Swift中的HTTP(十三) 基础鉴权对 web api 的 HTTP 请求通常需要有某种凭据。最简单的身份验证类

对 web api 的 HTTP 请求通常需要有某种凭据。最简单的身份验证类型是基本访问身份验证，在这篇文章中，我们将把这个功能添加到我们的库中。

当我们阅读基本身份验证的规范（或维基百科文章）时，我们看到它只是添加了一个授权标头，以及一个 base64 编码的用户名和密码。添加标头值是我们之前见过的，因此我们的 BasicAuth 加载器在原则上与我们的 ApplyEnvironment 加载器相似。

配置

我们需要的第一件事是一个结构来表示用户名和密码：

public struct BasicCredentials: Hashable, Codable {
    public let username: String
    public let password: String

    public init(username: String, password: String) { ... }
}

我们还需要一个 HTTPRequestOption 以便单个请求可以指定唯一的凭据：

extension BasicCredentials: HTTPRequestOption {
    public static let defaultOptionValue: BasicCredentials? = nil
}

extension HTTPRequest {
    public var basicCredentials: BasicCredentials? {
        get { self[option: BasicCredentials.self] }
        set { self[option: BasicCredentials.self] = newValue }
    }
}

顺便说一句，让我们转向加载器。

加载器

加载器的行为很简单：当请求进来时，加载器会查看它是否指定了自定义凭据。如果是这样，它将它们转换为正确的授权标头并将请求发送到链中。如果没有，它会将任何本地存储的凭据应用于请求，然后继续发送。如果它没有任何凭据，它会在检索一些请求时暂停传入请求。

让我们把它存根：

public protocol BasicAuthDelegate: AnyObject {
    func basicAuth(_ loader: BasicAuth, retrieveCredentials callback: @escaping (BasicCredentials?) -> Void)
}

public class BasicAuth: HTTPLoader {
    public weak var delegate: BasicAuthDelegate?

    private var credentials: BasicCredentials?
    private var pendingTasks = Array<HTTPTask>()

    public override func load(task: HTTPTask) {
        if let customCredentials = task.request.basicCredentials {
            self.apply(customCredentials, to: task)
        } else if let mainCredentials = credentials {
            self.apply(mainCredentials, to: task)
        } else {
            self.pendingTasks.append(task)
            // TODO: ask delegate for credentials
        }
    }

    private func apply(_ credentials: BasicCredentials, to task: HTTPTask) {
        let joined = credentials.username + ":" + credentials.password
        let data = Data(joined.utf8)
        let encoded = data.base64EncodedString()
        let header = "Basic (encoded)"
        task.request[header: "Authorization"] = header
    }
}

这是基本的实现，但不太正确。例如，如果同时收到许多请求，我们最终会多次向委托人询问凭据。我们需要在管理某些状态方面做得更好：

public class BasicAuth: HTTPLoader {
    public weak var delegate: BasicAuthDelegate?

    private enum State {
        case idle
        case retrievingCredentials(Array<HTTPTask>)
        case authorized(BasicCredentials)
    }

    private var state = State.idle

    public override func load(task: HTTPTask) {
        if let customCredentials = task.request.basicCredentials {
            self.apply(customCredentials, to: task)
            super.load(task: task)
            return
        } 

        // TODO: make this threadsafe
        switch state {
            case .idle:
                // we need to ask for credentials
                self.state = .retrievingCredentials([task])
                self.retrieveCredentials()

            case .retrievingCredentials(let others):
                // we are currently asking for credentials and waiting for the delegate
                self.state = .retrievingCredentials(others + [task])

            case .authorized(let credentials):
                // we have credentials
                self.apply(credentials, to: task)
                super.load(task: task)
        }
    }

    private func retrieveCredentials() {
        if let d = delegate {
            // we've got a delegate! Ask it for credentials
            // these credentials could come from storage (eg, the Keychain), from a UI ("log in" page), or somewhere else
            // that decision is entirely up to the delegate
            DispatchQueue.main.async {
                d.basicAuth(self, retrieveCredentials: { self.processCredentials($0) })
            }
        } else {
            // we don't have a delegate. Assume "nil" credentials
            self.processCredentials(nil)
        }
    }

    private func processCredentials(_ retrieved: BasicCredentials?) {
        // TODO: make this threadsafe

        guard case .retrievingCredentials(let pending) = state else {
            // we got credentials, but weren't waiting for them; do nothing
            // this could happen if we were "reset()" while waiting for the delegate
            return
        }

        if let credentials = retrieved {
            state = .authorized(credentials)
            for task in pending {
                self.apply(credentials, to: task)
                super.load(task: task)
            }
        } else {
            // we asked for credentials but didn't get any
            // all of these tasks will fail
            state = .idle
            pending.forEach { $0.fail(.cannotAuthenticate) }
        }
    }

    private func apply(_ credentials: BasicCredentials, to task: HTTPTask) {
        ...
    }
}

这看起来好多了，但它仍然缺少一些关键的东西，我将留给你来实现：我们需要注意一个任务在挂起时被取消（即 .retrievingCredentials 状态）这些都不是线程安全的我们需要 reset(with:) 逻辑来使挂起的任务失败并返回到 .idle 状态还有一些其他场景值得考虑：

此加载程序假定请求将始终经过身份验证。您将如何更改它以允许请求完全绕过身份验证，因为他们不需要它？
在 URLComponents 上有一种方法可以指定用户和密码。由于 HTTPRequest 在后台使用 URLComponents，您如何更改此加载器以在那里查找可能的授权信息，而不是（或除此之外）basicCredentials 请求选项？你认为这应该是 API 吗？为什么或者为什么不？
此加载程序不会检测请求何时未通过身份验证，例如返回 401 Unauthorized 或 403 Forbidden 的响应。这个装载机应该尝试检测吗？为什么或者为什么不？

正如我们所见，基本身份验证归结为向我们的传出请求简单添加一个标头。通过延迟传入请求，我们可以转身询问应用程序的另一部分（通过代理）是否有任何凭证供我们使用。

这种模式将在下一篇文章中继续为我们服务，届时我们将研究通过 OAuth 2.0 进行身份验证的更复杂的场景。