How to get valid class instance after doing a qmlRegisterType?

Nelson_Piquet

@Wieland I am new to Qt. So I could be wrong in saying that qmlRegisterType instantiates anything

? Offline

@Nelson_Piquet How do you plan to use MyClass? Do you want to create instances from it in QML or do you only need a single MyClass object that is used from C++ and QML?

Nelson_Piquet

@Wieland I need a single MyClass object that is used from C++ and QML. My complete application will have only one instance of MyClass.

? Offline

Ok. Let's say this is MyClass:

#ifndef MYCLASS_H
#define MYCLASS_H

#include <QObject>
#include <QString>

class MyClass : public QObject
{
    Q_OBJECT
public:
    explicit MyClass(QObject *parent = 0);
    Q_INVOKABLE QString sayHello() const;
};

#endif // MYCLASS_H

#include "myclass.h"

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

QString MyClass::sayHello() const
{
    return "Hello!";
}

And this is main.qml:

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

import com.mycompany.myapplication 1.0

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

    Text {
        anchors.centerIn: parent
        text: myClass.sayHello()
    }
}

In main.cpp just do this:

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

#include "myclass.h"

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);
    qmlRegisterType<MyClass>("com.mycompany.myapplication", 1, 0, "MyClass");
    QQmlApplicationEngine engine;
    MyClass myClass;
    engine.rootContext()->setContextProperty("myClass", &myClass);
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));
    return app.exec();
}

Nelson_Piquet

@Wieland Thanks for such a vivid reply. if I have a method in MyClass where I want to set some data, then is following the correct point to set it ?

int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
qmlRegisterType<MyClass>("com.mycompany.myapplication", 1, 0, "MyClass");
QQmlApplicationEngine engine;
MyClass myClass;
myClass.setProperty("something"): //Is this the correct place to set it ?
engine.rootContext()->setContextProperty("myClass", &myClass);
engine.load(QUrl(QLatin1String("qrc:/main.qml")));
return app.exec();
}

? Offline

@Nelson_Piquet Yes, that's totally fine.

p3c0

@Nelson_Piquet To add to @Wieland's answer you don't have to register MyClass with qmlRegisterType as you already have access to MyClass in QML as a context property. Ofcourse unless you don't want to instantiate it as a QML component.

JosephMills

@Wieland I need a single MyClass object that is used from C++ and QML. My complete application will have only one instance of MyClass.

I would use qmlRegisterSingletonType and just register that once and that way it is only created once on your application.

for more on this and instantiation of this see docs here Here

for future if someone runs across this it is in the qqmlengine docs as of 5.8

As a side note .... This makes not sense to me at all.

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

#include "myclass.h"
int main(int argc, char *argv[])
{
 
   // At this point you are passing a object that makes a instance of your class
   // To be used in QML via qmlRegisterType 
   
  //  Example :
  //   MyClass{id: myclass }   
  //   Text{text: myclass.sayHello();}
  
   qmlRegisterType<MyClass>("com.mycompany.myapplication", 1, 0, "MyClass");


   // But than You are doing the same thing again just using up more memory.
   // IE this is not needed at all 
   MyClass myClass;
   engine.rootContext()->setContextProperty("myClass", &myClass);
}

So this is how I would do this

#ifndef MYCLASS_H
#define MYCLASS_H

#include <QObject>
#include <QString>

class MyClass : public QObject
{
    Q_OBJECT
public:
    explicit MyClass(QObject *parent = 0);
    Q_INVOKABLE QString sayHello() const;
};

#endif // MYCLASS_H
#include "myclass.h"

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

QString MyClass::sayHello() const
{
    return "Hello!";
}

In Qml

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

import com.mycompany.myapplication 1.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480

    Text {
        anchors.centerIn: parent
        text: MyClass.sayHello()
    }
}

Wrap it all up as a single instance.

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

#include "myclass.h"

static QObject *mySingleObject(QQmlEngine *engine, QJSEngine *scriptEngine)
{
    Q_UNUSED(engine)
    Q_UNUSED(scriptEngine)

    MyClass *myClass = new MyClass();
    return myClass;
}

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);
    qmlRegisterSingletonType<MyClass>("com.mycompany.myapplication", 1, 0, "MyClass" , mySingleObject);
    QQmlApplicationEngine engine;
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));
    return app.exec();
}

kshegunov

@JosephMills said in How to get valid class instance after doing a qmlRegisterType?:

  // At this point you are passing a object that makes a instance of your class
  // To be used in QML via qmlRegisterType 
   
  //  Example :
  //   MyClass{id: myclass }   
  //   Text{text: myclass.sayHello();}
  
   qmlRegisterType<MyClass>("com.mycompany.myapplication", 1, 0, "MyClass");

You're wrong. qmlRegisterType registers the type (i.e. class) in the QML engine. It creates no objects by itself. It makes it possible that you're able to instantiate objects of that type:

import com.mycompany.myapplication 1.0

MyClass {    //< This creates an object (instance of MyClass) and consumes memory
}

JosephMills

@kshegunov That is exactly what I said

Not sure how to make quote on the forum yet but ,

  // At this point you are passing a object that makes a instance of your class
  // To be used in QML via qmlRegisterType 
   
  //  ***Example:***
  //   MyClass{id: myclass }

Pardon if I was not clear enough. I will try to be more clear.

kshegunov

Nope, it's my fault, sorry. I need more coffee, as I am apparently still sleeping.

Nelson_Piquet

@JosephMills @Wieland @kshegunov I am trying around the suggestions. This is extremely good info around my question.

I'd said in my question that MyClass is derived from QQuickItem Looks as below

class MyClass : public QQuickItem
{
Q_OBJECT
public:
MyClass();
virtual ~ MyClass();
signals:
void visibilityChanged(bool visibility);
public Q_SLOTS:
void handleVisibilityChanged(bool visibility);
};

Do you guys mean that I should derive it from QObject instead of QQuickItem ? Or do solutions you guys provide work with QQuickItem derived classes as well ?

JosephMills

@Nelson_Piquet If you are going to make a Object or a Item I guess that is the question.

Example do you need the polymorphism of QQuickItem ? Like anchors ,opacity , x and y and all that that comes with QQuickItem. Or Do you need it as just a QObject ? This really depends on how you are designing the class that you are writing. Please note that QQuickItem Inherits: QObject and QQmlParserStatus. So maybe to move forward you can tell us a little bit more about the class that you are trying to expose to QML.

Things like is it all methods ? or only used to gather info ? Does it paint anything or take in other QQuickItems as Q_PROPERTY ect. Please also note that QQuickItem has visible that you have access to already.

Example:

void handleVisibilityChanged(const bool &visibility)
{
         if(isVisible() )
         {
                    setVisible(false);
         }
         else 
         {
                    setVisible(true);
         }  
}

As you can see I made it a const and also made it so that it is dereference. I am also wondering about your signal and why there is a value in it.

Cheers,
Joseph Mills

Nelson_Piquet

I really need my class to be derived from QQuickItem as I use the anchors ,opacity , x, y & much more. Its required to be displayed specifically on a wide range of mobile devices. This class is basically used to paint video data on to the screen with some open gl processing. It has a lot of methods & code to support that. All I need is the access to the object that in my main.cpp. The suggestions are really great but would have been best if the samples provided were with a class derived from QQuickItem. I am going to try out your suggestions anyways keeping MyClass derived from QQuickItem.

Shanna951

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