[Lazarus] Autosize behaviour

[Mattias Gaertner]

On Mon, 5 Apr 2010 18:26:36 +0200 (CEST)
Michael Van Canneyt <michael at freepascal.org> wrote:

>[...]
> I have been thinking about layout managers. I think that this should be an add-on to the currently existing layouting (to preserve delphi compatibility): 
> I imagine a component that one drops on a form. 
> One sets the 'target' control (control whose children should be managed) and some properties.
> 
> Depending on the layout one wants, a different component must be dropped:
> 
> TLayout
> +-  TAbsoluteLayout
> +-  THBoxLayout
> +-  TVBoxLayout
> +-  TTableLayout
> +-  TBorderLayout
>    ...
> Each descendent has different properties. Changing the layout just means
> dropping another component; something that can be done run-time.
> 
> Something like this exists in ExtJS, and it works very well.
> 
> The only thing I am unsure about is whether TControl should be made TLayout-aware.
> As far as I understand, a layouter only needs to be able to handle rectangles,
> within some restrictions (some callbacks to get min/max rectangles). 
> It need not know anything about TControl or it's properties, and as such could
> be implemented totally independent of the LCL.

See below.
 
> I discussed this idea once with Mattias Gaertner, but he seems to disagree
> on the TControl-unawareness, if I recall correctly.

With the new autosizing it is now possible to think about layout
managers.
A layout manager that handles the layout of children of a TWinControl
must implement the following:
- a computation of the total preferred space (children plus
  space). At the moment this is done by TControl.CalculatePreferredSize.
- a computation of the layout (position and size of each child)
  (AlignControls).
- a computation of the size (DoAutoSize)

The layouter needs access to Left, Top, Width, Height, ClientWidth,
ClientHeight, ReadBounds, BaseBounds, BaseParentClientSize,
AdjustSize, InvalidatePreferredSize.

Optionally a layouter could layout only a subset of the children (e.g.
those with Align=alCustom or only those with default Anchors).

So the LCL must call instead of its own functions the
layouter. Namely: DoAutoSize, AlignControl, CalculatePreferredSize,
ControlsAligned, CreateControlAlignList, CustomAlignInsertBefore,
CustomAlignPosition, DoAlignChildControls.

One solution would be a TWinControl property Layouter.

 
> Maybe the planned Lazarus day in the netherlands would be a good place to
> discuss this and settle things  once and for all :-)

:)

Mattias