GCD 并发流程控制

最近在项目中有一需求：假设有三个异步请求A、B、C，等这三个异步请求完成之后，通知程序执行D方法，但又不要阻塞线程！貌似面试的时候面试官也喜欢问这种问题，第一反应是用GCD处理，之前也看过一些GCD资料，但是没有实际操作过，今天网上又看了下GCD的资料，找到了简单的解决办法！

用最简单的办法去解决复杂的问题！！！

附以下代码：

---

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
/**创建dispatch组*/
dispatch_group_t group = dispatch_group_create();

for (NSInteger i = 0; i < 10; i++) {
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        /**此处是异步请求*/
        [WBRequestManager startGETRequestHost:(NSString *)host urlString:urlString argument:param completionBlock:^(WBResponseModel *model) {
            dispatch_group_leave(group);
        }];
    });
}
/**为group设置通知一个block，当group关联的block执行完毕后，就调用这个block*/
dispatch_group_notify(group, queue, ^{
    NSLog(@"It's done！！！");
});

PS: 手动管理group关联的block的运行状态（或计数），进入和退出group次数必须匹配

dispatch_group_enter(group);
dispatch_group_leave(group);

之前看GCD相关内容，将想要执行的代码写入下面block块中:

dispatch_group_async(group, queue, ^{
    /**此处写要执行的代码*/
});

说是和调用dispatch_group_enter(group); dispatch_group_leave(group);
等价的，经测试，其实不等价，dispatch_group_async里面只好执行同步的方法，如果是异步的请求，并不会等请求返回结果后再通知dispatch_group_notify进行处理！

---

信号量处理异步请求

dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

dispatch_group_async(group, queue, ^{ 
    [WBRequestManager startGETRequestHost:(NSString *)host urlString:urlString argument:param completionBlock:^(WBResponseModel *model) {
            dispatch_semaphore_signal(semaphore);
        }];
});

dispatch_group_async(group, queue, ^{ 
    [WBRequestManager startGETRequestHost:(NSString *)host urlString:urlString argument:param completionBlock:^(WBResponseModel *model) {
            dispatch_semaphore_signal(semaphore);
        }];
});

dispatch_group_notify(group, queue, ^{
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    
    dispatch_async(dispatch_get_main_queue(), ^{
        [self.tableView reloadData];
    });
});