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.)"

有没有办法让localizedDescription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?

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小开
最佳答案

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.

如果错误被转换，您可以提供更多的信息 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(还有domain和code)。例子:< / p > 
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
小开

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 .responseStatusError(status: let status, message: let message):
return "Error with status \(status) and message \(message) was thrown"
}
}

你甚至可以把它缩短成这样:

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

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" }


}
小开

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: "") }


}
小开

使用结构体也是一种选择。稍微优雅一点的静态本地化:

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)")
}
}
小开

这个对我很有用:

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

enum NetworkError: LocalizedError {
case noConnection


public var description: String {
///You can switch self here if you have multiple cases.
return "No internet connection"
}


// You need to implement `errorDescription`, not `localizedDescription`.
public var errorDescription: String? {
return description
}
}


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