一、利用NSTimer计时器

1.新建一个UIButton按钮，设置成属性，名为codeButton。（UIButton样式一定要为自定义，否则后面倒计时数秒时会出现闪烁现象）

2.定义一个NSTimer的属性，名为timer，同时定义一个用于计时的int变量time，设置初始值为60。

//启动一个定时器self.timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(operatePerSecond) userInfo:nil repeats:YES];//实现定时器中的方法- (void)operatePerSecond {    if (time == 1) {      [self.timer invalidate];      time = 60;      [self.codeButton setTitle:@"重新获取" forState:UIControlStateNormal];      self.codeButton.tintColor = [UIColor blackColor];      self.codeButton.enabled = YES;    }else {      time --;      [self.codeButton setTitle:[NSString stringWithFormat:@"%ds" ,time] forState:UIControlStateNormal];    }}

3.此时主要逻辑已经完成，但要记得：在本页面即将消失的时候也要停掉计时器self.timer。

二、利用GCD实现

1.定义一个用于计时的time（此时要用block修饰）---block int time = 60;

//倒计时时间  __block int timeout = 60;  dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);  dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);  dispatch_source_set_event_handler(timer, ^{    if(timeout == 1){      //倒计时结束，关闭      dispatch_source_cancel(timer);      dispatch_async(dispatch_get_main_queue(), ^{      timeout = 60;      [self.codeButton setTitle:@"重新获取" forState:UIControlStateNormal];      self.codeButton.tintColor = [UIColor blackColor];      self.codeButton.enabled = YES;      });    }else{      NSString *strTime = [NSString stringWithFormat:@"%ds",timeout];      dispatch_async(dispatch_get_main_queue(), ^{        [self.codeButton setTitle:strTime forState:UIControlStateNormal];      });      timeout--;    }  });dispatch_resume(timer);