How to update qml text from two different cpp ?

bladekel

I have an qml project.

There is a text(named id:cnt) on StackPage.qml and I need to update this text from firstclass.cpp and secondclass.cpp.

Q_PROPERTY defines are on firstclass.h and setCntText function is on firstclass.cpp.

I update the text from firstclass.cpp by setCntText(i) and try to update from secondclass.cpp by calling setCntText(0).

I can set the m_cntText variable from secondclass but couldnt update qml text(named id:cnt).

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

#include "firstclass.h"
#include "secondclass.h"

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    firstClass frmFirstClass;
    engine.rootContext()->setContextProperty("frmFirstClass",&frmFirstClass);

    secondClass frmSecondClass;

    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

firstclass.cpp

#include "firstclass.h"

firstClass::firstClass(QObject *parent) : QObject(parent)
{
    QTimer *timer1 = new QTimer();
    connect(timer1, &QTimer::timeout, [=]() {setter1();});
    timer1->start(2000);
}

int firstClass::cntText() const
{
    return m_cntText;
}

void firstClass::setCntText(int cntText)
{
    if (m_cntText == cntText)
        return;

    m_cntText = cntText;
    emit cntTextChanged(m_cntText);
}

void firstClass::setter1()
{
    static int i = 0;
    i++;
    qDebug() <<"counter1 : " << i;
    setCntText(i);
}

secondclass.cpp

#include "secondclass.h"

firstClass frmFirstClass;

secondClass::secondClass(QObject *parent) : QObject(parent)
{
    QTimer *timer = new QTimer();
    timer->setSingleShot(true);
    timer->start(1000);

    connect(timer, &QTimer::timeout, [=]() {
        QTimer *timer2 = new QTimer(this);
        connect(timer2,SIGNAL(timeout()),this,SLOT(setter2()));
        timer2->start(2000);
        timer->deleteLater();
    } );
}

void secondClass::setter2()
{
    frmFirstClass.setCntText(0);
    qDebug() << "Checking m_cntText = " << frmFirstClass.m_cntText;
}

firstclass.h

#ifndef FIRSTCLASS_H
#define FIRSTCLASS_H

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

class firstClass : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int cntText READ cntText WRITE setCntText NOTIFY cntTextChanged)


public:
    explicit firstClass(QObject *parent = nullptr);

    int cntText() const;

    int m_cntText;

signals:

    void cntTextChanged(int cntText);

public slots:

    void setCntText(int cntText);

private slots:

    void setter1();
};

#endif // FIRSTCLASS_H

secondclass.h

#ifndef SECONDCLASS_H
#define SECONDCLASS_H

#include <QObject>
#include <QTimer>
#include <QDebug>
#include "firstclass.h"

extern firstClass frmFirstClass;

class secondClass : public QObject
{
    Q_OBJECT

private:

public:
    explicit secondClass(QObject *parent = nullptr);

signals:

public slots:

private slots:

    void setter2();
};

#endif // SECONDCLASS_H

main.qml

import QtQuick 2.10
import QtQuick.Window 2.12
import QtQuick.Controls 2.0

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

    Rectangle{
        anchors.fill: parent
        color: "black"

        StackView{
            anchors.fill: parent
            initialItem : stackPage
        }

        Component{
            id:stackPage
            StackPage{}
        }
    }
}

StackPage.qml

import QtQuick 2.0

Item {

    Rectangle{
        anchors.centerIn: parent
        width: 200
        height: 200
        color: "orange"

        Text {
            id: cnt
            text: frmFirstClass.cntText
            verticalAlignment: Text.AlignVCenter
            horizontalAlignment: Text.AlignHCenter
            font.pointSize: 40
            anchors.fill: parent
        }

    }

}

jsulm

@bladekel said in How to update qml text from two different cpp ?:

firstClass frmFirstClass;

This is not going to work, as you're using different firstClass instance than the one which is really used (the one created in main()).
Pass the pointer to firstClass instance to secondClass constructor and use it then.

bladekel

@jsulm said in How to update qml text from two different cpp ?:

@bladekel said in How to update qml text from two different cpp ?:

firstClass frmFirstClass;

This is not going to work, as you're using different firstClass instance than the one which is really used (the one created in main()).
Pass the pointer to firstClass instance to secondClass constructor and use it then.

Can you please explain it ? because I dont know how to do it.....

J.Hilk

@bladekel
pleas read through this
http://www.cplusplus.com/doc/tutorial/classes/

to get a better understanding of what a class and a class instance is

bladekel

@J-Hilk said in How to update qml text from two different cpp ?:

@bladekel
pleas read through this
http://www.cplusplus.com/doc/tutorial/classes/

to get a better understanding of what a class and a class instance is

As your suggest, I have read it, and I think I understood what are they.

But still cant solve my problem...

Can you please suggest me something more....

J.Hilk

@bladekel
ok, now you'll understand that this

#include "secondclass.h"

firstClass frmFirstClass;

and this

QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    firstClass frmFirstClass;

are 2 different instances of firstClass.

The solution is what @jsulm posted
modify your 2ndclass constructor to accept, additionally to a parent a pointer to a firstClass instance.

You should read up on class constructors and pointers, if you're having trouble with this

https://www.tutorialspoint.com/cplusplus/cpp_constructor_destructor.htm
https://www.tutorialspoint.com/cplusplus/cpp_pointer_to_class.htm

bladekel

@J-Hilk said in How to update qml text from two different cpp ?:

@bladekel
ok, now you'll understand that this
#include "secondclass.h"

firstClass frmFirstClass;
and this
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    firstClass frmFirstClass;
are 2 different instances of firstClass.

The solution is what @jsulm posted
modify your 2ndclass constructor to accept, additionally to a parent a pointer to a firstClass instance.

You should read up on class constructors and pointers, if you're having trouble with this

https://www.tutorialspoint.com/cplusplus/cpp_constructor_destructor.htm
https://www.tutorialspoint.com/cplusplus/cpp_pointer_to_class.htm

So I think I understood the problem exactly.

When I did the things that you pointed, I have been creating two different first class so they can not communicate....

Am I right?

J.Hilk

@bladekel said in How to update qml text from two different cpp ?:

Am I right?

exactly, in the code you posted, firstClass exists 2 times. And those two instances do not now each other or interact with each other. They exist separately, like identical twins, operated at birth. One Lives with the mother, one with the father....

bladekel

@J-Hilk said in How to update qml text from two different cpp ?:

exactly, in the code you posted, firstClass exists 2 times. And those two instances do not now each other or interact with each other. They exist separately, like identical twins, operated at birth. One Lives with the mother, one with the father....

So what I have to do is I think, creating the instance with same adress.

jsulm

@bladekel said in How to update qml text from two different cpp ?:

creating the instance with same adress

No, simply pass the pointer to firstClass instance as additional parameter to secondClass constructor as already was suggested.

explicit secondClass(firstClass *frmFirstClass, QObject *parent = nullptr);

Important: Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

How to update qml text from two different cpp ?