running a time interval computation in the background

Question

I have a NSTimeInterval which is basically

NSTimeInterval interval = [date1 timeIntervalSinceDate: [NSDate date]];

I want this computation to be always running in the background when I am in a view X, so I can display a timer counting down in a UILabel.. how do I do this? It's like in the groupon iphone app, but it doesn't show it in details of seconds

Answer 1

You could use an NSTimer to get a method called at a set time interval (however this is all performed on the main thread, not on a background thread):

- (void)setupTimer {
    //Start a timer to call update updateInterval: every 1 second
    self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0
                                               target:self
                                               selector:@selector(updateInterval:)
                                             userInfo:nil
                                              repeats:YES];
}

- (void)updateInterval:(NSTimer *)timer {
    NSTimeInterval interval = [date1 timeIntervaleSinceDate:[NSDate date]];
    //Do other things like updating the label
}