swift - 从 Swift 异常生成错误表

Question

我想从 Swift 异常处理程序生成 NSAlert 样式的错误表。在过去使用 Objective-C 我可以简单地使用 [NSAlert alertWithError:] 现在使用 Swift 并且它自己的异常处理系统混合在一起，警报感觉相当笨拙:

enum Problems: ErrorType {
  case Bad
  case Worse
}

func f() throws {
  throw Problems.Worse
}

func g() {
  let errorMessage: String

  do {
    try f()
  }
  catch Problems.Bad {
    errorMessage = "This was bad"
  }
  catch Problems.Worse {
    errorMessage = "This was even worse"
  }
  catch {
    errorMessage = "This was unexpected"
  }

  guard errorMessage == nil else {
    let alert = NSAlert()
    alert.messageText = "Error"
    alert.informativeText = errorMessage
    alert.beginSheetModalForWindow(self, completionHandler: { [unowned self] (returnCode) -> Void in
      if returnCode == NSAlertFirstButtonReturn {
        // Handle the response
      }
    })
  }

如果有许多必须检查的错误条件，这感觉很笨拙，并且无法很好地扩展。有没有更好的办法？

Answer 1

Swift 允许您的 ErrorType 具有关联数据。这或多或少是 NSError 中 userInfo 的分解版本。如果你想要方便，你可以写一个函数来将 Problems 枚举转换为 NSError:

enum Problems: ErrorType {
    case Small
    case Bad(message: String?)
    case Worse(message: String?)

    func toNSError() -> NSError {
        var errorMessage = ""

        switch self {
        case .Small:
            errorMessage = "Small error"
        case let .Bad(msg):
            errorMessage = msg ?? "Default bad message"
        case let .Worse(msg):
            errorMessage = msg ?? "Default worse message"
        }

        return NSError(domain: NSCocoaErrorDomain, code: 1, userInfo: [NSLocalizedDescriptionKey: errorMessage])
    }
}

func f() throws {
    throw Problems.Worse(message: nil)
}


do {
    try f()
} catch let error as Problems {
    let err = error.toNSError()
    // now display NSAlert with err...
}

您可以使用 message: nil 抛出错误以获取默认消息，或者根据本地条件添加您自己的自定义消息:

throw Problems.Bad(message: "I have a bad feeling about this")