iOS 验证码按钮倒计时功能

// _CountdownTime 倒计时总时间；
//_timer 定时器
- (void)startTime:(UIButton *)VerificationCodeButton 
{
 __block NSInteger timeout = [_CountdownTime integerValue];
 dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
 _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);
 dispatch_source_set_event_handler(_timer, ^{
  if(timeout<=0){
   dispatch_source_cancel(_timer);
   dispatch_async(dispatch_get_main_queue(), ^{
    [VerificationCodeButton setTitle:@"重新获取" forState:UIControlStateNormal];
    VerificationCodeButton.userInteractionEnabled = YES;
    VerificationCodeButton.alpha = 1.0;
    VerificationCodeButton.backgroundColor = [UIColor whiteColor];
   });
  } else {
   NSString *strTime = [NSString stringWithFormat:@"%lds", (long)timeout];
   dispatch_async(dispatch_get_main_queue(), ^{
    [VerificationCodeButton setTitle:strTime forState:UIControlStateNormal];
    VerificationCodeButton.userInteractionEnabled = NO;
    VerificationCodeButton.backgroundColor = [UIColor lightTextColor];
   });
   timeout--;
  }
 });
 dispatch_resume(_timer);
}