C++ data didn't update in Qml

dziko147

Hello guys ,
I have a frontend Qml , i want to display some data .
What i do is i create a backend.cpp class

#ifndef BACKEND_H
#define BACKEND_H

#include <QObject>
#include <QQmlEngine>

class Backend : public QObject
{
Q_OBJECT
Q_PROPERTY(QString directiontext READ getdirectiontext WRITE setdirectiontext NOTIFY directiontextChanged)
QML_ELEMENT

public:
explicit Backend(QObject *parent = nullptr);
QString getdirectiontext() {return m_directiontext;}
;}

}
//Setter methode //

void setdirectiontext(QString text);
signals:
void directiontextChanged();

private :
QString m_directiontext;

};

#endif // BACKEND_H```

//Backend.cpp
#include"backend.h"
#include<QDebug>
Backend::Backend(QObject *parent ) :QObject(parent)
{
m_directiontext="test direct";

void Backend::setdirectiontext(QString text)
{
qDebug("signal direction called");
if(m_directiontext == text)
return;
m_directiontext = text;
emit directiontextChanged();
}

 
I used a quickwidget to display Qml file 
here is my Mainwindow

qmlRegisterType<Backend>("com.beckend", 1, 0, "Myback");

m_ui->quickWidget->setResizeMode(QQuickWidget::SizeRootObjectToView);
m_ui->quickWidget->setSource(QUrl(QStringLiteral("qrc:/qml/automotive.qml")));```

and my qmlfile

import com.beckend 1.0
Myback{
    id:back
}
Label {
    id: label5
    x: 879
    y: 192
    text: back.directiontext
}

I have another class for change , so i create a command.cpp class and this is my code .

Backend back;
QString Mode= ui->modecommande->currentText(); // this is a combobox
qDebug() << Mode << "this is Mode select commande" ;
back.setdirectiontext(Mode);

when the combobox value change the data should change in my qmlfile .

Currently , i couldn't do this .

Any help please
thank you

KroMignon

@dziko147 said in C++ data didn't update in Qml:

when the combobox value change the data should change in my qmlfile .
Currently , i couldn't do this .

This cannot work as it is.

Backend back;
QString Mode= ui->modecommande->currentText(); // this is a combobox
qDebug() << Mode << "this is Mode select commande" ;
back.setdirectiontext(Mode);

This will create a local variable back updates m_directiontext and destroy the variable on function end.
Why should QML be aware about this?

import com.beckend 1.0
Myback{
    id:back
}

Will create a QML component back, there is no link with C++ side.

The right way is to create a global instance of Backend, and register it into QML context.

qmlRegisterType<Backend>("com.beckend", 1, 0, "Myback");

m_ui->quickWidget->setResizeMode(QQuickWidget::SizeRootObjectToView);
// supposing m_ui has a member `m_back`
m_ui->quickWidget->rootContext()->setContextProperty("backend", &m_back);
m_ui->quickWidget->setSource(QUrl(QStringLiteral("qrc:/qml/automotive.qml")));```

Use this reference in QML and then all changes on C++ will be available on QML side.

Label {
    id: label5
    x: 879
    y: 192
    text: backend.directiontext
}

dziko147

@KroMignon thanks for your reply .
I tried this solution and it doesn't work . :/

KroMignon

@dziko147 said in C++ data didn't update in Qml:

I tried this solution and it doesn't work . :/

Did you change also change the update method?

QString Mode= ui->modecommande->currentText(); // this is a combobox
qDebug() << Mode << "this is Mode select commande" ;
ui->m_back.setdirectiontext(Mode);

dziko147

This post is deleted!

KroMignon

@dziko147 said in C++ data didn't update in Qml:

@KroMignon No ,
Can you suggest a method ?

I don't understand what you mean, please be more clear.
No what?

No you don't have change the update method?
After changing the update method, the QML didn't be update?

dziko147

@KroMignon the update method is in another class .
So how can get this m_back :)
i am really new in Qt developpment .
So excuse meif my question is stupid :)

dziko147

This post is deleted!

KroMignon

@dziko147 said in C++ data didn't update in Qml:

the update method is in another class .
So how can get this m_back :)
i am really new in Qt developpment .
So excuse meif my question is stupid :)

I don't want to hurt you, but this has nothing to do with Qt, this is C++ basic.

Do you have any knowledge about C++ programming?
Do you know what is a class?
Do you know what is a class method or member?

Please take time to got basic knowledge about the programming language you want to use.
There are many C++ tutorials available, take one and do at least the first step to understand what you are doing.

Basic C++ knowledge is mandatory if you want to program with Qt which is a C++ framework.

dziko147

@KroMignon ok i will :)
but can you provide a way to create this m_back instance :)

KroMignon

@dziko147 said in C++ data didn't update in Qml:

but can you provide a way to create this m_back instance :)

https://www.w3schools.com/cpp/cpp_classes.asp

dziko147

@KroMignon I know how to use class/object .
I know how to manipulate c++ .
But my problem is how can get this in my second class

m_ui->quickWidget->rootContext()->setContextProperty("backend", &m_back);

KroMignon

@dziko147 said in C++ data didn't update in Qml:

But my problem is how can get this in my second class

I don't understand which second class you are talking about?

dziko147

This post is deleted!

KroMignon

@dziko147 said in C++ data didn't update in Qml:

So my problem is how can i get the m_back in the commande.cpp

Again, this is C++ basic => variable scope and life-cycle: