Connecting Signal QML to C++ (Qt5) - c++

Question

I would like to change a the color of a rectangle when I click a button. They are both in the main.qml file. I'd like to send a signal to C++ backend to change the color of the rectangle. I can't seem to figure it out from the code given in the documentation
main.qml:
import QtQuick 2.4
import QtQuick.Controls 1.3
import QtQuick.Window 2.2
import QtQuick.Dialogs 1.2
ApplicationWindow {
title: qsTr("Hello World")
width: 640
height: 480
visible: true
id:root
signal mysignal()
Rectangle{
anchors.left: parent.left
anchors.top: parent.top
height : 100
width : 100
}
Button
{
id: mybutton
anchors.right:parent.right
anchors.top:parent.top
height: 30
width: 50
onClicked:root.mysignal()
}
}
main.cpp:
#include <QApplication>
#include <QQmlApplicationEngine>
#include<QtDebug>
#include <QQuickView>
class MyClass : public QObject
{
Q_OBJECT
public slots:
void cppSlot() {
qDebug() << "Called the C++ slot with message:";
}
};
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
MyClass myClass;
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
QPushButton *mybutton = engine.findChild("mybutton");
engine.connect(mybutton, SIGNAL(mySignal()),
&myClass, SLOT(cppSlot()));
return app.exec();
}
Any help would be appreciated!

Answer 1

QPushButton *mybutton = engine.findChild("mybutton");
First, QObject::findChild finds QObjects by object name, not id (which is local to a context anyhow). Hence in QML you need something like:
objectName: "mybutton"
Second, I think you need to perform that findChild not on the engine itself, but on its root objects as returned from QQmlApplicationEngine::rootObjects().
// assuming there IS a first child
engine.rootObjects().at(0)->findChild<QObject*>("myButton");
Third, a Button in QML is represented by a type that you don't have available in C++. So you can't just assign the result to a QPushButton *, but you need to stick with the generic QObject *.

Answer 2

I had to create a seperate class and header file and then connect it to the signal in main.cpp
main.cpp
#include <QApplication>
#include <QQmlApplicationEngine>
#include<QtDebug>
#include <QQuickView>
#include<QPushButton>
#include<myclass.h>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
QObject *topLevel = engine.rootObjects().at(0);
QQuickWindow *window = qobject_cast<QQuickWindow *>(topLevel);
MyClass myClass;
QObject::connect(window,
SIGNAL(mysignal()),
&myClass,
SLOT(cppSlot())
);
return app.exec();
}
myclass.h
#ifndef MYCLASS
#define MYCLASS
#include <QObject>
#include <QDebug>
class MyClass : public QObject
{
Q_OBJECT
public slots:
void cppSlot();
};
#endif // MYCLASS
myclass.cpp
#include<myclass.h>
void MyClass::cppSlot(){
qDebug()<<"Trying";
}