How to catch the signal emitted from C++ global function in QML

[Naveen_D]

@raven-worx
Thank you for your reply...i am new to this
can you please explain me what changes i have to make in the above example so that i can get an idea. thanks

[sierdzio]

int output(int a, int b)
{
qDebug()<<"output function called"<<endl;
return(a+b);
Globaltest t;
t.testSignal();
}

You are creating Globaltest instance after return statement - it will never be called.

As raven-workx mentioned, there are several ways to make it work.

• make Globaltest a singleton (discouraged - there is no reason to do it in this case)
• make one instance of Globaltest in your main.cpp and use it in both QML and C++. Make it available in QML using engine->rootContext()->setContextProperty(). You can respond to signals from that object in QML by using Connections QML type
• use Globaltest as a QML singleton
• etc.

All that is described in great detail in Qt documentation: here and here.

[Naveen_D]

@sierdzio i am posting the updated code here...please correct me if i am wrong somewhere...thanks
.cpp

#include "globaltest.h"

Globaltest testobj;

Globaltest::Globaltest(QObject *parent) : QObject(parent)
{
}

void Globaltest::test()
{
    qDebug()<<"test called"<<endl;
    int x= output(20,20);
    qDebug()<<x<<endl;
}

int output(int a, int b)
{
    qDebug()<<"output function called"<<endl;
    testobj.testSignal();
    return(a+b);
}

main.cpp

#include <QGuiApplication>
#include <QApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

#include "globaltest.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    Globaltest testobj;

    qmlRegisterType<Globaltest>("com.globalCpp",1,0,"Globaltest");


    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("TestObject",&testobj);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

main.qml

import QtQuick 2.7
import QtQuick.Window 2.2
import com.globalCpp 1.0

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Globaltest{
        id: global
        onTestSignal: {
            console.log("signal catched");
        }
    }

    MouseArea{
        id: mouse
        anchors.fill: parent
        onClicked: {
            global.test()
        }
    }
}

[sierdzio]

Now you have 3 separate instances of Globaltest, and you want a single one :-)

Try this:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    Globaltest testobj;

    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("TestObject",&testobj);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    testobj.test(); // This emits the signal. Not the nicest solution, but should work for now.

    return app.exec();
}

// QML
Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Connections {
      target: TestObject
      onTestSignal: console.log("Signal caught")
   }

    MouseArea{
        id: mouse
        anchors.fill: parent
        onClicked: {
            TestObject.test() // Emits from QML
        }
    }
}

[Naveen_D]

only using the global variable, is it possible to make signal slot communication from cpp to Qml?

[raven-worx]

@Naveen_D
@sierdzio already showed you how to

Connections {
target: TestObject
onTestSignal: console.log("Signal caught")
}

[Naveen_D]

@raven-worx i tried as said by @sierdzio but i am not able to catch the signal.
here is the code.
.cpp

#include "globaltest.h"

Globaltest testobj;
Globaltest::Globaltest(QObject *parent) : QObject(parent)
{
}

void Globaltest::test()
{
    qDebug()<<"test called"<<endl;
    output();
}

void output()
{
    qDebug()<<"output function called"<<endl;
    testobj.testSignal();
}

main.cpp

#include <QGuiApplication>
#include <QApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

#include "globaltest.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    Globaltest testobj;

    qmlRegisterType<Globaltest>("com.globalCpp",1,0,"Globaltest");


    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("TestObject",&testobj);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

main.qml

import QtQuick 2.7
import QtQuick.Window 2.2
import com.globalCpp 1.0

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Connections {
          target: TestObject
          onTestSignal: console.log("Signal caught")
       }
    MouseArea{
            id: mouse
            anchors.fill: parent
            onClicked: {
                TestObject.test()
            }
        }
}

[raven-worx]

@Naveen_D
you are creating two different instances of your your object. One in the main() and one in the cpp file.
You need to ensure that you operate on the same instance. Take a look at the Singleton pattern:

class Globaltest {
public:
    static Globaltest* Globaltest::Instance() {  //static method
          static Globaltest* instance = new Globaltest;
          return instance;
    }
private:
     Globaltest();  //hidden constructor --> only allow to use our static Instance() method
};

[Naveen_D]

@raven-worx without using singleton...it is not possible? becoz i have a global object of that class...

[raven-worx]

@Naveen_D
sure, but you need to make sure you are using the right object instance. The singleton is an easy and understandable mechanism, which prevents some possible poitfalls.
As i said in your posted example you are using 2 different instances. Define the global object as global static object in the header file instead of the source file.

[Naveen_D]

@raven-worx
if i define the global object as global static object in the header file instead of the source file. it is not necessary to again create an object in main.cpp and use context property?..without this i can directly catch the signal using signal handler in qml ?

[raven-worx]

@Naveen_D said in How to catch the signal emitted from C++ global function in QML:

it is not necessary to again create an object in main.cpp and use context property?

the context property of course is still necessary. But not the duplicate object creation. Instead reference the static global one from the header file wherever you need it.

[Naveen_D]

@raven-worx i want to use this global obj in global function to emit the signal from that function.
the way i am declaring the global function is right ?

#ifndef GLOBALTEST_H
#define GLOBALTEST_H

#include <QObject>
#include <QDebug>

void output();
class Globaltest : public QObject
{
    Q_OBJECT
    static Globaltest *s_instance;
public:
    explicit Globaltest(QObject *parent = 0);

signals:
    void testSignal();


public slots:
    Q_INVOKABLE void test();
};

#endif // GLOBALTEST_H

[Naveen_D]

Hi, as you said i have only one instance of the object in the below code but when i use this object in set context property i am getting QVariant error...the error is

/home/ubuntu/Qt5.7.0/5.7/gcc_64/include/QtCore/qvariant.h:471: error: 'QVariant::QVariant(void)' is private
inline QVariant(void ) Q_DECL_EQ_DELETE;

and if i remove the '&' and run the code i get this qml error
QML debugging is enabled. Only use this in a safe environment.
qrc:/main.qml:20: TypeError: Cannot call method 'test' of null

can anyone tell what is wrong in the code and what i need to change
Thanks

.h

#ifndef GLOBALTEST_H
#define GLOBALTEST_H

#include <QObject>
#include <QDebug>

void output();
class Globaltest : public QObject
{
    Q_OBJECT

public:
    explicit Globaltest(QObject *parent = 0);

signals:
    void testSignal();


public slots:
    Q_INVOKABLE void test();
};
extern Globaltest *MySender;
#endif // GLOBALTEST_H

.cpp

#include "globaltest.h"

Globaltest *MySender;
Globaltest::Globaltest(QObject *parent) : QObject(parent)
{
}

//Globaltest *Globaltest::s_instance = 0;

void Globaltest::test()
{
    qDebug()<<"test called"<<endl;
    output();
}

void output()
{
    qDebug()<<"output function called"<<endl;
    MySender= new Globaltest;
    MySender->testSignal();
}

main.cpp

#include <QGuiApplication>
#include <QApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

#include "globaltest.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    qmlRegisterType<Globaltest>("com.globalCpp",1,0,"Globaltest");

    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("TestObject",&MySender); //Getting error here //
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

main.qml

import QtQuick 2.7
import QtQuick.Window 2.2
import com.globalCpp 1.0

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Connections {
          target: TestObject
          onTestSignal: console.log("Signal caught")
       }

    MouseArea{
            id: mouse
            anchors.fill: parent
            onClicked: {
                TestObject.test()
            }
        }
}

[sierdzio]

You are declaring MySender to be a pointer, so using '&' in setContextProperty() is wrong. '&' is a way to extract a pointer from non-pointer variable. Here you already have a pointer, so no need for '&'.

You declare MySender variable, but you never set it to any value (meaning: you never create an object of type Globaltest). So QML engine - rightfully - complains that the object is null (== not set).

You need to set the value of MySender somewhere in your code (before call to setContextProperty), in other words:

MySender = new GlobalTest;

[Naveen_D]

@sierdzio ya that i came to know since i am using pointer var no need of '&' but in cpp before emitting a signal i am allocating memory for that global object...

[raven-worx]

@Naveen_D
to add up to @sierdzio
also remove the line MySender= new Globaltest; from your output() method.
Instead call it once before you set it as context property.

[sierdzio]

@raven-worx said in How to catch the signal emitted from C++ global function in QML:

also remove the line MySender= new Globaltest; from your output() method.

Doh, I did not even look at that method when writing my reply. Good hint, thanks for being so attentive @raven-worx :-)

[Naveen_D]

@raven-worx @sierdzio
yes when i allocate memory before setcontext property it worked...
thanks alot for your help...:-)

[sierdzio]

Cool, good to know :-) Happy coding!