swift - 如何在单个失败时中止整个调度组操作而不返回多个失败完成处理程序？

Question

我在循环中使用调度组来跟踪一组网络请求，因此当它们全部成功完成时，成功完成处理程序仅返回一次。

但是，如果发生单个故障，我想中止整个操作并仅返回一次故障完成处理程序。我面临的问题是所有失败完成处理程序都会多次返回。这不是我想要的。

我的代码看起来像这样。

class NetworkClient {
    // ...
    func fetchBlogPosts(completion: @escaping (Result<[BlogPost], NetworkClientError>) -> Void) {
        let dispatchGroup = DispatchGroup()
        var blogPosts     = [BlogPost]()
        
        for (index, value) in blogPostJSONURLs.enumerated() {
            dispatchGroup.enter()
            
            guard let jsonURL = URL(string: blogPostJSONURLs[index]) else {
                dispatchGroup.leave()
                completion(.failure(.invalidURL))
                return
            }
            
            let dataTask = URLSession.shared.dataTask(with: jsonURL) { data, response, error in
                if error != nil {
                    dispatchGroup.leave()
                    completion(.failure(.errorReturned))
                    return
                }
                
                guard let httpResponse = response as? HTTPURLResponse, (200...299).contains(httpResponse.statusCode) else {
                    dispatchGroup.leave()
                    completion(.failure(.serverError))
                    return
                }
                
                guard let data = data else {
                    dispatchGroup.leave()
                    completion(.failure(.noData))
                    return
                }
                
                do {
                    let blogPost = try JSONDecoder().decode(BlogPost.self, from: data)
                    blogPosts.append(blogPost)
                    dispatchGroup.leave()
                } catch {
                    dispatchGroup.leave()
                    completion(.failure(.failedToDecodeJSON))
                }
            }
            
            dataTask.resume()
        }
        
        dispatchGroup.notify(queue: .main) {
            completion(.success(blogPosts))
        }
    }
}