Cascading properties

[Mark81]

To improve readability of my code I want to bind the value of a property to another.
For example, in QML I define:

property real _voltage

Component.onCompleted() {
    _voltage = MyApp.data["123"]
}

MyApp class handles data as follow:

public:
    Q_PROPERTY(QVariantMap data READ data NOTIFY dataChanged)
    QVariantMap data() const { return _mapData; }

private:
    QVariantMap _mapData;

signals:
    void dataChanged();

whenever I change the value of an item of _mapData I emit the dataChanged() signal.
In QML if I bind something to the MyApp.data() function it works fine (i.e. the text property is updated correctly) but if I use _voltage it doesn't update.

I mean: MyApp.data["123"] updates according to the actual value in MyApp while voltage keep the value it had when I assigned it (_voltage = MyApp.data["123"]).

Instead I expect it should update too because it's bound to MyApp.data["123"]. Is there a way to do what I want?

[sierdzio]

@Mark81 said in Cascading properties:

_voltage = MyApp.data["123"]

That's an assignment (imperative), not a binding.

This is a declarative assignment (binding):

property real _voltage: MyApp.data

If you need a more advanced binding, use Binding component.

[Mark81]

@sierdzio Oh thank you. I admit I didn't get the difference! I thought it was the very same thing, just at different time.

[sierdzio]

@Mark81 said in Cascading properties:

@sierdzio Oh thank you. I admit I didn't get the difference! I thought it was the very same thing, just at different time.

Here is how to remember it easily:

• a = b assigns value of b to a at that exact moment when it is called (imperative assignment, same as in C++ or JS)
• a: b updates a when b changes (declarative assignment)