iOS-GCD的实践_ios gcd实习视频加载-优快云博客

本文链接：https://blog.youkuaiyun.com/samuelandkevin/article/details/71482051

本文探讨了dispatch_group_enter()与dispatch_group_leave()在多线程任务中的使用，并通过示例代码展示了它们如何确保所有子任务按预期顺序完成。此外，还介绍了dispatch_semaphore在控制并发任务数量方面的应用。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

1.dispatch_group_enter(),dispatch_group_leave() 的应用

简单来说，它们分别代表进入group和离开group.想想以下代码打印的顺序是多少？

   dispatch_group_t group = dispatch_group_create();
    NSLog(@"1");
    dispatch_group_enter(group);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"2");
        dispatch_group_leave(group);
    });
    NSLog(@"3");

    dispatch_group_enter(group);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"4");
        dispatch_group_leave(group);
    });
    NSLog(@"5");
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"6");

    });

正确的打印结果如下图： (1,3,2,5,4,6)

2.dispatch_semaphore的应用

设计一个并发数量为n（n>0,为整数）的DEMO

    int n = 5;//假设并发数量为5
    dispatch_group_t group = dispatch_group_create();
    dispatch_semaphore_t sem = dispatch_semaphore_create(n);
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    for(int i=0 ;i <100;i++){
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        dispatch_group_async(group, queue, ^{
            NSLog(@"%i",i);
            sleep(2);
            dispatch_semaphore_signal(sem);
        });
    }
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

为了验证结果，抛出证据来：
设计一个并发数量为n的DEMO

3.死锁案例

//用SDWebImage来判断本地是否存在高清图片，运行此代码发现界面控件无法响应事件。
- (BOOL)hasHighQImage{
    __block BOOL hasHighQImage = NO;
    SDWebImageManager *manager = [SDWebImageManager sharedManager];
    dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    [manager diskImageExistsForURL:[self highQualityImageURLForIndex:self.currentImageIndex] completion:^(BOOL isInCache) {
        hasHighQImage = isInCache;
        dispatch_semaphore_signal(sem);
    }];
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    return hasHighQImage;
}