[Gtk2] About bug 11523: analysis and proper fix(es)

[Luiz Americo Pereira Camara]

The problem occurs because each device context instance has a text cache 
while the layout is shared between all device contexts that have the 
same font.

In TextOutCacheGtk2.zip there's a project that helps to show the problem.

Considering that "Single Line" option is set:
Str1 is draw to Bitmap1.Canvas:
 - The text cache of Bitmap1.Canvas is empty and is set to Str1 data
 - Str1 is set as the layout text
Str2 is draw to Bitmap2.Canvas:
 - The text cache of Bitmap2.Canvas is empty and is set to Str2 data
 - Str2 is set as the layout text
Str1 is draw (again) to Bitmap1.Canvas:
 - The text cache of Bitmap1.Canvas has Str1 data so it does not set the 
layout text
 - The layout still have Str2 as the text...

I can't explain what's going on when "Single Line" option is not set...

The fix is simple, instead of caching the text data at each time the 
layout text is set, retrieve the text directly from the layout using 
pango_layout_get_text (unlike the *set_text function this is pretty 
cheap since it just returns a pointer of the stored text). With this 
approach i have two wons: no data copy/duplication and avoid the out of 
sync between cache and layout texts.

I implemented  in three ways (see the patches):

1) Stores the OldFont (Pangolayout) and compare the Old/New strings 
using strcomp*

Than i noticed that is not necessary to store the old font and comes 
with the 2nd implementation. There's also no more need to be a method of 
TGtk2DeviceContext.

2) Just compare the two strings using StrComp*

* Is better to use StrLComp instead of StrComp

But i lost the shortcut evaluation of length that prevents a string 
comparation. So...

3) Stores the old length in the layout object using a gquark* and 
compare both strings with comparebytes (i know that both strings are the 
same size)

** The old length can be also stored in a field of font GdiObject


Luiz
 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: TextOutCacheGtk2.zip
Type: application/zip
Size: 2836 bytes
Desc: not available
URL: <http://lists.lazarus-ide.org/pipermail/gtk2/attachments/20080712/e239c59e/attachment-0001.zip>
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: gtk2settext_ver1.diff
URL: <http://lists.lazarus-ide.org/pipermail/gtk2/attachments/20080712/e239c59e/attachment-0003.ksh>
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: gtk2settext_ver2.diff
URL: <http://lists.lazarus-ide.org/pipermail/gtk2/attachments/20080712/e239c59e/attachment-0004.ksh>
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: gtk2settext_ver3.diff
URL: <http://lists.lazarus-ide.org/pipermail/gtk2/attachments/20080712/e239c59e/attachment-0005.ksh>