GCD实现倒计时

之前面试中，好多面试官，问使用GCD如何实现倒计时，我当时也没写过，所以一时不知道怎么说，所以结束之后，我实现一下GCD的倒计时。

- (void)startTime:(UIButton *)sender {
    __block int timeout=60; //倒计时时间
    //创建队列
    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){
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                [sender setTitle:@"重新获取" forState:UIControlStateNormal];
                sender.userInteractionEnabled = YES;
            });
        }else{
            int seconds = timeout % 61;
            NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
            dispatch_async(dispatch_get_main_queue(), ^{
                [UIView beginAnimations:nil context:nil];
                [UIView setAnimationDuration:1];
                [sender setTitle:[NSString stringWithFormat:@"发送中(%@秒)",strTime] forState:UIControlStateNormal];
                [UIView commitAnimations];
                sender.userInteractionEnabled = NO;
            });
            timeout--;
        }
    });
    dispatch_resume(_timer);
}

github中demo:    https://github.com/ZY201611/ZPStrikeLine