如何在Swift中提供带有Error类型的本地化描述？

Question

我正在使用Swift 3语法定义自定义错误类型，我想提供一个用户友好的错误描述，该错误由localizedDescription对象的Error属性返回。我该怎么做？

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。”）？请注意，此处的错误对象是Error类型而不是MyError。当然，我可以将对象强制转换为MyError

(error as? MyError)?.localizedDescription

但是有没有办法让它工作而不会转换为我的错误类型？

Answer 1

如Xcode 8 beta 6发行说明中所述，

Swift定义的错误类型可以通过采用新的LocalizedError协议提供本地化的错误描述。

在你的情况下：

public enum MyError: Error {
    case customError
}

extension MyError: LocalizedError {
    public var errorDescription: String? {
        switch self {
        case .customError:
            return NSLocalizedString("A user-friendly description of the error.", comment: "My error")
        }
    }
}

let error: Error = MyError.customError
print(error.localizedDescription) // A user-friendly description of the error.

如果错误转换为NSError（始终可以），您可以提供更多信息：

extension MyError : LocalizedError {
    public var errorDescription: String? {
        switch self {
        case .customError:
            return NSLocalizedString("I failed.", comment: "")
        }
    }
    public var failureReason: String? {
        switch self {
        case .customError:
            return NSLocalizedString("I don't know why.", comment: "")
        }
    }
    public var recoverySuggestion: String? {
        switch self {
        case .customError:
            return NSLocalizedString("Switch it off and on again.", comment: "")
        }
    }
}

let error = MyError.customError as NSError
print(error.localizedDescription)        // I failed.
print(error.localizedFailureReason)      // Optional("I don\'t know why.")
print(error.localizedRecoverySuggestion) // Optional("Switch it off and on again.")

通过采用CustomNSError协议，错误可以提供userInfo字典（以及domain和code）。例：

extension MyError: CustomNSError {

    public static var errorDomain: String {
        return "myDomain"
    }

    public var errorCode: Int {
        switch self {
        case .customError:
            return 999
        }
    }

    public var errorUserInfo: [String : Any] {
        switch self {
        case .customError:
            return [ "line": 13]
        }
    }
}

let error = MyError.customError as NSError

if let line = error.userInfo["line"] as? Int {
    print("Error in line", line) // Error in line 13
}

print(error.code) // 999
print(error.domain) // myDomain

Answer 2

我还要补充一下，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 {
  var errorDescription: String {
    switch self {
    case .responseStatusError(status: let status, message: let message):
      return "Error with status \(status) and message \(message) was thrown"
  }
}

你甚至可以这样缩短：

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

Answer 3

现在有两种错误采用协议，您的错误类型可以采用这些协议，以便为Objective-C - LocalizedError和CustomNSError提供附加信息。这是一个采用它们的示例错误：

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 4

使用结构可以是一种替代方案。静态本地化有点优雅：

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)")
   }
}

Answer 5

这是更优雅的解决方案：

  enum ApiError: String, LocalizedError {

    case invalidCredentials = "Invalid credentials"
    case noConnection = "No connection"

    var localizedDescription: String { return NSLocalizedString(self.rawValue, comment: "") }

  }