C++ updating data to QML

wickwire

Hi,

I've been searching for this issue for a while now, at the moment I'm more confused than ever:

Issue:

Display data in QML, that is being collected with C++, from the OS, updated in timed intervals

OS: GNU/Linux
Qt5 and QML

So far I have managed to obtain the following:

C++ invokes the wanted system binary and collects the output
I'm using a QTimer to do this every 1000msec so the variable gets updated in C++
main.cpp has a setContextProperty definition, so that main.qml can reach the C++ variable

So I'm able to use QML to reach the variable value in C++ and display/format it, but I'm not being able to refresh/update the data on QML. I'm only getting the first hit.

I know that C++ QTimer is working because I've added a qDebug definition which shows the updated data, every second, at the console log.

I will try to publish the code, however what would be the best way to achieve the intented result?

I've also considered inverting my reasoning: use QML to invoke C++on demand, when needed, and have the timer on the QML side - didn't work...

Thanks in advance

N3Roaster

Is there any reason the variable couldn't be a property on a QObject with a NOTIFY signal (emit when you collect new data) on the C++ side? Then just have the QML bind to that property and things should just work.

hpollak

I had an similar problem.

See my post: http://qt-project.org/forums/viewthread/23449/#109214

I found the solution in "Dokumentation":http://qt-project.org/doc/qt-5.0/qtcore/queuedcustomtype.html

The example is Widgetbased, but it's allso working with qml.

wickwire

Thank you both for your input, I've been at it for a while and I decided to start from scratch:

This example attemps the following:

C++

Clock has the task of attributing a random value to a variable
main has a Timer which invokes the function count() every second, generating a new random
each execution is sent to the console log
main has a property binding (hopefully that's the name) which attempts to expose the recurrent value to QML
QML has its own timer, with which I'd be hoping to have the C++ value refreshed (maybe not needed, the intent would be to make sure QML refreshed the variable - and a QML-own counter just to show activity)
the objective is to display the same randoms show in the console logs (from C++) on the white canvas from QML.

Quite possibly my notions may still be off as I'm learning, so I'm taking your given help in trying to get there.

Hopefully I will get a proper example working and when I do, I will update the code here!

Many thanks!

Using Qt Creator, I created a new project "Qt Quick 2 Application (Built-in Elements)

Headers:

clock.h

@#ifndef CLOCK_H
#define CLOCK_H

#include <QObject>

class Clock : public QObject
{
Q_OBJECT

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

signals:
void counterUpdated();

public slots:
void count();

};

#endif // TIMER_H
@

clock.cpp

@#include "clock.h"
#include <QDebug>

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

void Clock::count(){
long plus = random();
qDebug() << plus;
//emit counterUpdated();
}
@

main.cpp

@#include <QtGui/QGuiApplication>
#include "qtquick2applicationviewer.h"
#include <QTimer>
#include <clock.h>
#include <QDebug>
#include <QQmlContext>
#include <QObject>

int main(int argc, char *argv[])
{
QString randomitum;
QGuiApplication app(argc, argv);
QtQuick2ApplicationViewer viewer;
Clock *tintin = new Clock;
QTimer *tim = new QTimer;
QObject::connect(tim,SIGNAL(timeout()),tintin,SLOT(count()));
tim->start(1000);

viewer.rootContext()->setContextProperty("qmlCounter", randomitum);
viewer.setMainQmlFile&#40;QStringLiteral("qml/fringe/main.qml"&#41;);
viewer.showExpanded();

return app.exec();

}
@

main.qml

@import QtQuick 2.0

Rectangle {
width: 360
height: 360
property int flip : 0
Text {
id: textum
anchors.centerIn: parent
}

Timer {
    interval: 1000; running: true; repeat: true
    onTriggered: { flip = flip +1; textum.text = qmlCounter + " : " + flip }
}

MouseArea {
    anchors.fill: parent
    onClicked: {
        Qt.quit();
    }
}

}
@

warjan

Do N3Roaster said. From "http://qt-project.org/doc/qt-5.0/qtqml/qtqml-cppintegration-exposecppattributes.html":http://qt-project.org/doc/qt-5.0/qtqml/qtqml-cppintegration-exposecppattributes.html

For maximum interoperability with QML, any property that is writable should have an associated NOTIFY signal that is emitted whenever the property value has changed.