Can I get 60fps with a 10ms drawRect?

Question

I've asked a more general question about how CADisplayLink and drawRect behave here, but as that question doesn't have any answers yet, I thought I'd ask a more specific one: has anyone managed to get an app to run at 60fps with a drawRect method that takes ~10ms to execute?

In my app, I configure a CADisplayLink like so:

displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(update)];
[displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];

and the update method simply calls setNeedsDisplay on my custom UIView (as well as checking for skipped frames):

- (void)update
{
    static CFTimeInterval timestamp = 0;

    // check for skipped frame
    CFTimeInterval newTimestamp = displayLink.timestamp;
    if ((newTimestamp - timestamp) > (displayLink.duration * (displayLink.frameInterval + 0.5)))
        NSLog(@"Frame skipped!");
    timestamp = newTimestamp;

    [myView setNeedsDisplay];
}

This begins to skip frames as soon as drawRect takes more than 4 or 5 milliseconds. For my purposes, it will need up to around 10-11ms, but I thought that given 1/60fps = 16.7ms this should be no problem.

Has anyone achieved this? What am I doing wrong?

Answer 1

I just don't understand why I can't use the full 16ms between frames

Other stuff gets drawn when the screen refreshes besides the one view you're working on. (For example, the status bar clock.) Also, anything in drawRect: is not hardware accelerated. So some of that computational power is doing other stuff (background downloads, iCloud sync, etc.)

You'll never reliably get 60fps using drawRect:. You can try to improve performance using caching and some of Apple's other drawing tips. You might also get better performance using Core Animation or SceneKit, both of which are highly optimized.