GCD定时器验证码倒计时

- (IBAction)butClick:(UIButton *)but {
    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);
    __block NSInteger time = count;
    dispatch_source_set_event_handler(_timer, ^{
        if(time<=0){
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                [but setTitle:@"重新获取" forState:UIControlStateNormal];
                but.userInteractionEnabled = YES;
                but.alpha = 1.0;
                but.backgroundColor = [UIColor whiteColor];
            });
        } else {
            NSString *strTime = [NSString stringWithFormat:@"%lds", (long)time];
            dispatch_async(dispatch_get_main_queue(), ^{
                [but setTitle:strTime forState:UIControlStateNormal];
                but.userInteractionEnabled = NO;
                but.backgroundColor = [UIColor lightTextColor];
            });
            time--;
        }
    });
    dispatch_resume(_timer);
}