我使用DispatchGroup尝试对我的客户端运行2个网络请求，并在两个请求都完成后返回结果。

我遇到一个问题，就是有时DispatchGroup中一个请求的完成处理程序被调用两次，而另一个请求根本没有被调用。

一个例子是-

    func fetchProfileWithRelatedArticle(onSuccess: @escaping (User,[RelatedArticle]) -> Void,onError: @escaping (Error) -> Void) {
        let dispatchGroup = DispatchGroup()

        var user: User?
        var articles: [RelatedArticle] = []

        var errors: [Error] = []

        dispatchGroup.enter()
        fetchProfileForUser(onSuccess: {
            user = $0
            print("fetchProfile:",$0)
            print("123")
            dispatchGroup.leave()
        },onError: { error in
            errors.append(error)
            dispatchGroup.leave()
        })

        dispatchGroup.enter()
        getarticlesForUser(onSuccess: {
            articles = $0
            print("getarticlesForUser:",$0)
            print("456")
            dispatchGroup.leave()
        },onError: { error in
            errors.append(error)
            dispatchGroup.leave()
        })

        dispatchGroup.notify(queue: .main) {
            guard let user = user,errors.isEmpty else { return }

            onSuccess(user,articles)
        }
    }

我在这里获取用户个人资料，还获取他们撰写的文章列表。这些通过完成处理程序返回并显示在其他位置。

在大多数情况下，此方法有效，但是有时这些请求之一会调用其自身的完成处理程序两次，而另一个请求则不会。

我怀疑这可能是我的访问令牌到期的原因，如果我短暂离开应用程序会出现这种情况。我的访问令牌的寿命为2分钟。

如果请求收到401响应，我的网络客户端中将有以下方法，该方法请求一个新令牌，然后再次调用该调用。我相信这可能无法按照我的意愿进行。

            if response.statusIs401() {
                self?.refreshHandler { success in
                    guard success else { completion(.failure(TokenError.refused)); return }
                    self?.request(resource,completion)
                }
                return
            }

我怀疑更新对我的调度组返回的请求执行了某些操作后，再次调用了该方法。

是否可以这种方式链接请求？

struct NoContent: Codable { }
typealias RefreshHandler = (@escaping (Bool) -> Void) -> ()
typealias TokenGetter = () -> [String: String]

protocol ClientType: class {
    associatedtype Route: RouterType
    func request<T: Codable>(_ resource: Route,_ completion: @escaping  (Result<T>)-> Void)
}

class Client<Route: RouterType>: ClientType {

    enum APIError: Error {
        case unknown,badResponse,jsonDecoder,other
    }

    enum TokenError: String,Error {
        case expired = "access Token Expired"
        case refused = "Refresh Token Failed"
    }

    private(set) var session: SessionType
    private(set) var tokenGetter: TokenGetter
    private(set) var refreshHandler: RefreshHandler

    private lazy var decoder: JSONDecoder = {
        let decoder = JSONDecoder()
        decoder.dateDecodingStrategy = .iso8601withFractionalSeconds
        return decoder
    }()

    init(session: SessionType,tokenGetter: @escaping TokenGetter,refreshHandler: @escaping RefreshHandler) {
        self.session = session
        self.tokenGetter = tokenGetter
        self.refreshHandler = refreshHandler
    }

    func request<T: Codable>(_ resource: Route,_ completion: @escaping  (Result<T>)-> Void) {

        let request = URLRequest(
            resource: resource,headers: tokenGetter()
        )

        URLSession.shared.dataTask(with: request) { [weak self] data,response,error in
            guard error == nil else { completion(.failure(APIError.unknown)); return }
            guard let response = response as? HTTPURLResponse else { completion(.failure(APIError.badResponse)); return }

            if response.statusIs401() {
                self?.refreshHandler { success in
                    guard success else { completion(.failure(TokenError.refused)); return }
                    self?.request(resource,completion)
                }
                return
            }

            if response.statusIsSuccess() {
                guard let self = self,let data = self.deserializeNoContentResponse(data: data) else { completion(.failure(APIError.badResponse)); return }
                do {
                    let value = try self.decoder.decode(T.self,from: data)
                    DispatchQueue.main.async {
                        completion(.success(value))
                    }
                } catch let error {
                    print(error)
                }
                return
            }

            completion(.failure(APIError.other))
        }.resume()
    }

    // some calls return a 200/201 with no data
    private func deserializeNoContentResponse(data: Data?) -> Data? {

        if data?.count == 0 {
            return "{ }".data(using: .utf8)
        }

        return data
    }
}