secondsCountDown = 60;//60秒倒计时 
 countDownTimer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(timeFireMethod) userInfo:nil repeats:YES]; 
-(void)timeFireMethod{ 
  secondsCountDown--; 
  if(secondsCountDown==0){ 
   [countDownTimer invalidate]; 
  } 
} 

__block int timeout=300; //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_release(_timer); 
     dispatch_async(dispatch_get_main_queue(), ^{ 
//设置界面的按钮显示 根据自己需求设置 
       。。。。。。。。 
     }); 
   }else{ 
     int minutes = timeout / 60; 
     int seconds = timeout % 60; 
     NSString *strTime = [NSString stringWithFormat:@"%d分%.2d秒后重新获取验证码",minutes, seconds]; 
     dispatch_async(dispatch_get_main_queue(), ^{ 
       //设置界面的按钮显示 根据自己需求设置 
。。。。。。。。 
     }); 
     timeout--; 
   } 
 }); 
 dispatch_resume(_timer);