Question

        recordingTimer = [NSTimer scheduledTimerWithTimeInterval:0.03
                                                          target:self
                                                        selector:@selector(recordingTimerSelector:)
                                                        userInfo:nil
                                                         repeats:YES];

This needs to be very accurate. I’ve been experimenting with it, and it seems to skip a beat every now and again. Is there a better way to run a method every 0.03 seconds?

Answer 1

NSTimer has a resolution of 50 to 100 milliseconds, or 0.05 to 0.1 seconds. You could try CADisplayLink.

CADisplayLink* link = [CADisplayLink displayLinkWithTarget:self selector:@selector(doSomething)];
        link.frameInterval = 2;//thirty times a second
        [link addToRunLoop:[NSRunLoop mainRunLoop]
                         forMode:NSDefaultRunLoopMode];

That would call your method thirty times a second.

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EDIT: I searched around for CADisplayLink’s resolution but couldn’t find much, so I did some testing myself.

I modified some existing code that logs the frame rate using mach_absolute_time() to print the interval in milliseconds between calls to a method I had set up to be called by a CADisplayLink. I did a test on a few hundred time intervals between calls for both CADisplayLink and NSTimer. CADisplayLink had an average deviance (from the target time) of three tenths of a millisecond while NSTimer had an average deviation of half of a millisecond. That’s a decent difference in accuracy considering only a few sprites were rendering to the screen. CADisplayLink was 40% more accurate.

So, my conclusion is that if you want a method to be called 60/n (where n is a whole integer) times per second, CADisplayLink is vastly superior. You could try running on a different thread as well, where less is going on. That might increase the accuracy of the calls as well.