了解 AlamoFire OAuth 示例

Question

我能够获得 AlamoFire 提供的 OAuth 示例的有效实现。不过，我希望了解某些代码行及其工作原理。

完整示例：

class OAuth2Handler: RequestAdapter, RequestRetrier {
    private typealias RefreshCompletion = (_ succeeded: Bool, _ accessToken: String?, _ refreshToken: String?) -> Void

    private let sessionManager: SessionManager = {
        let configuration = URLSessionConfiguration.default
        configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders

        return SessionManager(configuration: configuration)
    }()

    private let lock = NSLock()

    private var clientID: String
    private var baseURLString: String
    private var accessToken: String
    private var refreshToken: String

    private var isRefreshing = false
    private var requestsToRetry: [RequestRetryCompletion] = []

    // MARK: - Initialization

    public init(clientID: String, baseURLString: String, accessToken: String, refreshToken: String) {
        self.clientID = clientID
        self.baseURLString = baseURLString
        self.accessToken = accessToken
        self.refreshToken = refreshToken
    }

    // MARK: - RequestAdapter

    func adapt(_ urlRequest: URLRequest) throws -> URLRequest {
        if let urlString = urlRequest.url?.absoluteString, urlString.hasPrefix(baseURLString) {
            var urlRequest = urlRequest
            urlRequest.setValue("Bearer " + accessToken, forHTTPHeaderField: "Authorization")
            return urlRequest
        }

        return urlRequest
    }

    // MARK: - RequestRetrier

    func should(_ manager: SessionManager, retry request: Request, with error: Error, completion: @escaping RequestRetryCompletion) {
        lock.lock() ; defer { lock.unlock() }

        if let response = request.task?.response as? HTTPURLResponse, response.statusCode == 401 {
            requestsToRetry.append(completion)

            if !isRefreshing {
                refreshTokens { [weak self] succeeded, accessToken, refreshToken in
                    guard let strongSelf = self else { return }

                    strongSelf.lock.lock() ; defer { strongSelf.lock.unlock() }

                    if let accessToken = accessToken, let refreshToken = refreshToken {
                        strongSelf.accessToken = accessToken
                        strongSelf.refreshToken = refreshToken
                    }

                    strongSelf.requestsToRetry.forEach { $0(succeeded, 0.0) }
                    strongSelf.requestsToRetry.removeAll()
                }
            }
        } else {
            completion(false, 0.0)
        }
    }

    // MARK: - Private - Refresh Tokens

    private func refreshTokens(completion: @escaping RefreshCompletion) {
        guard !isRefreshing else { return }

        isRefreshing = true

        let urlString = "\(baseURLString)/oauth2/token"

        let parameters: [String: Any] = [
            "access_token": accessToken,
            "refresh_token": refreshToken,
            "client_id": clientID,
            "grant_type": "refresh_token"
        ]

        sessionManager.request(urlString, method: .post, parameters: parameters, encoding: JSONEncoding.default)
            .responseJSON { [weak self] response in
                guard let strongSelf = self else { return }

                if 
                    let json = response.result.value as? [String: Any], 
                    let accessToken = json["access_token"] as? String, 
                    let refreshToken = json["refresh_token"] as? String 
                {
                    completion(true, accessToken, refreshToken)
                } else {
                    completion(false, nil, nil)
                }

                strongSelf.isRefreshing = false
            }
    }
}

问题：

[weak self] succeeded, accessToken, refreshToken in
                    guard let strongSelf = self else { return }

1. [weak self] 和 guard for strongSelf 的用途是什么？

       requestsToRetry.append(completion)
   
       if !isRefreshing {
           refreshTokens { [weak self] succeeded, accessToken, refreshToken in
               guard let strongSelf = self else { return }
   
               //Implementation
   
               strongSelf.requestsToRetry.forEach { $0(succeeded, 0.0) }
               strongSelf.requestsToRetry.removeAll()
           }
       }
   

2. 此请求重试如何工作？ requestsToRetry 只是 RequestRetryCompletion = (_ shouldRetry: Bool, _ timeDelay: TimeInterval) 的数组，它如何知道要重试的请求？

strongSelf.lock.lock()

1. NSLock 是否不允许在此方法执行时任何其他线程访问 self (OAuth2Handler)？

Answer 1

1) 正如 Fonix 所评论的那样，您强烈引用 self 以避免如果 self 为 nil 您开始收集保留周期..

我指的是：

[weak self] ... in
guard let strongSelf = self else { return }

由于self 将被捕获在异步分派的块中，self 将被隐式保留并在块完成后再次释放，换句话说，self 将一直延伸到块完成后.编写此代码，您可以避免延长 self 的生命周期，并在 self 等于 nil 时决定不执行该块

2）根据您提到的行：

if let response = request.task?.response as? HTTPURLResponse, response.statusCode == 401 {
            requestsToRetry.append(completion)
            ..

有一个名为requestsToRetry 的数组包含您需要重新启动的所有请求。在此代码中，您将所有具有 401 状态代码的请求附加到数组（当服务器返回状态代码 401 时） 使用代码forEach 循环遍历requestToRetry 数组：

strongSelf.requestsToRetry.forEach { $0(succeeded, 0.0) }
strongSelf.requestsToRetry.removeAll()

并启动所有项目。循环结束后，您删除所有项目。

事实上，消息来源报道：

public typealias RequestRetryCompletion = (_ shouldRetry: Bool, _ timeDelay: TimeInterval) -> Void

public protocol RequestRetrier {
    func should(_ manager: SessionManager, retry request: Request, with error: Error, completion: @escaping RequestRetryCompletion)
}

您可以找到更多详情here

3）正如您所说，经常面临的并发问题是与从不同线程访问/修改共享资源有关的问题。 lock.lock() 是在修改项目时锁定其他执行块的解决方案。 defer 中的代码在离开函数解锁块之前被调用。