[Development] Namespacing QML context properties

[Alberto Mardegan]

On 12/09/2013 12:54 AM, Alan Alpert wrote:
> On Sun, Dec 8, 2013 at 5:38 AM, Alberto Mardegan
> <mardy at users.sourceforge.net> wrote:
[...]
> What do you mean "current context"? The problem you seem to be hitting
> is that there is another symbol in the current context that you want
> to avoid, so that it finds a symbol higher up the tree. That would
> mean something like engine.rootContext(), but I don't think that would
> make sense in QML.

I don't really want the engine.rootContext(), but actually
engine.contextForObject(this). Please have a look at this example, and
suppose that I want to use this as a delegate in a ListView, in order to
provide a different delegate for each element in the list, depending on
some property in the model:

Item {
    ListView {
        model: ...
        delegate: loader
    }
    Component {
        id: delegate1
        Rectangle { color: model.color; ... }
    }
    Component {
        id: delegate2
        Text { text: model.text; ... }
    }
    Loader {
        id: loader
        // property variant model: model
        sourceComponent: model.someProperty ? delegate1 : delegate2
    }
}

Now, if I don't remove the comment, the actual delegates loaded by the
Loader (delegate1 and delegate2) won't see the "model" property; but if
I uncomment the line, it won't work either, because inside the Loader's
definition the property named "model" will obscure the context property
having the same name.
How would you solve this (without of course renaming variables or moving
components around: suppose that I have no control over the listview or
delegates' code)?

And another simpler case, this:

Item {
    ListView {
        model: ...
        delegate: ComboBox {
            text: model.displayName
        }
    }
}

And suppose that a certain point the authors of the ComboBox element
change it by adding a "model" property to it. That would break the code
above, because "model" wouldn't refer anymore to the "model" injected by
the ListView, but to the new ComboBox property!
But if you had a way to select your local context with some prefix, no
breakage could ever happen:

Item {
    ListView {
        model: ...
        delegate: ComboBox {
            text: qmlContext.model.displayName
        }
    }
}

I hope that the question is now clearer. :-)

Ciao,
  Alberto

-- 
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