Passing QVariant from QML to C++ - c++

Question

I am trying to send a list from QML to c++. I've tried with a string and an integer with success but when I try with a QVariant I get the error:
QObject::connect: No such slot MyClass::cppSlot(QVariant) in ../test_2206/main.cpp:31
My main.qml
import QtQuick 2.4
import QtQuick.Layouts 1.1
import Material.ListItems 0.1 as ListItem
import Material.Extras 0.1
import QtQuick.Controls 1.3 as QuickControls
import QtQuick.Controls 1.4
import Material 0.2
Window {
visible: true
property var listCloud: ["nuage1", "nuage2", "nuage3", "nuage4"]
id: item
width: 100; height: 100
signal qmlSignal(variant msg)
/* MouseArea {
anchors.fill: parent
onClicked: item.qmlSignal("Hello from QML")
}*/
Rectangle{
id:sousRect
color:"red"
width:100; height:100
Button{
id:buttonTest
onClicked: {
item.qmlSignal(listCloud)
}
}
}
}
My main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlProperty>
#include <QQmlComponent>
#include <QDebug>
#include "myclass.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
QQmlComponent component(&engine, QUrl(QStringLiteral("qrc:/main.qml")));
QObject *object = component.create();
MyClass myClass;
QObject::connect(object, SIGNAL(qmlSignal(QVariant)),
&myClass, SLOT(cppSlot(QVariant)));
return app.exec();
}
My myclass.h
#ifndef MYCLASS_H
#define MYCLASS_H
#include <QDebug>
#include <QString>
#include <QList>
class MyClass : public QObject
{
Q_OBJECT
public:
MyClass(QObject *parent = 0);
signals:
public slots:
void cppSlot(QVariant &msg);
};
#endif // MYCLASS_H
My myclass.cpp
#include "myclass.h"
MyClass::MyClass(QObject *parent):
QObject(parent)
{
}
void MyClass::cppSlot(QVariant &msg){
qDebug() << "c++: bien reçu chef " << msg;
}
I don't understand why I can't put a QVariant parameter in this signal.
Any help is welcome :)

Answer 1

Change your slot to receive his parameter by value, rather than by reference. Like this:
void cppSlot(QVariant msg);
I think it has something to do with QML refusing to pass Window's property by reference. Anyway, Qt often sends signal by-value even if your signal/slot signature[s] declare arguments by-reference. For more on the topic see this and that

Answer 2

Don't use references between QML and C++, they won't work.
void cppSlot(QVariant & msg);
You can also create Q_INVOKABLE function in C++ and call it directly from QML. Here is how this looks like:
main.cpp:
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlProperty>
#include <QQmlComponent>
#include <QDebug>
#include "myclass.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
auto myobject = new MyClass(&app);
engine.rootContext()->setContextProperty(QStringLiteral("myobject"), myobject);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine.rootObjects().isEmpty())
{
return -1;
}
return app.exec();
}
mycass.h:
#ifndef MYCLASS_H
#define MYCLASS_H
#include <QDebug>
#include <QString>
#include <QList>
class MyClass : public QObject
{
Q_OBJECT
public:
MyClass(QObject *parent = 0);
Q_INVOKABLE void cppSlot(QVariant msg);
};
#endif // MYCLASS_H
myclass.cpp:
#include "myclass.h"
MyClass::MyClass(QObject *parent):QObject(parent){}
void MyClass::cppSlot(QVariant msg)
{
qDebug() << "c++: bien reçu chef " << msg;
}
main.qml:
import QtQuick 2.4
import QtQuick.Layouts 1.1
import Material.ListItems 0.1 as ListItem
import Material.Extras 0.1
import QtQuick.Controls 1.3 as QuickControls
import QtQuick.Controls 1.4
import Material 0.2
Window
{
visible: true
property var listCloud: ["nuage1", "nuage2", "nuage3", "nuage4"]
id: item
width: 100; height: 100
Rectangle
{
id:sousRect
color:"red"
width:100; height:100
Button
{
id:buttonTest
onClicked: myobject.cppSlot(listCloud)
}
}
}
But if you like to use SIGNAL SLOTS there is Connections Object in QML. Your main.qml will look then like that:
Window
{
visible: true
property var listCloud: ["nuage1", "nuage2", "nuage3", "nuage4"]
id: item
width: 100; height: 100
Connections
{
target: buttonTest
onClicked: myobject.cppSlot(listCloud)
}
Rectangle
{
id:sousRect
color:"red"
width:100; height:100
Button
{
id:buttonTest
}
}
}
Q_INVOKABLE is nothing else as an public SLOT, it will be called through meta-object system.