xcode4 - 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 here it is:

The user enter hour and minutes through textfields. I convert those to integers and display them in a "countDownLabel".

1. How can i make the count down to zero?
2. It would be nice to show the seconds which is something that the user didn't imported but i guess it will not be hard to show.
3. How could somebody stop this procedure with a press of a UIButton?

I know that i am asking a lot...I would be really grateful if someone could help!

int intHourhs;
intHourhs=([textHours.text intValue]);
int intMinutes;
intMinutes=([textMinutes.text intValue]);
int *intSeconds;
intSeconds=0;

NSString *stringTotalTime=[[NSString alloc] initWithFormat:@"%.2i:%.2i:%.2i",intHourhs,intMinutes,intSeconds]; 
[countDownLabel setFont:[UIFont fontWithName:@"DBLCDTempBlack" size:45]];
countDownLabel.text=stringTotalTime;

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 */

}