You would like to perform a specific task repeatedly with a certain delay. For instance, you want to update a view on your screen every second that your application is running.

Use a timer:

- (void) paint:(NSTimer *)paramTimer{
  /* Do something here */
  NSLog(@"Painting");
}

- (void) startPainting{
  
  self.paintingTimer = [NSTimer 
                        scheduledTimerWithTimeInterval:1.0
                        target:self
                        selector:@selector(paint:) 
                        userInfo:nil
                        repeats:YES];
  
}

- (void) stopPainting{
  if (self.paintingTimer != nil){
    [self.paintingTimer invalidate];    
  }
}

- (void)applicationWillResignActive:(UIApplication *)application{
  [self stopPainting];
}

- (void)applicationDidBecomeActive:(UIApplication *)application{
  [self startPainting];
}