第一步，拖两个空间textfiled和button到storyboard上的viewcontroller上。

第二步，拖线，链接到.h文件中代码如下：

1	@property (weak, nonatomic) IBOutlet UIButton *l_timeButton;

第三步，在,m文件中为l_timeButton设置监听器，监听点击事件。

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- ( void )viewDidLoad {      [super viewDidLoad];      [_l_timeButton addTarget:self action:@selector(startTime) forControlEvents:UIControlEventTouchUpInside]; }

第四步，在方法“startTime”中实现倒计时功能

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-( void )startTime{      __block int timeout=30; //倒计时时间      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_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行      dispatch_source_set_event_handler(_timer, ^{          if (timeout<=0){ //倒计时结束，关闭              dispatch_source_cancel(_timer);              dispatch_async(dispatch_get_main_queue(), ^{                  //设置界面的按钮显示 根据自己需求设置                  [_l_timeButton setTitle:@ "发送验证码" forState:UIControlStateNormal];                  _l_timeButton.userInteractionEnabled = YES;              });          } else {              int seconds = timeout % 60;              NSString *strTime = [NSString stringWithFormat:@ "%.2d" , seconds];              dispatch_async(dispatch_get_main_queue(), ^{                  //设置界面的按钮显示 根据自己需求设置                  //NSLog(@"____%@",strTime);                  [UIView beginAnimations:nil context:nil];                  [UIView setAnimationDuration:1];                  [_l_timeButton setTitle:[NSString stringWithFormat:@ "%@秒后重新发送" ,strTime] forState:UIControlStateNormal];                  [UIView commitAnimations];                  _l_timeButton.userInteractionEnabled = NO;              });              timeout--;          }      });      dispatch_resume(_timer); }

一个验证码倒计时的功能就写好了