swift - 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 语法定义自定义错误类型，并且我想提供由 Error 的 localizedDescription 属性返回的错误的用户友好描述> 对象。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，这里的错误对象是 Error 类型，而不是 MyError 类型。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转换为我的错误类型？

Answer 1

如 Xcode 8 beta 6 发行说明中所述，

Swift-defined error types can provide localized error descriptions by adopting the new LocalizedError protocol.

就您而言:

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.

如果错误已转换，您可以提供更多信息到 NSError (这总是可能的):

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(protocol)，错误可以提供一个userInfo字典(还有一个domain和code)。示例:

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