[haiku-appserver] TextLayoutEngine

  • From: Stephan Aßmus <superstippi@xxxxxx>
  • To: haiku-appserver@xxxxxxxxxxxxx
  • Date: Wed, 16 Nov 2005 21:27:21 +0100


I started to work on extracting a class for general glyph processing. It 
would use the AGG glyph cache and be a global instance class. Whenever some 
app_server object needs to iterate over glyphs, it will go through the 
global TextLayoutEngine. This object has a glyph cache on its backend. It 
will use a template class "GlyphConsumer" such as this:

template<class GlyphConsumer>
TextLayoutEngine::LayoutGlyphs(const char* utf8String, int32 length, 
GlyphConsumer& consumer);

A glyph consumer can be anything. The TextLayoutEngine expects the 
GlyphConsumer template to export a function ConsumeGlyph(const 
agg::glyph_cache* glyph);

A GlyphConsumer can be an object which sums up the width of the glyphs 
layouted by the TextLayoutEngine. It could be the AGGTextRenderer. The 
point is that we would get rid of all this code duplication to iterate over 
glyphs at all those places.

The global TextLayoutEngine instance would have two functions for granting 
exclusive access to it:

Acquire(const ServerFont& font);

Between Acquire(), which also configures the font and Release(), The 
TextLayoutEngine can be used by just one client. The clients can be 
ServerFont objects in their StringWidth(), GetEscapements(), etc calls. But 
also the Painter objects, of which I plan to give each ServerWindow object 
another instance.

Later, we could also make the caching backend a MultiLocker object. You 
would start with a ReadLock() and switch to WriteLock() access as soon as 
you need to add something to the cache. But let's do one step at a time.

I think this change would greatly clean up the text layouting code.
Any objections or suggestions?

Best regards,

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