ios - 了解 AlamoFire OAuth 示例

Question

我能够获得 AlamoFire 提供的 OAuth 示例的有效实现。但是，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strongSelf 的guard 的目的是什么？

       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. 这个请求重试是如何工作的？ requestsToRetry 只是一个 RequestRetryCompletion = (_ shouldRetry: Bool, _ timeDelay: TimeInterval) 的数组 它如何知道要重试哪些请求？

strongSelf.lock.lock()

1. 在执行此方法时，NSLock 是否不允许任何其他线程访问 self (OAuth2Handler)？

Answer 1

1) 正如 Fonix 所评论的那样，您对 self 有很强的引用，以避免如果 self 为 nil 您开始收集保留周期..

我是指:

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

由于 self 将在异步调度的 block 中被捕获， self 将被隐式保留并在 block 完成时再次释放，换句话说 self 将一直延伸到 block 完成之后。编写这段代码，您可以避免延长 self 的生命周期，并决定在 self 等于 nil 时不执行该 block

2)根据你提到的台词:

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

有一个名为 requestsToRetry 的数组，其中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，您将移除所有项目。

事实上，消息来源报告:

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

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

您可以找到更多详细信息 here

3) 正如您所说，经常面临的并发问题与从不同线程访问/修改共享资源有关。lock.lock() 是一种在项目被修改时锁定其他执行 block 的解决方案。 defer 中的代码在离开函数以解锁 block 之前被调用。