Cocoa - NSTimer initWithFireDate: multiple seconds too late?

Question

I have a simple app that doesn't do much more (at the moment) than create a timer:

     _time = [[NSTimer alloc] initWithFireDate: _nextDate
                                 interval: 0.0f
                                   target: self
                                 selector: @selector(trackTime)
                                 userInfo: nil
                                  repeats: NO];
[[NSRunLoop currentRunLoop] addTimer:_time forMode:NSRunLoopCommonModes];

In the trackTime function I checked to see how much difference there is between the stated firing date (_nextDate) and the actual firing date:

     NSTimeInterval secondsBetween = [_nextDate timeIntervalSinceDate:[NSDate date]];

From the results, it appears that the function is called 2 to 4 seconds too late.

Is this normal? Do I need a different runloop mode? Is there anything else I can do to get a better timing?

Answer 1

NSTimer doesn't guarantee the interval because it depends on the runloop mechanism. You can see the details about the runloop (https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/Multithreading/RunLoopManagement/RunLoopManagement.html#//apple_ref/doc/uid/10000057i-CH16-SW1)

Maybe you can try dispatch_after from GCD.