iOS计时器

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];

    NSDate *date1 = [NSDate date];
    sleep(2);
    NSDate *date2 = [NSDate date];
    //怎样将date2与date1的时间间隔表示为：00:00:10 的形式
   NSTimeInterval time = [date2 timeIntervalSinceDate:date1];
    NSDate *date = [NSDate dateWithTimeIntervalSinceReferenceDate:time];
    
    NSDateFormatter *df = [[NSDateFormatter alloc] init];
    [df setDateFormat:@"HH:mm:ss"];
    NSTimeZone *gmt = [NSTimeZone timeZoneWithAbbreviation:@"GMT"];
    [df setTimeZone:gmt];
    NSString *str = [df stringFromDate:date];
    NSLog(@"%@",str);
    //结果：00:00:10
    
    timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(update) userInfo:nil repeats:YES];
    
    
    
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    return YES;
}

-(void)update
{
    static int i = 0;
    NSLog(@"update:%d",i++);
    if (i>10) {
        i=0;
        [timer setFireDate:[NSDate distantFuture]];
        [self other];
    }
}

-(void)other
{
    NSLog(@"stop 2 seconds");
    sleep(2);
    [timer setFireDate:[NSDate date]];
}