如何在Swift中提供一个错误类型的本地化描述?

我用Swift 3语法定义了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.)"

localizedDescription是否有办法返回我的自定义错误描述(“一个用户友好的错误描述。”)?注意，这里的错误对象的类型是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使它工作而不强制转换为错误类型呢?

当前回答

正如Xcode 8 beta 6发布说明中所述，

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.

如果转换了错误，则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，错误可以提供 userInfo字典(以及域和代码)。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

2016-08-27 01:09:40

其他回答

现在有两种错误接受协议，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" }

}

2017-07-22 16:21:28

我还要补充一点，如果你的误差有这样的参数

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

你可以像这样在本地化描述中调用这些参数:

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

2017-06-11 06:22:13

这个对我很有用:

NSError(domain: "com.your", code: 0, userInfo: [NSLocalizedDescriptionKey: "Error description"])

2020-08-26 03:31:33

这里有一个更优雅的解决方案:

  enum ApiError: String, LocalizedError {

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

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

  }

2017-07-29 19:22:54