can't draw fast enough to keep up with touchesMoved?

Question

I am trying to implement simple paint functionality in my iPhone app. I tried updating a bitmap with a bitmap brush, and I also tried this tutorial.

Both methods have

Answer 1

You can't directly call drawRect:. To refresh your screen on demand, try calling [self setNeedsDisplay] from your touchesMoved method, which will setup the proper contexts for a call to drawRect:.

Answer 2

It sounds to me like you're not giving the main run loop a chance to update the display. Your drawing code may be taking longer to execute than the time between touch events, so the display is never updated. When you lift your finger, it does the updating because it's no longer burdened with your drawing.

You might consider optimizing your drawing to speed it up (drawing only within the dirty region of the screen, for example), using something like NSOperationQueue to queue up the heavy calculations of your drawing to run on a background thread, or selectively dropping touch drawing events to keep your response smooth.

One additional possibility is placing your heavy drawing code in a separate method and calling it via performSelector:withObject:afterDelay, with a 10 millisecond (or smaller) delay. This might give the main run loop a chance to update the display with its current state. I haven't tested this, but if I remember correctly I've seen this work.