NSTimer Gotchas With Threads/RunLoops

Sometimes when you notice that the NSTimer isn't working (i.e. the scheduled task doesn't fire), it can be caused by runloop issues.

By default, it runs in the current run loop from where it is scheduled. If you schedule it from the wrong loop, it might not fire as the loop could be blocked somewhere else.

Which method you use to initialize the timer also makes a difference. The "scheduledTimerWithTimeInterval" creates a timer that runs in the current loop. If the current loop is not suitable for the timer to run, the timer may not fire and you have no idea why.

If you have a problem with the above, you will have to use the "timerWithTimeInterval" methods. However, you have to remember to add the timer to the run loop that you want to add on.

For example, to schedule a timer on the main loop from another loop:

NSTimer *timer = [NSTimer timerWithTimeInterval:waiTime target:self selector:@selector(fireMethod:) userInfo:nil repeats:YES]; [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes]

The various behaviors are documented in the class reference.

Reference: https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/nstimer_Class/Reference/NSTimer.html