Objective-C : NSTimer and countdown

Question

I know that I should understand Apple\'s documentation for NSTimer, but I don\'t! I have also read a lot of questions about it but can not find one that suites my case. Well

Answer 1

First you should calculate what your countdown time is, in seconds and put it in a instance variable (ivar)

NSTimeInterval totalCountdownInterval;

I think in order to keep good accuracy (NSTimer firing can be off by as much as 100ms and errors will add up) you should record the date at which the countdown started, and put it in another ivar:

NSDate* startDate = [NSDate date];

Then you can have a timer firing at regular (here 1 second) intervals calling a method on your class repeatedly

NSTimer* timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(checkCountdown:) userInfo:nil repeats:YES];

And in that method you check the elapsed time against the total countdown time and update the interface

-(void) checkCountdown:(NSTimer*)_timer {

    NSTimeInterval elapsedTime = [[NSDate date] timeIntervalSinceDate:startDate];

    NSTimeInterval remainingTime = totalCountdownInterval - elapsedTime;

    if (remainingTime <= 0.0) {
        [_timer invalidate];
    }

    /* update the interface by converting remainingTime (which is in seconds)
       to seconds, minutes, hours */

}