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



  • 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 ")
        }
    }
    
    


  • 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



  • 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")
       }
    }
    


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



  • @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. 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



  • @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;
    }
    
    


  • 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.



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



  • 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.



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



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



  • 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.



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



  • 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
    Yes it works. I have tried before.



  • @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
    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



  • @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.



  • @VincentLiu what are the different ways...?



  • Hello, can anyone please help me out with this...
    thanks


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