Problem with QML and C++ properties

[enrico5th]

Hi everybody.

I'm trying to load a QML component depending on the value of a certain variable declared as a property in a C++ class but when I start debugging the application QML does not recognize the C++ object containing that property.
Does anyone can help? I'll put the code down here.

This is the main.cpp file:

#include <QtGui/QGuiApplication>
#include <QtQml>
#include "app.h"
 
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
 
    cApp viewer;
    viewer.setMainQmlFile(QStringLiteral("qml/ClemDemo/main.qml"));
    viewer.showExpanded();
 
    return app.exec();
}

#include <QtGui/QGuiApplication>
#include <QtQml>
#include "app.h"

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

    cApp viewer;
    viewer.setMainQmlFile(QStringLiteral("qml/ClemDemo/main.qml"));
    viewer.showExpanded();

    return app.exec();
}

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This is the cApp class derived from the class generated by the QtQuick wizard in order to use a particular nativeEvent function:

#include <QtQml>
#include "qtquick2applicationviewer.h"
#include "myclass.h"
 
class cApp : public QtQuick2ApplicationViewer
{
    Q_OBJECT
 
    bool nativeEvent(const QByteArray &eventType, void *message, long *result);
 
public:
    cMyClass* myClass ;
    explicit cApp(QWindow *parent = 0);
    virtual ~cApp();
 
public slots:
    void device_in();
    void device_out();
};
 
cApp::cApp(QWindow *parent) : QtQuick2ApplicationViewer(parent)
{
    myClass = new cMyClass;
    rootContext()->setContextProperty("myqmlclass", myClass );
    connect(myClass , SIGNAL(device_connected()), this, SLOT(device_in()));
    connect(myClass , SIGNAL(device_removed()), this, SLOT(device_out()));
}
 
bool cApp::nativeEvent(const QByteArray &eventType, void *message, long *result)
{
    return myClass ->nativeEvent(eventType, message, result);
}
 
void cApp::device_in()
{
    myClass ->setDeviceConnected(true);
}
 
void cApp::device_out()
{
    myClass ->setDeviceConnected(false);
}

#include <QtQml>
#include "qtquick2applicationviewer.h"
#include "myclass.h"

class cApp : public QtQuick2ApplicationViewer
{
    Q_OBJECT

    bool nativeEvent(const QByteArray &eventType, void *message, long *result);

public:
    cMyClass* myClass ;
    explicit cApp(QWindow *parent = 0);
    virtual ~cApp();

public slots:
    void device_in();
    void device_out();
};

cApp::cApp(QWindow *parent) : QtQuick2ApplicationViewer(parent)
{
    myClass = new cMyClass;
    rootContext()->setContextProperty("myqmlclass", myClass );
    connect(myClass , SIGNAL(device_connected()), this, SLOT(device_in()));
    connect(myClass , SIGNAL(device_removed()), this, SLOT(device_out()));
}

bool cApp::nativeEvent(const QByteArray &eventType, void *message, long *result)
{
    return myClass ->nativeEvent(eventType, message, result);
}

void cApp::device_in()
{
    myClass ->setDeviceConnected(true);
}

void cApp::device_out()
{
    myClass ->setDeviceConnected(false);
}

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This is the class i need to use into QML:

class cMyClass: public QObject
{
    Q_OBJECT
 
    friend class cApp;
 
protected:
    bool nativeEvent(const QByteArray &eventType, void *message, long *result);
 
public:
    cMyClass();
    virtual ~cMyClass(){}
 
public:
    bool DeviceConnected;
    Q_PROPERTY(bool connected READ isConnected WRITE setDeviceConnected NOTIFY somethingHappened)
 
    void setDeviceConnected(bool status){ DeviceConnected = status;}
    bool isConnected(){ return DeviceConnected; }
 
signals:
    void somethingHappened();
 
    void device_connected();
    void device_removed();
};
 
cMyClass::cMyClass()
{
    //Connections for attach/detach notification of a particular device
    // .............
}
 
bool cMyClass::nativeEvent(const QByteArray &eventType, void *message, long *result)
{
    //Handle messages of device attached/detached
}

class cMyClass: public QObject
{
    Q_OBJECT

    friend class cApp;

protected:
    bool nativeEvent(const QByteArray &eventType, void *message, long *result);

public:
    cMyClass();
    virtual ~cMyClass(){}

public:
    bool DeviceConnected;
    Q_PROPERTY(bool connected READ isConnected WRITE setDeviceConnected NOTIFY somethingHappened)

    void setDeviceConnected(bool status){ DeviceConnected = status;}
    bool isConnected(){ return DeviceConnected; }

signals:
    void somethingHappened();

    void device_connected();
    void device_removed();
};

cMyClass::cMyClass()
{
    //Connections for attach/detach notification of a particular device
    // .............
}

bool cMyClass::nativeEvent(const QByteArray &eventType, void *message, long *result)
{
    //Handle messages of device attached/detached
}

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And here is the QML code:

import QtQuick 2.0
 
BorderImage {
    id: main_window
    source: "qrc:/main/sfondo.jpg"
    width: 1680; height: 932
    horizontalTileMode: BorderImage.Stretch
    verticalTileMode: BorderImage.Stretch
 
    Component
    {
        id: devices
           Rectangle {
                id: device_side
                width: 400; height: 500
                color: "#C0A3A3A3"      // #AARRGGBB where AA is Alpha channel (for transparency)
                radius: 8
             }
    }
    Loader { id: device_loader }
    myqmlclass.onSomethingHappened: { device_loader.sourceComponent = devices }
}

import QtQuick 2.0

BorderImage {
    id: main_window
    source: "qrc:/main/sfondo.jpg"
    width: 1680; height: 932
    horizontalTileMode: BorderImage.Stretch
    verticalTileMode: BorderImage.Stretch

    Component
    {
        id: devices
           Rectangle {
                id: device_side
                width: 400; height: 500
                color: "#C0A3A3A3"      // #AARRGGBB where AA is Alpha channel (for transparency)
                radius: 8
             }
    }
    Loader { id: device_loader }
    myqmlclass.onSomethingHappened: { device_loader.sourceComponent = devices }
}

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The error is that myqmlclass is not recognized as a QML property. This is what I can see on the application output:

"Cannot assign to non-existent property "myqmlclass"
myqmlclass.onSomethingHappened: { device_loader.sourceComponent = devices }"

[anda_skoa]

somethingHappend is a signal, not a property.

You have two options:

1) property binding

Loader {
    id: device_loader
    sourceComponent = myqmlclass.connected ? devices : null
}

Loader {
    id: device_loader
    sourceComponent = myqmlclass.connected ? devices : null
}

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2) Connection element

Connection {
    target: myqmlclass
    onSomethingHappend: ....
}

Connection {
    target: myqmlclass
    onSomethingHappend: ....
}

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Cheers,
_