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，我想提供一个用户友好的错误描述，该错误由< code>Error对象的< code > localized description 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.)"

有没有办法让本地化的描述返回我的自定义错误描述（“错误的用户友好描述”）？请注意，此处的错误对象类型为错误，而不是“我的错误”。当然，我可以把物体扔到我的错误

(error as? MyError)?.localizedDescription

但是有没有一种方法可以使它在不转换为我的错误类型的情况下工作？

Answer 1

现在有两个错误采用协议，您的错误类型可以采用它们，以便为 Objective-C 提供更多信息 —

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 2

我还要补充一点，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"
  }
}

你甚至可以像这样把它变短:

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

Answer 3

如 Xcode 8 测试版 6 发行说明中所述，

通过采用新的localized error协议，Swift定义的错误类型可以提供本地化的错误描述。

在您的情况下：

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（这总是可能的），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，错误可以提供一个用户信息dictionary（以及一个域代码）。例子：

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