How to update qml control periodically from c++ code

[Ahti]

I want to update qml slider every second from c++ code how would I do it ?

main.qml:

 Slider {               
              id: new_slider
              from: 0
              to: 100
              live: true               
              value: "0"
              stepSize: 1.0
         }

Example.h:

QTimer *timer;

Example.cpp:

Example::Example(QObject *parent) : QObject(parent)
{
    timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()),this,SLOT(update_slider()));
    timer->start(1000);
}
Example::~Example(){
    timer->stop();
}
void Example::update_slider(){
    // code for updating qml slider
}

how would I update the slider every second in c++ ? please help

[LeLev]

@Ahti hi
here are ways to Expose Attributes of C++ Types to QML you can use the first approach

class Example: public QObject
{
    Q_OBJECT
 // create a  qproperty to expose it to qml
    Q_PROPERTY(int val READ val WRITE setVal NOTIFY valChanged)
public:

    void update_slider(){ // update
     setVal(m_val+1);
    }

    void setVal(const int &v) { 
        if (v != m_val) {
            m_val = v;
            emit valChanged();
        }
    }
    int val() const {
        return m_val;
    }

signals:
    void valChanged(int);
private:
    QTimer *timer;
    int m_val;
};

//expose class object to qml

int main(int argc, char *argv[]) {
    QGuiApplication app(argc, argv);
...
    Example ex;
    view.engine()->rootContext()->setContextProperty("mycppObj", &ex);
  ..

    return app.exec();
}
// use the obect in qml 
 Slider {               
              id: new_slider
              from: 0
              to: 100
              live: true               
              value: mycppObj.val //this will update
              stepSize: 1.0
         }

[Ahti]

@LeLev That doesn't update the slider after timeout

[LeLev]

@Ahti ok , what we do now ? could you provide more info instead of asking other people to answer ?

[LeLev]

..tested example

#ifndef EXAMPLE_H
#define EXAMPLE_H

#include <QObject>
#include <QTimer>
#include <QDebug>

class Example : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int val READ val WRITE setVal NOTIFY valChanged)
public:
    explicit Example(QObject *parent = nullptr);
    void setVal(const int &v) {
        if (v != m_val) {
            m_val = v;
            emit valChanged(v);
        }
    }
    int val() const {
        return m_val;
    }
public slots:
    void update_slider(){ // update
        qDebug()<< "adding 1";
     setVal(m_val+1);
      qDebug()<< m_val;
    }
signals:
    void valChanged(int);
private:
    QTimer *timer;
    int m_val=0;
};
#endif // EXAMPLE_H
#include "example.h"
//.cpp
Example::Example(QObject *parent) : QObject(parent)
{
    timer = new QTimer(this);
        // old connect(timer, SIGNAL(timeout()),this,SLOT(update_slider()));
        QObject::connect(timer,&QTimer::timeout,this,&Example::update_slider);
        timer->start(1000);
}
//main
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QDebug>
#include <QQmlContext>
#include <example.h>
int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    Example e;

    QQmlApplicationEngine engine;

    engine.rootContext()->setContextProperty("ex",&e);

    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}
//qml
import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12
import QtQuick.Layouts 1.12

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Slider{
        id:sl
        anchors.centerIn: parent
        from: 0
        to : 500

        value: ex.val
    }

}

[Ahti]

Okay thanks it works now :) And I have changed time to 500 milliseconds. But doesn't timer makes qt projects slower ? Isn't there any other better way ? like binding to an event which I think would reduce computation power because you don't have to periodically repeat the computation.