1 NSTimer

启动定时器

timer = [NSTimer scheduledTimerWithTimeInterval:2 target:self selector:@selector(updateWithTimer:) userInfo:nil repeats:YES];

- (void) updateWithTimer:(NSTimer*) timer {
    NSLog(@"update from  timer!");
}

停止定时器

[timer invalidate];
timer = nil;

2 重复的调用某个block

@interface ViewController ()	 { 
	dispatch_source_t timer;
}
@end

/ Get the queue to run the blocks on
dispatch_queue_t queue = dispatch_get_main_queue();


// Create a dispatch source, and make it into a timer that goes off every second
timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC, 0);
   
// When the timer goes off, heal the player
dispatch_source_set_event_handler(timer, ^{
    GivePlayerHitPoints();
});


// Dispatch sources start out paused, so start the timer by resuming it
dispatch_resume(timer);

//To cancel the timer, just set the timer variable to nil:
timer = nil; ##3 CADisplayLink

当屏幕每次重绘的时候，CADisplayLink会发送消息。

displaylink = [CADisplayLink displayLinkWithTarget:self selector:@selector(update:)];
[displaylink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];

-(void) update:(CADisplayLink*) displaylink {
    
    NSLog(@"update from display link") ;
}

停止CADisplayLink

[displayLink invalidate];
displayLink = nil;

暂停 CADisplayLink

displayLink.paused = YES;

44 计算时间间隔

-(void) testDiffTime {
    NSDate* old = [NSDate date] ;
    NSDate* now = [NSDate date] ;
    
    NSTimeInterval timeElapsed = [now timeIntervalSinceDate:old] ;
    float minutes = timeElapsed / 60.0  ;
    float hours = timeElapsed / 3600.0 ;
    float seconds = fmodf(timeElapsed, 60.0) ;
}

5 在将来的某一个时刻执行任务

double timeToWait = 10.0;
dispatch_time_t delayTime = dispatch_time(DISPATCH_TIME_NOW,
(int64_t)(timeToWait * NSEC_PER_SEC));
dispatch_queue_t queue = dispatch_get_main_queue();
dispatch_after(delayTime, queue, 
		^(void){ // Time's up. Kaboom.
			ExplodeBomb();
		});