Why isn't Quartz double buffering my drawInContext()?

Question

I am rendering a simple line drawing (a line with some text in the middle) in a CALayer subclass via drawInContext(). I update this layer as the user is performing a gesture by calling setNeedsDisplay on it. The effect that I am seeing is what I might expect if there were no double buffering going on... i.e. I see parts of new rendering overlapping parts of old rendering. When I stop updating (complete the gesture) the system "catches up" and I always see the correct final result, but during the updates I see inconsistent results... This effect is not subtle and sometimes it is extreme... e.g. if I keep updating fast enough I can keep stale parts of the drawing on the screen for seconds while the new parts are drawing ahead...

I don't understand this at all. If Quartz is doing buffering then it seems that it is not blitting the result to the screen in its entirety or it is miscalculating the affected area.

Things I've tried:

1) I am disabling implicit animations and doing all of the drawing within a CATransaction

2) I am not making a mistake in my drawing... It's literally just two lines with some text in between... there is no way that I'm rendering the intermediate artifacts.

3) I have tried limiting the rate of updates by skipping most of them... but even at the lower rate I see artifacts until I stop updating and let the system catch up.

4) BTW, this happens identically in the simulator and on the device (iPad).

Is it necessary for me to draw into an offscreen buffer myself and copy it to the screen in its entirety? I thought that I had read that Quartz does this for me.

Update: As usual, after hours of banging my head against the wall I find the (partial) answer 5 minutes after posting the question. I realized that I was using a CATiledLayer in order to get my layer re-rendered on zoom. If I switch it back to a regular CALayer the glitches go away. So I guess what I am seeing artifacts of the separate tiles rendering. Now I am trying to figure out how to deal with this...

Answer 1

So, it turns out that I had three problems:

1) CATiledLayer explicitly fades in new tile content with a default time of 0.25 seconds... This was causing havoc with my drawing. I overrode this in my CATiledLayer subclass:

+ (CFTimeInterval)fadeDuration {
    NSLog(@"got fade duration");
    return 0;
}

2) I also had to adjust the maximum tile size up (I set it to 1024x1024 though I don't know what size it is actually using).

3) I was making adjustments to my layer's frame periodically during the updates and that seemed to cause additional problems for the tiled layer. I am making changes to stop that.

With all of those changes the performance seems acceptable now.