iOS GCD实现发送验证码倒计时

下面的代码是从网上找来的，进行了稍微修改，将下面代码放到一个工具类中，直接就可以调用了。

#pragma mark 倒计时
+ (void)startTime:(NSInteger)time sendAuthCodeBtn:(UIButton *)sendAuthCodeBtn {
    if (time > 59 || time < 1) {
        time = 59;
    }
    __block NSInteger timeout = time; //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_async(dispatch_get_main_queue(), ^{
                //设置界面的按钮显示
                [sendAuthCodeBtn setTitle:@"发送验证码" forState:UIControlStateNormal];
                // iOS 7
                [sendAuthCodeBtn setTitle:@"发送验证码" forState:UIControlStateDisabled];
                sendAuthCodeBtn.enabled = YES;
            });
        } else {
            NSInteger seconds = timeout % 60;
            NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
            dispatch_async(dispatch_get_main_queue(), ^{
                [sendAuthCodeBtn setTitle:[NSString stringWithFormat:@"重发(%@)",strTime] forState:UIControlStateNormal];
                // iOS 7
                [sendAuthCodeBtn setTitle:[NSString stringWithFormat:@"重发(%@)",strTime] forState:UIControlStateDisabled];
                sendAuthCodeBtn.enabled = NO;
            });
            timeout--;
        }
    });
    dispatch_resume(_timer);
}