嵌套的完成块没有被称为 objc答案

【问题标题】：Nested completionBlock is not getting called objc嵌套的完成块没有被称为 objc
【发布时间】：2017-11-12 20:21:58
【问题描述】：

这是我的嵌套块，请看一下：

- (void)getVideoList:(NSDictionary*)videoData
     completionBlock:(void (^)(NSMutableArray *))
completionBlock {
    NSArray *videos = (NSArray*)[videoData objectForKey:@"items"];
    NSMutableArray* videoList = [[NSMutableArray alloc] init];

    for (NSDictionary *videoDetail in videos) {
        if (videoDetail[@"id"][@"videoId"]){
            [self initializeDictionary:videoDetail completionBlock:^(YoutubeVideo * utubeVideo) {
                [videoList addObject:utubeVideo];
//                NSLog(@"zuuudo %@", utubeVideo.channelProfileImageURL);
            }];
        }
    }
    completionBlock(videoList);
}

- (void)initializeDictionary:(NSDictionary *)dictionary completionBlock:(void (^)(YoutubeVideo *))
completionBlock {
    YoutubeVideo *youtubeVideo = [[YoutubeVideo alloc] init];

    youtubeVideo.videoTitle = dictionary[@"snippet"][@"title"];
    youtubeVideo.videoID = dictionary[@"id"][@"videoId"];
    youtubeVideo.channelID = dictionary[@"snippet"][@"channelId"];
    [self getChannelProfilePictureForChannelID:youtubeVideo.channelID completionBlock:^(NSMutableArray *channelList) {
        NSLog(@"[channelList objectAtIndex:0] %@", [channelList objectAtIndex:0]);
        youtubeVideo.channelProfileImageURL = [channelList objectAtIndex:0];
    }];
    youtubeVideo.channelTitle = dictionary[@"snippet"][@"channelTitle"];
    youtubeVideo.videoDescription = dictionary[@"snippet"][@"description"];
    youtubeVideo.pubDate = [self dateWithJSONString:dictionary[@"snippet"][@"publishedAt"]];
    youtubeVideo.thumbnailURL = dictionary[@"snippet"][@"thumbnails"]
    [@"high"][@"url"];
    completionBlock(youtubeVideo);
}

- (void)getChannelProfilePictureForChannelID:(NSString*)channelID completionBlock:(void (^)(NSMutableArray *))completionBlock
{
    NSString *URL = [NSString stringWithFormat:@"https://www.googleapis.com/youtube/v3/channels?part=snippet&fields=items/snippet/thumbnails/default&id=%@&key=%@", channelID, apiKey];
    NSURLRequest *request = [[NSURLRequest alloc] initWithURL:[NSURL URLWithString:[URL stringByAddingPercentEncodingWithAllowedCharacters:[NSCharacterSet URLQueryAllowedCharacterSet]]]];

    NSURLSession *session = [NSURLSession sharedSession];
    [[session dataTaskWithRequest:request
                completionHandler:^(NSData *data,
                                    NSURLResponse *response,
                                    NSError *error) {
                    if (!error){
                        [self getChannelProfileImageList:[NSJSONSerialization JSONObjectWithData:data options:0 error:nil] completionBlock:
                         ^(NSMutableArray * channelList) {
                             // return the final list
                             completionBlock(channelList);
                         }];
                    }
                    else {
                        // TODO: better error handling
                        NSLog(@"error = %@", error);
                    }
                }] resume];
}

- (void)getChannelProfileImageList:(NSDictionary*)channelData
     completionBlock:(void (^)(NSMutableArray *))
completionBlock {
    NSArray *channels = (NSArray*)[channelData objectForKey:@"items"];
    NSMutableArray *channelList = [[NSMutableArray alloc] init];

    for (NSDictionary *channelDetail in channels) {
        [self initializeDictionaryForChannelProfileImage:channelDetail completionBlock:^(NSString *chnlProfileImageURL) {
            [channelList addObject:chnlProfileImageURL];
        }];
        //[channelList addObject:[self initializeDictionaryForChannelProfileImage:channelDetail]];
        //[channelList addObject:[[YoutubeVideo alloc] initWithDictionaryForChannelProfileImage:channelDetail]];
    }
    completionBlock(channelList);
}

- (void)initializeDictionaryForChannelProfileImage:(NSDictionary *)dictionary completionBlock:(void (^)(NSString *))
completionBlock
{
    _channelProfileImageURL = dictionary[@"snippet"][@"thumbnails"]
    [@"default"][@"url"];

    completionBlock(_channelProfileImageURL);
}

问题出在这个- (void)initializeDictionary:(NSDictionary *)dictionary completionBlock:(void (^)(YoutubeVideo *)) completionBlock { } 块中，有下面的块

[self getChannelProfilePictureForChannelID:youtubeVideo.channelID completionBlock:^(NSMutableArray *channelList) { NSLog(@"[channelList objectAtIndex:0] %@", [channelList objectAtIndex:0]); youtubeVideo.channelProfileImageURL = [channelList objectAtIndex:0]; }];

当这行代码没有执行时，块返回值NSSting值。

youtubeVideo.channelProfileImageURL = _channelProfileImageURL;
NSLog(@"youtubeVideo.channelProfileImageURL %@", youtubeVideo.channelProfileImageURL);

它在执行其余代码后被调用：

    youtubeVideo.channelTitle = dictionary[@"snippet"][@"channelTitle"];
    youtubeVideo.videoDescription = dictionary[@"snippet"][@"description"];
    youtubeVideo.pubDate = [self dateWithJSONString:dictionary[@"snippet"][@"publishedAt"]];
    youtubeVideo.thumbnailURL = dictionary[@"snippet"][@"thumbnails"]
    [@"high"][@"url"];

所以值没有插入到我的对象模型中。请给我一个建议。提前致谢。
祝你有美好的一天。

【问题讨论】：

标签： objective-c thread-safety grand-central-dispatch completion-block

【解决方案1】：

在执行其余代码后被调用

您将异步执行与期望代码将同步执行混为一谈：

- (void)initializeDictionary:(NSDictionary *)dictionary 
             completionBlock:(void (^)(YoutubeVideo *))completionBlock
{

这是一个典型的异步方法声明，其中completionBlock 参数应该在initializeDictionary 的所有工作完成后异步调用。

    YoutubeVideo *youtubeVideo = [[YoutubeVideo alloc] init];

    youtubeVideo.videoTitle = dictionary[@"snippet"][@"title"];
    youtubeVideo.videoID = dictionary[@"id"][@"videoId"];
    youtubeVideo.channelID = dictionary[@"snippet"][@"channelId"];

三个同步分配。

    [self getChannelProfilePictureForChannelID:youtubeVideo.channelID 
                               completionBlock:^(NSMutableArray *channelList)
       {
          NSLog(@"[channelList objectAtIndex:0] %@", [channelList objectAtIndex:0]);
          youtubeVideo.channelProfileImageURL = [channelList objectAtIndex:0];
       }
    ];

这是对另一个异步方法的嵌套调用，该方法将在完成后调用其完成块。在它返回时，它可能还没有调用它的竞争块。

    youtubeVideo.channelTitle = dictionary[@"snippet"][@"channelTitle"];
    youtubeVideo.videoDescription = dictionary[@"snippet"][@"description"];
    youtubeVideo.pubDate = [self dateWithJSONString:dictionary[@"snippet"][@"publishedAt"]];
    youtubeVideo.thumbnailURL = dictionary[@"snippet"][@"thumbnails"]
    [@"high"][@"url"];

另外四个同步分配...

    completionBlock(youtubeVideo);

然后你调用initializeDictionary:的完成块之前你知道getChannelProfilePictureForChannelID:已经完成并调用它的完成块。

如果你正在编写一个异步方法，它本身需要调用一个异步方法，那么你必须在嵌套异步方法的完成中完成你的方法......

是的，这有点令人困惑！让我们重新安排你的方法：

- (void)initializeDictionary:(NSDictionary *)dictionary 
             completionBlock:(void (^)(YoutubeVideo *))completionBlock
{
    YoutubeVideo *youtubeVideo = [[YoutubeVideo alloc] init];

    youtubeVideo.videoTitle = dictionary[@"snippet"][@"title"];
    youtubeVideo.videoID = dictionary[@"id"][@"videoId"];
    youtubeVideo.channelID = dictionary[@"snippet"][@"channelId"];

    youtubeVideo.channelTitle = dictionary[@"snippet"][@"channelTitle"];
    youtubeVideo.videoDescription = dictionary[@"snippet"][@"description"];
    youtubeVideo.pubDate = [self dateWithJSONString:dictionary[@"snippet"][@"publishedAt"]];
    youtubeVideo.thumbnailURL = dictionary[@"snippet"][@"thumbnails"]
    [@"high"][@"url"];

先做所有同步赋值，再做嵌套异步调用：

    [self getChannelProfilePictureForChannelID:youtubeVideo.channelID 
                               completionBlock:^(NSMutableArray *channelList)
       {
          NSLog(@"[channelList objectAtIndex:0] %@", [channelList objectAtIndex:0]);
          youtubeVideo.channelProfileImageURL = [channelList objectAtIndex:0];

此时getChannelProfilePictureForChannelID 的完成块已经完成了你想要它做的事情，现在做initializeDictionary: 在getChannelProfilePictureForChannelID 完成后需要做的任何剩余工作。这种情况下不多，直接调用initializeDictionary:比赛即可：

          completionBlock(youtubeVideo);
       }
    ];
}

HTH

附录

从您的 cmets 来看，我认为您误解了异步链接的工作原理。让我们看看以下是否有帮助。

你写的方法有格式：

A - block of work to do before async nested call
B - async call
   nested async completion block
   C - block of work to do after nested call completes
D - second block of work
E - call our async completion block

调用此方法时，A、B、D、E 会依次执行，然后方法返回。你不知道 C 什么时候执行，也不能保证它会在 E 之前执行，事实上，异步网络调用很可能不会（所以你甚至不可能得到意外的正确性）。

要执行一系列异步操作，您需要通过延续块将它们链接起来。因此，您可以将方法更改为：

A - block of work to do before async nested call
B - async call
   nested async completion block
   C - block of work to do after nested call completes
   D - second block of work
   E - call our async completion block

将 D & E 放入嵌套完成块中。现在，当您调用您的方法时，只有 A & B 在它返回之前执行。在稍后的某个时间点，嵌套的完成块被异步执行，C 和 D 被执行。最后执行E，原调用的完成块，从而完成工作。您现在已经保证了正确性，只有在嵌套的异步调用完成后才会执行 E。

注意：我在阅读您的代码时注意到的是，在嵌套调用之后似乎不需要执行块 D（代码中的四个赋值），因此我将您的代码重新排列为：

A & D - block of work to do before async nested call
B - async call
   nested async completion block
   C - block of work to do after nested call completes
   E - call our async completion block

将 D 提升到顶部。

当您有一个本身依赖于另一个异步方法的异步方法时，这种异步调用链是基础 - 在每个阶段您都必须使用完成块链以正确的顺序执行代码。

HTH

【讨论】：

非常感谢您清晰明了的解释。这很容易理解。但问题是[self getChannelProfilePictureForChannelID 在所有 4 次同步执行之后仍然被调用。我的意思是它已经返回了completionBlock(youtubeVideo);。我注意到的另一件事是 getChannelProfilePictureForChannelID 这个 [weakSelf getChannelProfileImageList:[NSJSONSerialization JSONObjectWithData:data options:0 error:nil] completionBlock: completionBlock 当时没有打电话。在 initializeDictionary 中完成所有方法后调用它。
那么我怎样才能设法从这里getChannelProfilePictureForChannelID 进一步调用那些asynchronize completionBlock，有点同步方式。
@Tulon - 添加了附录，希望对您有所帮助。