Objective C / iPhone: How to add a timer to let the text update?

I want to write an application that allow user to count down the time. The time will be displayed using label. I want to update the label every second, how can I do it? or which API is suitable for me to do this?开发者_如何学Python thz in advance.

- (IBAction)startCountdown:(id)sender
{
    NSTimer *countdownTimer = [NSTimer scheduledTimerWithTimeInterval:1 target:self     selector:@selector(advanceTimer:) userInfo:nil repeats:YES];
    NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
    [runLoop addTimer:countdownTimer forMode:NSDefaultRunLoopMode];
}

- (void)advanceTimer:(NSTimer *)timer
{
    [countdown setIntegerValue:59];
}

+1up ;)