Build basic Qt interface with QML - c++

Question

I am working on a program which uses an user interface created in QML. The program at it's core have two threads: the main thread in which UI is running and the second thread which will handle all the rest of the work. So program have once class for interfacing with UI and a second class which it's the backend process. The problem is with connecting the slots/signals from UI class to the second class on the second thread.
The code is the following:
main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include "QmlInterface.h"
#include "MainClass.h"
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
qmlRegisterType<QmlInterface>("CppInterface",1,0,"CppInterface");
/* Ui Stuff */
QmlInterface qml;
qml.SetupUI();
/* Main class */
MainClass work;
QObject::connect(&qml, SIGNAL(onButtonClicked()), &work, SLOT(on_ButtonClicked()) );
return app.exec();
}
QmlInterface.h
#ifndef QMLINTERFACE_H
#define QMLINTERFACE_H
#include <QObject>
#include <QDebug>
#include <QQmlApplicationEngine>
class QmlInterface : public QObject
{
Q_OBJECT
public:
explicit QmlInterface();
virtual ~QmlInterface();
void SetupUI();
public slots:
Q_INVOKABLE void buttonClicked();
signals:
void onButtonClicked();
private:
QQmlApplicationEngine *engine;
};
#endif // QMLINTERFACE_H
QmlInterface.cpp
#include "QmlInterface.h"
QmlInterface::QmlInterface()
{
}
QmlInterface::~QmlInterface()
{
}
void QmlInterface::SetupUI()
{
engine = new QQmlApplicationEngine;
engine->load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine->rootObjects().isEmpty())
{
qDebug() << "Failed to load UI";
}
}
void QmlInterface::buttonClicked()
{
qDebug() << "Button clicked! Signal to be emited!";
emit onButtonClicked();
}
MainClass.h
#ifndef MAINCLASS_H
#define MAINCLASS_H
#include <QThread>
#include <QtDebug>
class MainClass : public QThread
{
Q_OBJECT
public:
MainClass();
virtual ~MainClass() {}
public slots:
void on_ButtonClicked();
private:
void run();
};
#endif // MAINCLASS_H
MainClass.cpp
#include "MainClass.h"
MainClass::MainClass()
{
}
void MainClass::on_ButtonClicked()
{
qDebug() << "Button click received in main class!";
}
void MainClass::run()
{
while(1)
{
QThread::sleep(1);
}
}
And finally main.qml
import QtQuick 2.11
import QtQuick 2.8
import QtQuick.Controls 2.1
import QtQuick.Window 2.1
import CppInterface 1.0
Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
CppInterface
{
id: cpp
}
Button
{
text: "Click me"
onPressed: {
cpp.buttonClicked();
}
}
}
The connection between QML and QmlInterface works fine! The problem is with connection between QmlInterface and MainClass.
To be more specific, the problem is that connect() function called in main.cpp seems not to be able to link given signal with given slot from MainClass:
QObject::connect(&qml, SIGNAL(onButtonClicked()), &work, SLOT(on_ButtonClicked()) );

Answer 1

The problem is that you have created 2 instances of QmlInterface:
QmlInterface qml;
qml.SetupUI();
and
CppInterface
{
id: cpp
}
And you have connected the signal of the first instance, and you are emitting the signal using the second instance.
So instead of creating 2 QmlInterface just create one, for convenience you do not create the object in QML and only export the object created in C++ using setContextProperty():
// ...
#include <QQmlContext>
// ...
void QmlInterface::SetupUI()
{
engine = new QQmlApplicationEngine;
engine->load(QUrl(QStringLiteral("qrc:/main.qml")));
if (engine->rootObjects().isEmpty())
qDebug() << "Failed to load UI";
else
// export
engine->rootContext()->setContextProperty("cpp", this);
}
*.qml
import QtQuick 2.11
import QtQuick.Controls 2.1
import QtQuick.Window 2.1
Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
Button
{
text: "Click me"
onPressed: cpp.buttonClicked();
}
}
On the other hand it is not necessary to register as a type to QmlInterface so you can remove it:
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
/* Ui Stuff */
QmlInterface qml;
qml.SetupUI();
/* Main class */
MainClass work;
QObject::connect(&qml, &QmlInterface::onButtonClicked, &work, &MainClass::on_ButtonClicked );
return app.exec();
}