Creating a C++ identifier for a dynamically generated object

Snorlax

This post is deleted!

lemons

You could do something like this:

class BackEnd : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString userName READ userName WRITE setUserName NOTIFY userNameChanged)

public:

//main.cpp
#include <QQmlContext>

//...

QQmlApplicationEngine engine;

BackEnd myBackend;
engine.rootContext()->setContextProperty("BACKEND",  &myBackend);

//main.qml - quick and dirty example
import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12
import QtQuick.Layouts 1.12

Window {
    id: app
    visible: true
    width: 640
    height: 480

    ColumnLayout {
        RowLayout {

            Column {
                spacing: 10
                Text {
                    font.bold: true
                    text: "Value from C++ backend:"
                }

                Text {
                    // automatically re-evaluates based on the Q_PROPERTY NOTIFY signal
                    text: BACKEND.userName
                }
            }
            Rectangle {
                border.width: 2
                border.color: "green"
                width: childrenRect.width
                height: childrenRect.height
                TextField {
                    id: unserNameInput
                    padding: 10
                    color: "black"
                    placeholderText: "Enter Username"
                }
            }
        }
        Button {
            Layout.alignment: Qt.AlignHCenter
            padding: 20
            text: "SAVE"
            onClicked: {
                // uses the Q_PROPERTY WRITE method
                BACKEND.userName = unserNameInput.text
            }
        }
    }

    // in case you need any additional signal handling
    Connections {
        target: BACKEND
        function onUserNameChanged() {
            console.debug("BackEnd signal:", "userNameChanged")
        }
    }
}

fcarney

@lemons said in Creating a C++ identifier for a dynamically generated object:

engine.rootContext()->setContextProperty("BACKEND", &myBackend);

It is a little off putting that your "backend" is yelling at you. ;-)
Good example, btw.

lemons

@fcarney yeah, I am somehow used to capitalize the context properties :D
for me, this makes the origin more clear, so I can better differentiate in the auto-completion menu...

Snorlax

This post is deleted!

lemons

@Snorlax you have to set the context property before loading the initial url in main.cpp

int main(int argc, char *argv[])
{
    // ...
    QApplication app(argc, argv);
    // ...
    QQmlApplicationEngine engine;
    // ...
    BackEnd myBackend;
    engine.rootContext()->setContextProperty("BACKEND",  &myBackend);
    // ...
    // context property used in connection must be set before this line
    engine.load(url);
    // ...
    return app.exec();
}

Snorlax

This post is deleted!

lemons

@Snorlax the signal is only triggered when the signal from the c++ file is emitted.
Therefore, you have to enter something in the input field and click the save button.

As the example returns if you set the same name again, it won't emit a signal if you click the save button multiple times without changing the input field value:

void BackEnd::setUserName(const QString &userName)
{
    // do not proceed if current name is equal to new name
    if (userName == m_userName)
        return;
    
    m_userName = userName;
    emit userNameChanged();
}

Snorlax

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