从 REST API 客户端返回一个通用的 swift 对象

Question

我正在实现一个 API 客户端，它将调用我的后端 API，并返回适当的对象或错误。

这是我到目前为止：

public typealias JSON = [String: Any]
public typealias HTTPHeaders = [String: String]

public enum RequestMethod: String {
    case get = "GET"
    case post = "POST"
    case put = "PUT"
    case delete = "DELETE"
}

public class APIClient {
    public func sendRequest(_ url: String,
                             method: RequestMethod,
                             headers: HTTPHeaders? = nil,
                             body: JSON? = nil,
                             completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void) {
        let url = URL(string: url)!
        var urlRequest = URLRequest(url: url)
        urlRequest.httpMethod = method.rawValue

        if let headers = headers {
            urlRequest.allHTTPHeaderFields = headers
            urlRequest.setValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")
        }

        if let body = body {
            urlRequest.httpBody = try? JSONSerialization.data(withJSONObject: body)
        }

        let session = URLSession(configuration: .default)
        let task = session.dataTask(with: urlRequest) { data, response, error in
            completionHandler(data, response, error)
        }

        task.resume()
    }
}

好的，所以我想要做的是这样的事情：

apiClient.sendRequest("http://example.com/users/1", ".get") { response in
    switch response {
    case .success(let user):
         print("\(user.firstName)")
    case .failure(let error):
         print(error)
    }
}

apiClient.sendRequest("http://example.com/courses", ".get") { response in
    switch response {
    case .success(let courses):
        for course in courses {
            print("\(course.description")
        }
    case .failure(let error):
         print(error)
    }
}

因此， apiClient.sendRequest() 方法必须将响应 json 解码为所需的 swift 对象，并返回该对象或错误对象。

我有这些结构：

struct User: Codable {
    var id: Int
    var firstName: String
    var lastName: String
    var email: String
    var picture: String
}

struct Course: Codable {
    var id: Int
    var name: String
    var description: String
    var price: Double
}

我也定义了这个 Result 枚举：

public enum Result<Value> {
    case success(Value)
    case failure(Error)
}

我被卡住的地方是，我不确定如何在 sendRequest() 中调整我的 completionHandler，以便我可以将它与 User 对象或 Course 对象或任何其他自定义对象一起使用。我知道我必须以某种方式使用泛型来实现这一点，并且我已经在 C# 中使用了泛型，但我在 Swift 4 中还不是很舒服，所以我们感谢任何帮助。

编辑：另外，我想知道如何将 sendRequest() 的响应向上冒泡到 ViewController 中的调用代码的一级，以便 ViewController 可以访问成功和失败的结果（以异步方式） .

Answer 1

这是您可以使用的方法，它将实际的 HTTP 工作转发到现有方法，并仅处理 json 解码：

public func sendRequest<T: Decodable>(for: T.Type = T.self,
                                      url: String,
                                      method: RequestMethod,
                                      headers: HTTPHeaders? = nil,
                                      body: JSON? = nil,
                                      completion: @escaping (Result<T>) -> Void) {

    return sendRequest(url, method: method, headers: headers, body:body) { data, response, error in
        guard let data = data else {
            return completion(.failure(error ?? NSError(domain: "SomeDomain", code: -1, userInfo: nil)))
        }
        do {
            let decoder = JSONDecoder()
            try completion(.success(decoder.decode(T.self, from: data)))
        } catch let decodingError {
            completion(.failure(decodingError))
        }
    }
}

，可以这样调用：

apiClient.sendRequest(for: User.self,
                      url: "https://someserver.com",
                      method: .get,
                      completion: { userResult in
                        print("Result: ", userResult)
})

，或者像这样：

apiClient.sendRequest(url: "https://someserver.com",
                      method: .get,
                      completion: { (userResult: Result<User>) -> Void in
                        print("Result: ", userResult)
})

, 通过指定完成签名并省略第一个参数。无论哪种方式，我们都让编译器推断其他东西的类型，如果我们提供足够的信息来这样做的话。

在多个方法之间拆分职责使它们更可重用，更易于维护和理解。

假设您将 api 客户端包装到另一个公开一些更通用方法的类中，这些方法隐藏了 api 客户端的复杂性，并允许通过仅传递相关信息从控制器调用，您最终可能会得到一些像这样的方法：

func getUserDetails(userId: Int, completion: @escaping (Result<User>) -> Void) {
    apiClient.sendRequest(for: User.self,
                          url: "http://example.com/users/1",
                          method: .get,
                          completion: completion)
}

，它可以简单地从控制器调用，如下所示：

getUserDetails(userId: 1) { result in
    switch result {
    case let .success(user):
        // update the UI with the user details
    case let .failure(error):
        // inform about the error
    }
}

通过在 上添加另一个重载sendRequest()，也可以轻松添加对解码数组的更新支持，以下是从答案开头开始的代码的小型重构版本：

private func sendRequest<T>(url: String,
                            method: RequestMethod,
                            headers: HTTPHeaders? = nil,
                            body: JSON? = nil,
                            completion: @escaping (Result<T>) -> Void,
                            decodingWith decode: @escaping (JSONDecoder, Data) throws -> T) {
    return sendRequest(url, method: method, headers: headers, body:body) { data, response, error in
        guard let data = data else {
            return completion(.failure(error ?? NSError(domain: "SomeDomain", code: -1, userInfo: nil)))
        }
        do {
            let decoder = JSONDecoder()
            // asking the custom decoding block to do the work
            try completion(.success(decode(decoder, data)))
        } catch let decodingError {
            completion(.failure(decodingError))
        }
    }
}

public func sendRequest<T: Decodable>(for: T.Type = T.self,
                                      url: String,
                                      method: RequestMethod,
                                      headers: HTTPHeaders? = nil,
                                      body: JSON? = nil,
                                      completion: @escaping (Result<T>) -> Void) {

    return sendRequest(url: url,
                       method: method,
                       headers: headers,
                       body:body,
                       completion: completion) { decoder, data in try decoder.decode(T.self, from: data) }
}

public func sendRequest<T: Decodable>(for: [T].Type = [T].self,
                                      url: String,
                                      method: RequestMethod,
                                      headers: HTTPHeaders? = nil,
                                      body: JSON? = nil,
                                      completion: @escaping (Result<[T]>) -> Void) {

    return sendRequest(url: url,
                       method: method,
                       headers: headers,
                       body:body,
                       completion: completion) { decoder, data in try decoder.decode([T].self, from: data) }
}

现在你也可以做这样的事情：

func getAllCourses(completion: @escaping (Result<[Course]>) -> Void) {
    return apiClient.sendRequest(for: User.self,
                                 url: "http://example.com/courses",
                                 method: .get,
                                 completion: completion)
}

// called from controller
getAllCourses { result in
    switch result {
    case let .success(courses):
        // update the UI with the received courses
    case let .failure(error):
        // inform about the error
    }
}