Editing C++ properties from QML with updates from C++

swegmann

I currently try to implement a custom QML control that allows me to change values of C++ object properties but also is updated when the C++ object property changes on its own. I wonder if there really is no other way to do this.

So this is a working solution that I already have:

MyClass.h

Q_PROPERTY(int myCppValue READ myCppValue WRITE setMyCppValue NOTIFY myCppValueChanged)

MyCustomControl.qml

Item {
    property int value

    MouseArea {
        anchors.fill: parent
        onClicked: value = value + 1
    }
}

And now I instantiate this control as follows while assuming a MyClass instance is available as context property with the name "myclass":

MyCustomControl {
    id: my_control
    value: myclass.myCppValue

    /* from here things get messy */
    Binding {
        target: myclass
        property: "myCppValue"
        value: my_control.value
    }

    Binding {
        target: my_control
        property: "value"
        value: myclass.myCppValue
    }
}

My problem seems to be, that I cannot create an alias property in MyCustomControl that actually reads and modifies the value myclass.myCppValue. As soon as MyCustomControl assigns a new value to "value" I loose my connection to the myclass property. So the only way I could circumvent this problem is to create two bindings that connect these two values and just assign myclass.myCppValue as property value of my_control to get a nice initial value.

Is this the best you can do?

richardo

You could go the more imperative road, because now you have kind of endless update cycle:

    Connections {
            target: myclass
            onMyCppValueChanged: if (my_control.value !== myclass.myCppValue) my_control.value = myclass.myCppValue
    }

    Connections {
            target: my_control
            onValueChanged: if (my_control.value !== myclass.myCppValue) myclass.myCppValue = my_control.value
    }

Then you would not have the cyclic update calles, that might lead to a problem.

swegmann

@richardo I appreciate your input. Still I'm not sure I really like this approach that much better.

As for the cycle, I couldn't see any of those usual cyclic property warnings that QML gives you in those cases. I think this cycle is already properly broken by using the correct construction in setMyCppValue:

void setMyCppValue(int newValue)
{
    if (newValue!=myCppValue)
    {
        myCppValue = newValue;
        emit myCppValueChanged();
    }
}