QML variable itegrated from C++ (property?)

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!