//方法一:

    //创建定时器

    NSTimer *timer=[NSTimer timerWithTimeInterval:2.0 target:self selector:@selector(nextCilcked) userInfo:nil repeats:YES];

    //利用消息循环来开启定时器

    //创建消息循环

    NSRunLoop *runLoop =[NSRunLoop currentRunLoop];

    //Mode:1: NSDefaultRunLoopMode;

    //     2: NSRunLoopCommonModes ;

    //将定时器添加到到runLoop

    [runLoop addTimer:timer forMode:NSDefaultRunLoopMode];

    //方法二：

   //创建定时器

    NSTimer *timer1 =[NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(nextCilcked) userInfo:nil repeats:YES];

    //开启定时器

    [timer1 setFireDate:[NSDate distantPast]];

    //暂停定时器

    [timer setFireDate:[NSDate distantFuture]];

    //取消定时器(永久性停止)

    [timer invalidate];

    //释放计时器

    timer = nil;

}

-(void)nextCilcked

{

    NSLog(@"timer执行中......");

}