QML variable itegrated from C++ (property?)

[shokarta]

Hello gurus,

I have already asked similar question, but because i was too lost, with options, better would be starting newone because after reframing the basic idea, i guess the proposed solution would be way easier...

so lets say I have this in QML:

property bool someVariable

and via QML Timer, onTick i update the variable from c++ function:

onTriggered: { someVariable = cpp.variableRefresh(); }

in c++, i have the class registered:

qmlRegisterType<CPP>("CPP", 1, 0, "CPP");

and the boolean in the CPP class is:

bool variableRefresh() { return true; }

So as I can asume this is very silly way to update someVariable from QML via C++,
but if I wish to go otherway around, something like:

void variableRefresh() { QML.someVariable = true; }

Is there a way to do it?
Can setContextProperty() or setProperty() be used?
I have tried to google this all day long, but at the end of the day I have to ask because it made me even more confused...

[ODБOï]

@shokarta said in QML variable itegrated from C++ (property?):

Im creating a c++ instance of MSSQL class,
so basicaly, I declare the MSSQL class from main.cpp via:
qmlRegisterType<MSSQL>("MSSQL", 1, 0, "MSSQL");

you are Registering a new QML type

@shokarta said in QML variable itegrated from C++ (property?):

ApplicationWindow {
id: mainWindow
property bool dbOnline

MSSQL { id: sql }

and here you create an instance of your object in QML.

you can simply do like this

ApplicationWindow {
    id: mainWindow
    
   property bool dbOnline : sql.isOnline // < bind your property  dbOnline  to Qproperty of your object

	MSSQL { id: sql }

    Timer {
        id: dbRefresh
        interval: 30000
        running: true
        repeat: true
        onTriggered: { dbOnline=sql.checkConnection(); }
    }
}

and in checkConnection method you assign the qpoperty

void MSSQL::checkConnection(){
...
  setIsOnline(true or false )
}

[sierdzio]

All QML components are QObjects. You can get the root QObject of your QML scene, find your desired QML object, and manipulate it via setProperty() as you wish.

However, in most cases, it is completely unnecessary to do so.

If you have a property or variable which changes from time to time, make it into a property (Q_PROPERTY with NOTIFY signal). Expose that object to QML and react to it changing (onYourSignalName handler). This way you:

• don't need a timer at all
• change the value in QML only when it actually changes (big performance gain!), instead of calling the same method via meta object system periodically

[shokarta]

@sierdzio said in QML variable itegrated from C++ (property?):

make it into a property (Q_PROPERTY with NOTIFY signal). Expose that object to QML and react to it changing (onYourSignalName handler)

Can I kindly ask you to show me code how can I do this?
This will be the first time I would do so, so I wish to learn how to do this properly, this would be an extreme gain for my practice and I can see the benefit of modifying my apps this way

[jsulm]

@shokarta Easy to find using Google: https://doc.qt.io/qt-5/properties.html

[sierdzio]

Here's a high level overview: docs.

[shokarta]

im sorry i just dont understand how can i implement that whatsoever.
According to https://stackoverflow.com/questions/27584803/how-to-set-qml-properties-from-c
it seems simple to implement this, but only from main.cpp from main().
But when I need to change the property from my custom class, then im just lost...

is it too much to ask for example based on my scenario?
That way I can understand the better then just blindly keep trying something which makes no sence to me :(

main.cpp

#include <QQmlApplicationEngine>
#include <QGuiApplication>
#include <QtQml>
#include <QCoreApplication>
#include <QtCore>
#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QtSql>
#include <QtDebug>
#include <QTextStream>
#include "mssql.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    qmlRegisterType<MSSQL>("MSSQL", 1, 0, "MSSQL");

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

mssql.h:

#ifndef MSSQL_H
#define MSSQL_H

#include <QObject>
#include <QSqlDatabase>

class MSSQL : public QObject
{
    Q_OBJECT

public:
    explicit MSSQL(QObject *parent = nullptr);
    Q_INVOKABLE bool checkConnection(bool skipRefresh = false);

private:

signals:

public slots:

};

#endif // MSSQL_H

mssql.cpp:

#include <QtSql>
#include <QSqlQuery>
#include <QObject>
#include <QSqlDatabase>
#include "main.cpp"
#include <QSettings>
#include <QtDebug>
#include "mssql.h"
#include <QQmlComponent>


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

bool MSSQL::checkConnection(bool skipRefresh)
{
	//here I have to modify the QML property mainWindow.dbOnline = true;
}

main.qml:

import QtQuick 2.12
import QtQuick.Controls 2.0
import MSSQL 1.0

ApplicationWindow {
    id: mainWindow
    property bool dbOnline //this to be updated from c++ checkConnection()
    ....
}

[ODБOï]

@shokarta

#ifndef MSSQL_H
#define MSSQL_H

#include <QObject>

class MSSQL : public QObject
{
    Q_OBJECT
    Q_PROPERTY(bool isOnline READ isOnline WRITE setIsOnline NOTIFY isOnlineCHanged)
public:
    explicit MSSQL(QObject *parent = nullptr);

    bool isOnline()const{
        return m_isOnline;
    }
    void setIsOnline(const bool &onLine){
        if(m_isOnline==onLine)return;
        m_isOnline = onLine;
        emit isOnlineCHanged(/*m_isOnline*/);
    }

signals:
    void isOnlineCHanged(/*bool*/);

private :
    bool m_isOnline=false;
};

#endif // MSSQL_H

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include "mssql.h"

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

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    MSSQL myDb;

    engine.rootContext()->setContextProperty("myDatabase",&myDb);

    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();
}

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12
Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    property bool dbOnline : myDatabase.isOnline

    onDbOnlineChanged: console.log("database is", dbOnline ? "online" : "offline")

    Button{
        text: "test"
        onClicked: myDatabase.isOnline = !myDatabase.isOnline
    }
}

[shokarta]

Hello, and thanks a lot!

mostly I do understand it quite alright, however what I do not understand is:

engine.rootContext()->setContextProperty("myDatabase",&myDb);

• this obviously throws error: use of undeclared identifier 'myDb'

and also still dont understand how will change the QML variable from mssql.cpp, speficifly from this bool/void:

bool MSSQL::checkConnection(bool skipRefresh)
{
	//here I have to modify the QML property mainWindow.dbOnline = true;
}

• because here i have some code, which returns true/false because i update the value like dbOnline=MSSQL::checkConnection(),
• but if i change it to be void, then anytime i wish to update the dbOnline, i can do it directly from this c++ function

[ODБOï]

@shokarta said in QML variable itegrated from C++ (property?):

engine.rootContext()->setContextProperty("myDatabase",&myDb);

this obviously throws error: use of undeclared identifier 'myDb'

How and where do you create object from MSSQL class ?

@shokarta said in QML variable itegrated from C++ (property?):

and also still dont understand how will change the QML variable from mssql.cpp, speficifly from this bool/void:

use the setter method (WRITE)
Q_PROPERTY(bool isOnline READ isOnline WRITE setIsOnline NOTIFY isOnlineCHanged)
setIsOnline(yourValue)
then
isOnlineCHanged signal will be emitted (NOTIFY )
because setIsOnline is defined like this

void setIsOnline(const bool &onLine){
        if(m_isOnline==onLine)return;
        m_isOnline = onLine;
        emit isOnlineCHanged(/*m_isOnline*/);
    }

then qml will be updated automatically
property bool dbOnline : myDatabase.isOnline

reading the documentation linked by jsulm and sierdzio might help

[shokarta]

@LeLev said in QML variable itegrated from C++ (property?):

How and where do you create object from MSSQL class ?

Im creating a c++ instance of MSSQL class,

so basicaly, I declare the MSSQL class from main.cpp via:

qmlRegisterType<MSSQL>("MSSQL", 1, 0, "MSSQL");

then in QML I load it via:

MSSQL { id: sql }

and from QML I declare then variable:

property bool dbOnline

which im updating via:

dbOnline = sql.checkConnection();

to summary, the whole code is:
main.cpp:

#include <QQmlApplicationEngine>
#include <QGuiApplication>
#include <QtQml>
#include <QCoreApplication>
#include <QtCore>
#include <QtSql>
#include <QtDebug>
#include "mssql.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    qmlRegisterType<MSSQL>("MSSQL", 1, 0, "MSSQL");

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    engine.load(url);

    return app.exec();
}

main.qml:

import QtQuick 2.12
import QtQuick.Controls 2.0
import MSSQL 1.0

ApplicationWindow {
    id: mainWindow
    property bool dbOnline

	MSSQL { id: sql }

    Timer {
        id: dbRefresh
        interval: 30000
        running: true
        repeat: true
        onTriggered: { dbOnline=sql.checkConnection(); }
    }
}

mssql.h:

#ifndef MSSQL_H
#define MSSQL_H

#include <QObject>
#include <QSqlDatabase>

class MSSQL : public QObject
{
    Q_OBJECT

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

    Q_INVOKABLE bool checkConnection();

private:

signals:

public slots:

};

#endif // MSSQL_H

mssql.cpp:

#include <QtSql>
#include <QSqlQuery>
#include <QSqlDatabase>
#include "main.cpp"
#include "mssql.h"

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

bool MSSQL::checkConnection()
{
    if(something)
    {
        // here I need to change the QML variable dbOnline to: true
        return true;
    }
    else {
        // here I need to change the QML variable dbOnline to: false
        return false;
    }
}

[ODБOï]

@shokarta said in QML variable itegrated from C++ (property?):

Im creating a c++ instance of MSSQL class,
so basicaly, I declare the MSSQL class from main.cpp via:
qmlRegisterType<MSSQL>("MSSQL", 1, 0, "MSSQL");

you are Registering a new QML type

@shokarta said in QML variable itegrated from C++ (property?):

ApplicationWindow {
id: mainWindow
property bool dbOnline

MSSQL { id: sql }

and here you create an instance of your object in QML.

you can simply do like this

ApplicationWindow {
    id: mainWindow
    
   property bool dbOnline : sql.isOnline // < bind your property  dbOnline  to Qproperty of your object

	MSSQL { id: sql }

    Timer {
        id: dbRefresh
        interval: 30000
        running: true
        repeat: true
        onTriggered: { dbOnline=sql.checkConnection(); }
    }
}

and in checkConnection method you assign the qpoperty

void MSSQL::checkConnection(){
...
  setIsOnline(true or false )
}

[shokarta]

@LeLev you are the best!