[Qt-qml] Advanced layout support

[Adriano Rezende]

On Tue, Oct 18, 2011 at 6:20 AM,  <jens.bache-wiig at nokia.com> wrote:
>> span, shouldn't be there since it's just used by the Grid. So, maybe a
>> GridLayout would be better in this case, to avoid adding specific
>> properties in the Layout attachment.
>
> Yes, you are right about that. I think my proposal would only really with splitters and row/column layouts. So it might not be such a good idea after all.

We still need to provide min/max properties somehow. Consider the
following (very common) use case:

// Button.qml
Item {
    minimumWidth: label.implicitWidth + margin // won't work but still
needed by layouts
    ....
}

Follows the possible approaches to solve the above use case:

1. Add min/max support in QtQuick

// Button.qml
Item {
    minimumWidth: label.implicitWidth + margin
    ....
}

* Advantages:
- Clear and explicit API
- Native support without 3rd party extensions
- We could add min/max support for the QML anchoring system (like
QGraphicsAnchorLayout)

* Disadvantages:
- I can't see any

2. Provide a 'Widget' element in QtComponents

// Button.qml
Widget {
    minimumWidth: label.implicitWidth + margin
    ...
}

* Advantages
- Clear and explicit API

* Disadvantages
- Add an intermediary element just to handle min/max, increasing the
hierarchy tree.
- Just widgets could be added to RowLayout, ColumnLayout and
GridLayout, diverging from Row, Column and Grid idea.

3. Provide a common attached property in QtComponents

// Button.qml
Item {
    SizeHint.minimumWidth: label.implicitWidth + margin // a better
name could be provided (Layout, SizeHint, ...)
    ...
}

* Advantages
- Don't need to add another component
- Any element could be inserted in RowLayout, ColumnLayout and GridLayout

* Disadvantages
- Not so explicit API

4. Handle these information per layout

// Button.qml
Item {
    ...
}

RowLayout {
    Button { RowLayout.minimumWidth: 100; }
}

* Advantages
- Each element can provide specific properties using the same attached property
- Provide a more explicit API than the previous one (similar to
ListView attached property)

* Disadvantages
- Turn min/max implementation specific (layout specific)
- It would be impossible to address min/max to any layout


Personally, I prefer the first approach. But if an immediate API is
necessary, I would go with the 3rd approach.

Still, some layout specific information are still needed, and we could
handle that like the following:

RowLayout {
    Item {
        SizeHint.minimumWidth: 100
        RowLayout.stretchFactor: 2.0 // just layout specific properties
    }
    Button {
        RowLayout.spacing: 8 // just layout specific properties
    }
}


Any feedback regarding these 4 approaches are welcome.


Br,
Adriano