How do I create an accurate timer event in Objective-C/iOS?

Question

I'm looking to create a countdown timer for SMPTE Timecode (HH:MM:SS:FF) on iOS. Basically, it's just a countdown timer with a resolution of 33.33333ms. I'm not so sure NSTimer is accurate enough to be counted on to fire events to create this timer. I would like to fire an event or call a piece of code every time this timer increments/decrements.

I'm new to Objective-C so I'm looking for wisdom from the community. Someone has suggested the CADisplayLink class, looking for some expert advice.

Answer 1

Try CADisplayLink. It fires at the refresh rate (60 fps).

CADisplayLink *displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(timerFired:)];
displayLink.frameInterval = 2;
[displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];

This will fire every 2 frames, which is 30 times per seconds, which seems to be what you are after.

Note, that this is tied to video frame processing, so you need to do your work in the callback very quickly.

Answer 2

You basically have no guarantees with either NSTimer or dispatch_after; they schedule code to triggered on the main thread, but if something else takes a long time to execute and blocks the main thread, your timer won't fire.

That said, you can easily avoid blocking the main thread (use only asynchronous I/O) and things should be pretty good.

You don't say exactly what you need to do in the timer code, but if all you need to do is display a countdown, you should be fine as long as you compute the SMPTE time based on the system time, and not the number of seconds you think should have elapsed based on your timer interval. If you do that, you will almost certainly drift and get out of sync with the actual time. Instead, note your start time and then do all the math based on that:

// Setup
timerStartDate = [[NSDate alloc] init];
[NSTimer scheduledTimer...

- (void)timerDidFire:(NSTimer *)timer
{
    NSTImeInterval elapsed = [timerStartDate timeIntervalSinceNow];
    NSString *smtpeCode = [self formatSMTPEFromMilliseconds:elapsed];
    self.label.text = smtpeCode;
}

Now you will display the correct time code no matter how often the timer is fired. (If the timer doesn't fire often enough, the timer won't update, but when it updates it will be accurate. It will never get out of sync.)

If you use CADisplayLink, your method will be called as fast as the display updates. In other words, as fast as it would be useful, but no faster. If you're displaying the time, that's probably the way to go.

Answer 3

If you are targeting iOS 4+, you can use Grand Central Dispatch:

// Set the time, '33333333' nanoseconds in the future (33.333333ms)
dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, 33333333);
// Schedule our code to run
dispatch_after(time, dispatch_get_main_queue(), ^{
    // your code to run here...
});

This will call that code after 33.333333ms. If is this going to be a loop sorta deal, you may want to use the dispatch_after_f function instead that uses a function pointer instead of a block:

void DoWork(void *context);

void ScheduleWork() {
    // Set the time, '33333333' nanoseconds in the future (33.333333ms)
    dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, 33333333);
    // Schedule our 'DoWork' function to run
    // Here I pass in NULL for the 'context', whatever you set that to will
    // get passed to the DoWork function
    dispatch_after_f(time, dispatch_get_main_queue(), NULL, &DoWork);
}

void DoWork(void *context) {
    // ...
    // Do your work here, updating an on screen counter or something
    // ...

    // Schedule our DoWork function again, maybe add an if statement
    // so it eventually stops
    ScheduleWork();
}

And then just call ScheduleWork(); when you want to start the timer. For a repeating loop, I personally think this is a little cleaner than the block method above, but for a one time task I definitely prefer the block method.

See the Grand Central Dispatch docs for more info.