[Interest] QML: Considerations on an interesting design pattern that relies on JS engine's garbage collection

[Ulf Hermann]

Hi,

> ```
> Flickable {
>    id: root
>    // ...
>    property Component defaultScrollBar: Component {
>      ScrollBar { }
>    }
> 
>    ScrollBar.vertical: {
>      if (condition && root.defaultScrollBar) {
>        return root.defaultScrollBar.createObject() // a parent is not 
> passed on purpose
>      }
>      return null
>    }
> ```

This works in the sense that your ScrollBar object is garbage-collected 
when overridden. This is because the ScrollBar attached object 
un-parents it in that case and the QML engine doesn't consider it 
undeletable. However, if you create a new Component object in every 
derived type you probably haven't saved a lot of memory (but I haven't 
measured it).

What we should actually do is expose deferred properties to QML. In C++ 
you can already have deferred properties, like the background of Label. 
The default values for those are only created if they haven't been 
overridden.

best,
Ulf