From Lazarus-ccr

[edit] Overview / Terminology

This page explains the LCL auto sizing algorithm. AutoSizing means here: Automatic resizing and repositioning of LCL controls.

If you are merely interested in how to use the AutoSize properties, see Autosize_/_Layout.

[edit] Properties

The following properties defines the behavior of the LCL autosizing:

• Current Left,Top,Width,Height,ClientWidth,ClientHeight
• Loaded Left,Top,Width,Height,ClientWidth,ClientHeight
• AutoSize
• Anchors
• AnchorSides
• Align
• BorderSpacing
• ChildSizing
• Constraints

For Delphi compatibility the LCL supports the method AdjustClientRect which is an extension to the ChildSizing.LeftRightSpacing/TopBottomSpacing properties.

[edit] Order

[edit] Overview

Basically AutoSizing works in this order:

• The constraints are applied to every step.
• First comes the Align in order alTop, alBottom, alLeft, alRight and finally alClient. For example alLeft has 3 aligned sides and one free side. The free side is handled by the below rules. BorderSpacing is applied to aligned sides. alCustom and alNone have no aligned sides.
• Next comes ChildSizing. If ChildSizing.Layout is not cclNone then all child controls with Align=alNone, Anchors=[akLeft,aklRight] and no AnchorSide controls are resized/positioned. These child controls are called on this page "non aligned controls".
• Next comes AutoSize. If ChildSizing.Layout is cclNone then all "non aligned controls" are moved to the top, left and the control is shrinked/enlarged to fit around all its childs (not only the "non aligned controls").
• Next comes the Anchors. Only those sides are anchored, which were not handled by Align (e.g. akRight on alLeft). If a AnchorSide control is set the side will be aligned to the side of the other control. Otherwise the default anchor rules apply, which are mostly similar to the Delphi ones.

[edit] Algorithm

The main method is TWinControl.AlignControls.

1. init RemainingClientRect to ClientRect and RemainingBorderSpace to Rect(0,0,0,0)
2. call AdjustClient to adjust RemainingClientRect
3. apply ChildSizing.LeftRightSpacing,ChildSizing.TopBottomSpacing
4. call DoAlign with alTop,alBottom,alLeft,alRight,alClient,alCustom,alNone
   1. DoAlign collects child controls with this Align value and call for each such control DoPosition
      1. DoPosition calculates the new Left,Top,Width,Height

[edit] Differences to Delphi

• Delphi has no BorderSpacing, AnchorSides, ChildSizing.
• Delphi does not support AutoSize and Align on all controls.
• Delphi's AutoSize algorithm does not support nested autosized controls with the Align property. For example a Panel1 with Align=alTop in a Panel2 and both panels have AutoSize=true. Panel2 will not shrink/enlarge horizontally, which means Panel1 will not shrink/enlarge neither. So, Panel1 is not autosized to the needed size for its childs.

[edit] FAQ

see Autosize_/_Layout.