如何在Swift中提供带有错误类型的本地化描述？

Question

我正在使用Swift 3语法定义一个自定义错误类型，并希望提供一个用户友好的错误描述，该描述由Error对象的localizedDescription属性返回。我该怎么做呢？

public enum MyError: Error {
  case customError

  var localizedDescription: String {
    switch self {
    case .customError:
      return NSLocalizedString("A user-friendly description of the error.", comment: "My error")
    }
  }
}

let error: Error = MyError.customError
error.localizedDescription
// "The operation couldn’t be completed. (MyError error 0.)"

有没有办法让localizedDescription返回我的自定义错误描述(“用户友好的错误描述。”)？注意，这里的error对象的类型是Error，而不是MyError。当然，我可以将对象强制转换为MyError

(error as? MyError)?.localizedDescription

但是，有没有一种方法可以让它在不强制转换为我的错误类型的情况下工作？

Answer 1

我还要补充的是，如果你的错误有这样的参数

enum NetworkError: LocalizedError {
  case responseStatusError(status: Int, message: String)
}

您可以在本地化描述中调用这些参数，如下所示：

extension NetworkError {
  public var errorDescription: String? {
    switch self {
    case .responseStatusError(status: let status, message: let message):
      return "Error with status \(status) and message \(message) was thrown"
  }
}

您甚至可以像这样将其缩短：

extension NetworkError {
  public var errorDescription: String? {
    switch self {
    case let .responseStatusError(status, message):
      return "Error with status \(status) and message \(message) was thrown"
  }
}

Answer 2

现在有两个错误采用协议，您的错误类型可以采用，以便向Objective-C - LocalizedError和CustomNSError提供额外的信息。下面是一个同时采用这两种方法的错误示例：

enum MyBetterError : CustomNSError, LocalizedError {
    case oops

    // domain
    static var errorDomain : String { return "MyDomain" }
    // code
    var errorCode : Int { return -666 }
    // userInfo
    var errorUserInfo: [String : Any] { return ["Hey":"Ho"] };

    // localizedDescription
    var errorDescription: String? { return "This sucks" }
    // localizedFailureReason
    var failureReason: String? { return "Because it sucks" }
    // localizedRecoverySuggestion
    var recoverySuggestion: String? { return "Give up" }

}

Answer 3

使用结构可以是另一种选择。静态本地化的一点优雅：

import Foundation

struct MyError: LocalizedError, Equatable {

   private var description: String!

   init(description: String) {
       self.description = description
   }

   var errorDescription: String? {
       return description
   }

   public static func ==(lhs: MyError, rhs: MyError) -> Bool {
       return lhs.description == rhs.description
   }
}

extension MyError {

   static let noConnection = MyError(description: NSLocalizedString("No internet connection",comment: ""))
   static let requestFailed = MyError(description: NSLocalizedString("Request failed",comment: ""))
}

func throwNoConnectionError() throws {
   throw MyError.noConnection
}

do {
   try throwNoConnectionError()
}
catch let myError as MyError {
   switch myError {
   case .noConnection:
       print("noConnection: \(myError.localizedDescription)")
   case .requestFailed:
       print("requestFailed: \(myError.localizedDescription)")
   default:
      print("default: \(myError.localizedDescription)")
   }
}