iOS 验证码按钮倒计时

// _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);
}