How to update qml text from two different cpp? - qt

Question

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
}
}
}

Answer 1

Your extern usage is incomplete, so you are effectively creating two instances of firstClass. As extern is a bit misleading at poitns, I suggest you to give a pointer to the "one-and-only" firstClass when creating secondClass (in main). This also gives the code a better representation of intended hierarchy.
Update the following files:
secondClass.h
#include "firstClass.h"
class secondClass : public QObject
{
Q_OBJECT
private:
public:
explicit secondClass(firstClass *theFirstClass, QObject *parent = nullptr);
signals:
public slots:
private slots:
void setter2();
private:
firstClass *myFirstClass_ = nullptr;
};
secondClass.cpp
#include "secondclass.h"
secondClass::secondClass(firstClass *theFirstClass, QObject *parent)
: QObject(parent)
, myFirstClass(theFirstClass)
{
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()
{
myFirstClass->setCntText(0);
qDebug() << "Checking m_cntText = " << myFirstClass->m_cntText;
}
main.cpp
int main(int argc, char *argv[])
{
...
firstClass frmFirstClass;
engine.rootContext()->setContextProperty("frmFirstClass", &frmFirstClass);
secondClass frmSecondClass(&frmFirstClass);
...
}
Disclaimer: I did not try to compile this