ios - GCD机制在哪里调用？

Question

我关注 RAY WENDERLICH GCD tutorial- part 2 ，我不明白:

第一次实现

    - (void)downloadPhotosWithCompletionBlock:(BatchPhotoDownloadingCompletionBlock)completionBlock
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ // 1
 
        __block NSError *error;
        dispatch_group_t downloadGroup = dispatch_group_create(); // 2
 
        for (NSInteger i = 0; i < 3; i++) {
            NSURL *url;
            switch (i) {
                case 0:
                    url = [NSURL URLWithString:kOverlyAttachedGirlfriendURLString];
                    break;
                case 1:
                    url = [NSURL URLWithString:kSuccessKidURLString];
                    break;
                case 2:
                    url = [NSURL URLWithString:kLotsOfFacesURLString];
                    break;
                default:
                    break;
            }
 
            dispatch_group_enter(downloadGroup); // 3
            Photo *photo = [[Photo alloc] initwithURL:url
                                  withCompletionBlock:^(UIImage *image, NSError *_error) {
                                      if (_error) {
                                          error = _error;
                                      }
                                      dispatch_group_leave(downloadGroup); // 4
                                  }];
 
            [[PhotoManager sharedManager] addPhoto:photo];
        }
        dispatch_group_wait(downloadGroup, DISPATCH_TIME_FOREVER); // 5
        dispatch_async(dispatch_get_main_queue(), ^{ // 6
            if (completionBlock) { // 7
                completionBlock(error);
            }
        });
    });
}

第二次实现:

    - (void)downloadPhotosWithCompletionBlock:(BatchPhotoDownloadingCompletionBlock)completionBlock
{
    // 1
    __block NSError *error;
    dispatch_group_t downloadGroup = dispatch_group_create(); 
 
    for (NSInteger i = 0; i < 3; i++) {
        NSURL *url;
        switch (i) {
            case 0:
                url = [NSURL URLWithString:kOverlyAttachedGirlfriendURLString];
                break;
            case 1:
                url = [NSURL URLWithString:kSuccessKidURLString];
                break;
            case 2:
                url = [NSURL URLWithString:kLotsOfFacesURLString];
                break;
            default:
                break;
        }
 
        dispatch_group_enter(downloadGroup); // 2
        Photo *photo = [[Photo alloc] initwithURL:url
                              withCompletionBlock:^(UIImage *image, NSError *_error) {
                                  if (_error) {
                                      error = _error;
                                  }
                                  dispatch_group_leave(downloadGroup); // 3
                              }];
 
        [[PhotoManager sharedManager] addPhoto:photo];
    }
 
    dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{ // 4
        if (completionBlock) {
            completionBlock(error);
        }
    });
}

我的第一个实现相关代码被 dispatch_async 包围，一切都非常清晰。

但是，第二个实现还不清楚!我不明白，GCD 机制是如何处理进入和离开通知之外的任何部分的？

Answer 1

第一个实现从

开始运行一个后台线程

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ // 1

所以整个功能都在那里运行。但是这个后台线程最终在这一行被阻塞(它等待所有照片被下载):

 dispatch_group_wait(downloadGroup, DISPATCH_TIME_FOREVER);

这一切都很好，因为您没有阻塞主线程。但它的实现有点笨拙，因为，

• 你阻塞了一个你真的不需要的线程
• 代码看起来有点难看，因为整个函数都有 dispatch_async。

所以第二个实现有两个好处:

• 它并没有那么“丑陋”并且更具可读性(因为您摆脱了 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{})
• 并且没有 block ，因为 dispatch_group_notify 作为异步完成 block 。