[Alephmodular-devel] Drawing Contexts - CDContext

[Br'fin <br...@ma...>]

Yes, I know it's old and crusty in here. But I never know quite what to 
do aside from occasionally making some headway on the Display 
Abstraction.

The general business I see CDContext dealing with will mainly be 
drawing lines, text, and filling areas. As such it will provide the 
support/features needed and provided by screen_drawing.cpp. As well as 
other areas where direct drawing commands are done upon a buffer. (For 
instance, asking a buffer to be erased/filled to a color)

CDContext

A CDContext, aka a Drawing Context, would own all of the high-level 
operations for operating on a buffer. For instance, using line 
primitives and displaying text.

Some systems, such as Macintosh, use a method of drawing commands that 
works as follows.
	Store the current drawing context
	Use the desired graphics port as the context
	perform drawing operations
	Swap the ports back to where they were before.

Modern systems are much more object orientated which helps also when 
dealing with threads and multiple processors. So we will have a 
CDContext for each buffer we wish to draw to. (However, each buffer 
will only have one context, but may have multiple copies of that 
context floating around (via reference counting))
	
In practice, using a CDContext would be like:

	std::auto_ptr<CDContext> context= CBuffer.get_drawing_context()
	context.draw_line...
	context.draw_text
	(context automatically freed)
	
Since the Macintosh scheme currently uses Quickdraw, the existing 
Macintosh method predates multithreading and other modern issues. We 
should accept that the Carbon based drawing methods may have problems 
with performing drawing operations between sets of drawing contexts all 
at once.

( for instance:
	dc1.draw_line
	dc2.draw_line
	dc1.draw_line
)

But that is probably fairly rare to begin with. And in the future may 
allow us to shift to a Quartz based drawing scheme. (Heh, anti-aliased 
text and map lines?)

If a CBuffer already has an existing CDContext, then it should return 
the same drawing context. Hence, CDContext should be reference counted.
Refer to the C++ faq for the basics: 
http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.21
-Jeremy Parsons