swift - 如何保证在使用 AlamoFire 的 RequestInterceptor 类时一次只运行一次重试？

Question

我正在实现一种使用带有 AlamoFire5 的 OAuth2 刷新 session token 的方法，并且我正在尝试找出如何解决这种情况:

1 - When some request fail a refreshToken request must start, that must be the only one refreshToken request running at a time. i.e. the other requests that failed should not be retried until that request finishes.

2 - If the refreshToken finishes with an error the app must restarts and all the other request that were waiting must be cancelled.

3 - If the the refreshToken request succeeds the token must be updated and all the other requests waiting must now continue.

我正在使用 AlamoFire 的 RequestInterceptor 类来尝试解决这个问题，到目前为止我的实现是这样的:

final class RequestInterceptor: Alamofire.RequestInterceptor {
    
    private let disposeBag = DisposeBag()
    private let lock = NSRecursiveLock()

    private var refreshTokenParameters: TokenParameters {
        TokenParameters(clientId: "pdappclient",
                grantType: "refresh_token",
                refreshToken: KeychainManager.shared.refreshToken)
    }
    
    private let storage: AccessTokenStorage

    init(storage: AccessTokenStorage) {
        self.storage = storage
    }

    func adapt(_ urlRequest: URLRequest, for session: Session, completion: @escaping (Result<URLRequest, Error>) -> Void) {
        var urlRequest = urlRequest

        urlRequest.setValue("Bearer " + storage.accessToken, forHTTPHeaderField: "Authorization")

        completion(.success(urlRequest))
    }

    func retry(_ request: Request, for session: Session, dueTo error: Error, completion: @escaping (RetryResult) -> Void) {
        lock.lock()
        defer { lock.unlock() }
        
        guard let response = request.task?.response as? HTTPURLResponse, response.statusCode == 401 else {
            return completion(.doNotRetryWithError(error))
        }
        
        let refreshTokenRequest: Single<TokenResponse> = NetworkManager.shared
            .fetchData(fromApi: IdentityServerAPI.token(parameters: self.refreshTokenParameters))

        refreshTokenRequest.subscribe(onSuccess: { token in
            self.lock.unlock()
            self.storage.accessToken = token.accessToken ?? ""
            completion(.retry)
        }, onError: { error in
            self.lock.unlock()
            completion(.doNotRetryWithError(error))
        }).disposed(by: disposeBag)
    }
}

如何使用 RequestInterceptor 解决这种情况？

Answer 1

您可以使用数组来存储重试闭包，用于在 token 刷新完成之前可能发生的请求，并使用 bool 值来了解正在进行的刷新操作。
你最终会得到这样的结果:

final class RequestInterceptor: Alamofire.RequestInterceptor {
    
    private let disposeBag = DisposeBag()
    private let lock = NSRecursiveLock()

    private var refreshTokenParameters: TokenParameters {
        TokenParameters(
            clientId: "pdappclient",
            grantType: "refresh_token",
            refreshToken: KeychainManager.shared.refreshToken
        )
    }
    
    private let storage: AccessTokenStorage
    
    private var retryQueue = [(RetryResult) -> Void]()
    private var isTokenRefreshing = false

    init(storage: AccessTokenStorage) {
        self.storage = storage
    }

    func adapt(_ urlRequest: URLRequest, for session: Session, completion: @escaping (Result<URLRequest, Error>) -> Void) {
        var urlRequest = urlRequest

        urlRequest.setValue("Bearer " + storage.accessToken, forHTTPHeaderField: "Authorization")

        completion(.success(urlRequest))
    }

    func retry(_ request: Request, for session: Session, dueTo error: Error, completion: @escaping (RetryResult) -> Void) {
        lock.lock()
        defer { lock.unlock() }
        
        guard let response = request.task?.response as? HTTPURLResponse, response.statusCode == 401 else {
            completion(.doNotRetryWithError(error))
            return
        }
        retryQueue.append(completion)
        
        if !isTokenRefreshing {
            isTokenRefreshing = true

            let refreshTokenRequest: Single<TokenResponse> = NetworkManager.shared
                .fetchData(fromApi: IdentityServerAPI.token(parameters: self.refreshTokenParameters))

            refreshTokenRequest.subscribe(onSuccess: { token in
                self.lock.lock()
                defer { self.lock.unlock() }
                
                self.storage.accessToken = token.accessToken ?? ""
                
                self.retryQueue.forEach { $0(.retry) }
                self.retryQueue.removeAll()
                
                self.isTokenRefreshing = false
            }, onError: { error in
                self.lock.lock()
                defer { self.lock.unlock() }
                
                self.retryQueue.forEach { $0(.doNotRetryWithError(error)) }
                self.retryQueue.removeAll()
                
                self.isTokenRefreshing = false
            }).disposed(by: disposeBag)
        }
    }
}

请注意，如 defer文件指出:

A defer statement is used for executing code just before transferring program control outside of the scope that the defer statement appears in.

所以，先封defer语句将在 onSuccess 之前执行或 onError关闭。
这就是为什么我们需要再次锁定 onSuccess 内的源和 onError关闭。