NSTimer stops from time to time - xcode

Question

I've made a game in Xcode 6, using several NSTimers for different things, like a scorer timer, countdown timer, and to move my objects around. The problem is that sometimes (it seems like) the NSTimers stop for like half a second which makes it look like it lags. Example: When the character is moving, it stops for a tiny second and then continues to move. It happens so fast, but it is noticable, and it annoys me so much. I want it to be completely smooth. Any help would be appreciated!

Answer 1

A couple of thoughts:
If you're having a small delay in the timer processing, the most likely issue is that you have something blocking the main queue. Take a careful look at your code and see if you can find anything that could block the main queue.
You can actually use Instruments to find places in your app where the thread might be blocked. If I recall correctly, WWDC 2112 video Building Concurrent User Interfaces on iOS shows the trick with Instruments to find where your app is blocked. It's a bit dated, but the techniques for finding where the main thread blocks still apply.
It's unlikely, but you might want to consider checking the run loop modes that your timer is running on. For example, the default:
[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:#selector(tick:) userInfo:nil repeats:YES];
This can pause during certain animations. You might consider using a broader array of run loop modes, e.g.:
NSTimer *timer = [NSTimer timerWithTimeInterval:1.0 target:self selector:#selector(tick:) userInfo:nil repeats:YES];
[[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
This results in a timer that is less susceptible to certain delays during particular types of animations. It just depends upon what else your app is doing when you see the delay in the user interface.
When using a timer to update animations, better than a NSTimer is a CADisplayLink. For example, define a few properties:
#property (nonatomic, strong) CADisplayLink *displayLink;
#property (nonatomic) CFTimeInterval startTime;
Then you can write code to start and stop the display link:
- (void)startDisplayLink
{
self.displayLink = [CADisplayLink displayLinkWithTarget:self selector:#selector(handleDisplayLink:)];
self.startTime = CACurrentMediaTime();
[self.displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
}
- (void)stopDisplayLink
{
[self.displayLink invalidate];
self.displayLink = nil;
}
- (void)handleDisplayLink:(CADisplayLink *)displayLink
{
CFTimeInterval elapsed = CACurrentMediaTime() - self.startTime;
// update your UI, not on the basis of "this is called x times per second",
// but rather, on the basis that `elapsed` seconds have passed
}
The key in good animation code is that you don't just assume that your routine will be called at a specified frequency, but rather that you update the UI based upon the number of elapsed seconds. This way, a slow device that drops a few frames and a fast device will yield the same animation, the latter would just be a little smoother than the former.
The merits of display links, though, are discussed briefly in WWDC 2014 video - Building Interruptible and Responsive Interactions. There are other longer discussions of the topic that are eluding me at this point, but this might be a good place to get introduced to the topic (even though the vast majority of that video is on other topics).

Answer 2

You may want to try a high resolution timer, like Timer dispatch sources. It looks a bit scary at first, but actually quite easy to use. Sample code (with comments)
dispatch_source_t CreateDispatchTimer(uint64_t interval, uint64_t leeway, dispatch_queue_t queue , dispatch_block_t block) {
dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
if (timer) {
// Use dispatch_time instead of dispatch_walltime if the interval is small
dispatch_source_set_timer(timer, dispatch_walltime(NULL, 0), interval, leeway);
dispatch_source_set_event_handler(timer, block);
dispatch_resume(timer);
}
return timer;
}
void MyCreateTimer()
{
dispatch_source_t aTimer = CreateDispatchTimer(30 * NSEC_PER_SEC, 1 * NSEC_PER_SEC, dispatch_get_main_queue(), ^{
NSLog(#"Timer fired!");
});
// Keep a reference if you want to, say, stop it somewhere in the future
}
EDIT:
In XCode, if you type dispatch in the editor, it will suggest a snippet called dispatch_source timer - GCD: Dispatch Source (Timer), which will generate the template code for the timer.