Offline
                                        Naveen_DN Offline
                                        Naveen_D
                                        wrote on last edited by
                                        #19

                                        @VincentLiu the function in which i am emitting a signal is a global function so i need to create an obj of that class and emit.

                                        Naveen_D

                                        1 Reply Last reply
                                        0
                                        • V VincentLiu

                                          @Naveen_D
                                          Hi, first of all, I should say that I use different ways to do this. However, according to some similar experience on it. I guess you shouldn't use a new VoiceRecognition object in your .cpp file. I don't think distinct object derived from the same class can communicate this way. Please correct me if I am wrong. Thanks

                                          Naveen_DN Offline
                                          Naveen_DN Offline
                                          Naveen_D
                                          wrote on last edited by
                                          #20

                                          @VincentLiu what are the different ways...?

                                          Naveen_D

                                          1 Reply Last reply
                                          0

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