QML switch not working as expected with bind c++ property

bludger

Hi all,

Suppose I've a c++ object (derived from QObject) which exposes a property using the Q_PROPERTY macro. Whenever I bind this property to the checked property of a QML switch it reacts properly to the notify signal (changes from c++ code are properly propagated). But when I change the checked state of the switch by clicking on it, it will not call the respective set function of the bind property.

Some code to reproduce it:

CppObject.h

class CppObject : public QObject
{
    Q_OBJECT
    Q_PROPERTY(bool active READ active WRITE setActive NOTIFY activeChanged)

public:
    explicit CppObject(QObject *parent = nullptr) {}

    bool active() const { return m_active; }
    void setActive(bool active) { if (m_active != active) { m_active = active; emit activeChanged(); } }

signals:
    void activeChanged();

private:
    bool m_active;

};

Part of main.ccp

    CppObject obj;
    engine.rootContext()->setContextProperty("cppObject", &obj);

    engine.load(QUrl(QLatin1String("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();

main.qml

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

    Switch {
        id: switch2
        checked: cppObject.active
    }
}

I'm using Qt 5.9.1 and QtQuick.Controls 2.2. Does someone experience the same behaviour?

jeremy_k

Bindings are not bidirectional. If you want switch2.checked =>cppObject.active as well as cppObject.active => switch2.checked, the connection needs to be explicitly stated.

bludger

Thanks for the clarification @jeremy_k. I was always under the impression that property binding was bidirectional, mainly due to the Q_PROPERTY read and write parameters/functions. I think however that it is a bit lame that it can automatically read values by the getter but lacks the setter functionality.

jeremy_k

@bludger said in QML switch not working as expected with bind c++ property:

I think however that it is a bit lame that it can automatically read values by the getter but lacks the setter functionality.

The setter isn't missing. The program[mer] has to tell the runtime what to connect the input of that setter to. It's no less automatic than the getter, which also requires an explicit reference to the property.