Qt C++ Signal to Qml

Question

I am attempting to catch a C++ Qt Signal in Qml. I am able to send a signal, and catching a Qml Signal in Qt is working too; however, I cannot catch a Qt signal in Qml.

Which QObject::connect do i need?

minimal main.cpp:

    #include <QtGui/QGuiApplication>
    #include <QtQml/QQmlApplicationEngine>
    #include <QQmlContext>
    #include <QQuickWindow>
    
    #include "qmlcppapi.h"

    int main(int argc, char *argv[])
    {
        QGuiApplication app(argc, argv);
        qmlRegisterType<QmlCppApi>("com.handleQmlCppApi",1,0,"HandleQmlCppApi");
        QQmlApplicationEngine engine;
        const QUrl url(QStringLiteral("qrc:/qml/qmlfile.qml"));
        QmlCppApi api;
        engine.rootContext()->setContextProperty("api", &api);
        engine.load(url);
        QObject::connect(&api, &QmlCppApi::testStringSended,
                         &api, &QmlCppApi::printTestString);
        return app.exec();
    }

minimal gmlcppapi.hpp: Slot is only for showing if the signal is emitted

    #ifndef QMLCPPAPI_H
    #define QMLCPPAPI_H
    
    #include <QObject>
    #include <QDebug>
    
    class QmlCppApi : public QObject
    {
        Q_OBJECT
    
    public:
        Q_INVOKABLE void postTestString(QString TestString) {
            qDebug() << "cpp: recieved";
            emit testStringSended(TestString);
        }
    
    public slots:
        void printTestString(QString TestString) {
            qDebug() << "cpp: sended";
        }
    
    signals:
        void testStringSended(QString TestString);
    };
    
    #endif // QMLCPPAPI_H

minimal qmlfile.qml: The ToggleButton should execute the cpp function testStringSended. And the printTestString is firering a emit which should triggers the onTestStringSended

    import QtQuick 2.2
    import QtQuick.Window 2.1
    import QtQuick.Controls 1.4
    import QtQuick.Controls.Styles 1.4
    import QtQuick.Extras 1.4
    import com.handleQmlCppApi 1.0
    
    Window {
        visible: true
        ToggleButton {
            onClicked: {
                console.log("send")
                api.postTestString("TestString")
            }
        }
    
        HandleQmlCppApi {
            onTestStringSended: console.log("recieved")
        }
    }

output:

qml: send
cpp: recieved
cpp: sended

Why is my Qml not receiving the signal?

Answer 1

You have two instances of QmlCppApi that you created. One is in main.cpp, that you call api , and the other is in QML which is the unnamed HandleQmlCppApi object. You only need one of them. To catch a signal from api you need a Connections object, like this:

Connections {
    target: api

    onTestStringSended: console.log("recieved")
}