secondsCountDown = 60;//60秒倒計時  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; //倒計時時間  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); //每秒執行  dispatch_source_set_event_handler(_timer, ^{    if(timeout<=0){ //倒計時結束，關閉      dispatch_source_cancel(_timer);      dispatch_release(_timer);      dispatch_async(dispatch_get_main_queue(), ^{ //設置界面的按鈕顯示 根據自己需求設置        。。。。。。。。      });    }else{      int minutes = timeout / 60;      int seconds = timeout % 60;      NSString *strTime = [NSString stringWithFormat:@"%d分%.2d秒后重新獲取驗證碼",minutes, seconds];      dispatch_async(dispatch_get_main_queue(), ^{        //設置界面的按鈕顯示 根據自己需求設置 。。。。。。。。      });      timeout--;    }  });  dispatch_resume(_timer);