Re: Drawing to screen - nothing appears?

[Alexander Malmberg]

Steven Schlansker wrote:
> I'm trying to port an application that I've developed on Mac OS X to 
> GNUStep/Linux and GNUStep/Windows.  I'm currently running Gentoo with
> gnustep-base/gnustep-base-1.10.2_pre20041116 
> gnustep-base/gnustep-back-xlib-0.9.5_pre20041116 
> gnustep-base/gnustep-gui-0.9.5_pre20041116
>
> I'm using the libart backend, but xlib doesn't work either.
> The method I use for drawing goes like this:
>
> [NSWindow lockFocus]
> [NSColor set]
> NSRectFill (to clear the screen)
> for()
>    [NSColor set]
>    NSRectFill (to draw object)
> [NSWindow unlockFocus]
> [NSGraphicsContext flushGraphics]

Out of curiosity, could you provide some "real" example code that shows 
this? I'm not sure how to read the above pseudo-code (e.g. NSWindow has 
neither a +lockFocus method nor, if you mean an NSWindow instance, a 
-lockFocus method, so you should simply get an exception for that).

Anyway, the best way of drawing with -gui (as opposed to using the 
backend interface directly) is to leave all the focus and gstate handing 
to NSView and NSWindow and draw from -drawRect:, so if it's at all 
possible, I strongly recommend doing that. If you need to trigger a 
redraw at some point, you just call -setNeedsDisplay:.

With other methods, you are (more or less) bypassing -gui's drawing 
infrastructure, and this is risky and prone to weird behavior. However, 
in practice, I've had few problems with something like:

(theView is a view that covers the area I want to draw. It probably 
helps if theView is opaque and has an empty -drawRect:.)

[theView lockFocus];
/* Draw here. */
[theView unlockFocus];
[[theView window] flushWindow];

> Usually nothing shows in the window that I've created.  Sometimes a 
> rectangle appears as it should, but I can't figure out what triggers 
> this or why, and it disappears on the next refresh.

Most likely, -gui is going through the normal redraw mechanism in 
response to something, and various -drawRect:s are drawing the window 
background on top of your view. They expect your view's -drawRect: to 
redraw your parts, so the fix is to make sure your -drawRect: does that.

(Using an opaque view should help a bit here since it tells -gui that it 
doesn't _have_ to clear the window behind your view. It doesn't fix the 
problem, though; -gui might clear the background anyway.)

- Alexander Malmberg