Question

Say, for an iOS app, if a user slides his finger on the screen, and then 50,000 dots are recorded. If the drawing is done for all these dots in drawRect, then next time the user touched the 50,001st dot, at the end of touchesMoved the following line

[self.view setNeedsDisplay];

will cause drawRect to run again and have all 50,001 dots drawn again. So for every 1 new dot (for any new movement of finger), all 50,001 dots will need to be redrawn and it is not an efficient method.

I tried just drawing the last dot in drawRect, and it will not “add to” the existing view, but “wipe everything out” and then draw one dot.

Is there a way to

1) draw that 1 extra dot without needing to have drawRect called?
2) or, can drawRect draw one extra dot without first wiping the whole screen out?

Answer 1

One approach for this would be to render into an image when the touches end, and then keep adding to the image as more paths are generated. You can store the paths in an array if you need and undo buffer or otherwise need to regenerate the drawing.

(you will need more code than this, obviously)

 UIGraphicsBeginImageContextWithOptions(size, NO, 1.0);
 CGContextRef context = UIGraphicsGetCurrentContext();
 // do some drawing
 UIImage * image = UIGraphicsGetImageFromCurrentImageContext();
 UIGraphicsEndImageContext();

Oh – fwiw, I have found setNeedsDisplayInRect: to be odd/buggy in iOS5. The first call to it is the full view rect, not the rect passed in as a param. At least that is what I found when I tried to use it. Maybe there is some implementation detail I overlooked.