Core Graphics Lagging?

i'm trying make app draws line on window reflects position of 2 touches on trackpad. here's code:

oh, quick note, startpoint , endpoint both cgpoint ivars. more me experimenting nstouches , core graphics, i'm not concerned following proper practices , not having ivars.

code:

- (void)drawrect:(nsrect)rect {     cgcontextref mycontext = [[nsgraphicscontext currentcontext] graphicsport];     cgcontextsetlinecap(mycontext, kcglinecapsquare);     cgcontextsetrgbstrokecolor(mycontext, 0.0, 0.0, 0.0, 1.0);     cgcontextsetlinewidth(mycontext, 1.0);     cgcontextmovetopoint(mycontext, startpoint.x + 0.5, startpoint.y + 0.5);     cgcontextaddlinetopoint(mycontext, endpoint.x + 0.5, endpoint.y + 0.5);     cgcontextstrokepath(mycontext); }  - (void)touchesmovedwithevent:(nsevent *)event { 	nslog(@"touches moved."); 	nsmutableset *touches = [[event touchesmatchingphase:nstouchphaseany inview:self] mutablecopy]; 	nstouch *firsttouch = (nstouch *)[touches anyobject]; 	[touches removeobject:firsttouch]; 	nstouch *secondtouch = (nstouch *)[touches anyobject]; 	 	if (secondtouch) 	{ 		float startx = (firsttouch.normalizedposition.x)*(self.frame.size.width); 		float starty = (firsttouch.normalizedposition.y)*(self.frame.size.height); 		startpoint = cgpointmake(startx, starty); 		float endx = (secondtouch.normalizedposition.x)*(self.frame.size.width); 		float endy = (secondtouch.normalizedposition.y)*(self.frame.size.height); 		endpoint = cgpointmake(endx, endy); 		 		[self setneedsdisplay:yes]; 	} }

if user moves fingers enough, line fall behind. if user moves fingers enough long enough, can lift fingers , watch line continue move several seconds after user has stopped.

suggestions?

edit: i'm running in debugger in xcode on original macbook air (1.6 ghz). so, shouldn't matter... i'd rather if aborted drawing missed in-between , skipped should right now.

 

dumb question, have tried :

code:

		if(![self needsdisplay]) [self setneedsdisplay:yes];

 

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