Alamofire 对错误的处理考虑使用枚举,只要结果可能是有限的集合的情况下,其实枚举本身还是数据的一种载体，swift中，枚举有着很丰富的使用方法.
先总结一下swfit中enum中的用法：

1.正常用法

enum Movement {
    case Left
    case Right
    case Top
    case Bottom
}
let aMovement = Movement.Left
switch aMovement {
case .Left:
    print("left")
default:
    print("Unknow")
}
if case .Left = aMovement {
    print("Left")
}
if .Left == aMovement {
    print("Left")
}

2.声明为整型

enum Season: Int {
    case Spring = 0
    case Summer = 1
    case Autumn = 2
    case Winter = 3
}

.3.声明为字符串类型

enum House: String {
    case ZhangSan = "I am zhangsan"
    case LiSi = "I am lisi"
}
let zs = House.ZhangSan
print(zs.rawValue)
enum CompassPoint: String {
case North, South, East, West
}
let n = CompassPoint.North

4.声明为浮点类型

enum Constants: Double {
    case π = 3.14159
    case e = 2.71828
    case φ = 1.61803398874
    case λ = 1.30357
}
let pai = Constants.π
print(pai.rawValue)

5.其他类型

enum VNodeFlags : UInt32 {
    case Delete = 0x00000001
    case Write = 0x00000002
    case Extended = 0x00000004
    case Attrib = 0x00000008
    case Link = 0x00000010
    case Rename = 0x00000020
    case Revoke = 0x00000040
    case None = 0x00000080
}

6.enum包含enum

enum Character {

    enum Weapon {
        case Bow
        case Sword
        case Lance
        case Dagger
    }

    enum Helmet {
        case Wooden
        case Iron
        case Diamond
    }

    case Thief
    case Warrior
    case Knight
}

7.结构体和枚举

struct Scharacter {
    enum CharacterType {
        case Thief
        case Warrior
        case Knight
    }
    enum Weapon {
        case Bow
        case Sword
        case Lance
        case Dagger
    }
    let type: CharacterType
    let weapon: Weapon
}
let sc = Scharacter(type: .Thief, weapon: .Bow)
print(sc.type)

8.值关联

enum Trade {
    case Buy(stock: String, amount: Int)
    case Sell(stock: String, amount: Int)
}
let trade = Trade.Buy(stock: "Car", amount: 100)
if case let Trade.Buy(stock, amount) = trade {
    print("buy \(amount) of \(stock)")
}
enum Trade0 {
    case Buy(String, Int)
    case Sell(String, Int)
}
let trade0 = Trade0.Buy("Car0", 100)
if case let Trade0.Buy(stock, amount) = trade0 {
    print("buy \(amount) of \(stock)")
}

9.枚举中的函数

enum Wearable {
    enum Weight: Int {
        case Light = 2
    }

    enum Armor: Int {
        case Light = 2
    }

    case Helmet(weight: Weight, armor: Armor)


    func attributes() -> (weight: Int, armor: Int) {
        switch self {
        case .Helmet(let w, let a):
            return (weight: w.rawValue * 2, armor: a.rawValue * 4)

        }
    }
}

let test = Wearable.Helmet(weight: .Light, armor: .Light).attributes()
print(test)

enum Device {
    case iPad, iPhone, AppleTV, AppleWatch
    func introduced() -> String {
        switch self {
        case .AppleTV: return "\(self) was introduced 2006"
        case .iPhone: return "\(self) was introduced 2007"
        case .iPad: return "\(self) was introduced 2010"
        case .AppleWatch: return "\(self) was introduced 2014"
        }
    }
}
print (Device.iPhone.introduced())

10.枚举中的属性

enum Device1 {
    case iPad, iPhone
    var year: Int {
        switch self {
        case .iPad:
            return 2010
        case .iPhone:
            return 2007
    }
    }
}

let iPhone = Device1.iPhone
print(iPhone.year)

ParameterEncodingFailureReason

通过ParameterEncodingFailureReason我们能够很清楚的看出来这是一个参数编码的错误原因。大家注意reason这个词，在命名中，有或者没有这个词，表达的意境完全不同，因此，Alamofire牛逼就体现在这些细节之中。

public enum AFError: Error {
    /// The underlying reason the parameter encoding error occurred.
    ///
    /// - missingURL:                 The URL request did not have a URL to encode.
    /// - jsonEncodingFailed:         JSON serialization failed with an underlying system error during the
    ///                               encoding process.
    /// - propertyListEncodingFailed: Property list serialization failed with an underlying system error during
    ///                               encoding process.
    public enum ParameterEncodingFailureReason {
        case missingURL
        case jsonEncodingFailed(error: Error)
        case propertyListEncodingFailed(error: Error)
    }
 }

ParameterEncodingFailureReason本身是一个enum，同时，它又被包含在AFError之中，这说明枚举之中可以有另一个枚举。那么像这种情况我们怎么使用呢？看下边的代码：

let parameterErrorReason = AFError.ParameterEncodingFailureReason.missingURL

枚举的访问是一级一级进行的。我们再看这行代码：case jsonEncodingFailed(error: Error)。jsonEncodingFailed(error: Error)并不是函数，就是枚举的一个普通的子选项。(error: Error)是它的一个关联值，相对于任何一个子选项，我们都可以关联任何值，它的意义就在于，把这些值与子选项进行绑定，方便在需要的时候调用。我们会在下边讲解如何获取关联值。

参数编码有一下几种方式：

把参数编码到URL中
把参数编码到httpBody中
Alamofire中是如何进行参数编码的，这方面的内容会在后续的ParameterEncoding.swift这一篇文章中给出详细的解释。那么编码失败的原因可能为：

missingURL 给定的urlRequest.url为nil的情况抛出错误
jsonEncodingFailed(error: Error) 当选择把参数编码成JSON格式的情况下，参数JSON化抛出的错误
propertyListEncodingFailed(error: Error) 这个同上
综上所述，ParameterEncodingFailureReason封装了参数编码的错误，可能出现的错误类型为Error，说明这些所谓一般是调用系统Api产生的错误。

MultipartEncodingFailureReason

public enum MultipartEncodingFailureReason {
        case bodyPartURLInvalid(url: URL)
        case bodyPartFilenameInvalid(in: URL)
        case bodyPartFileNotReachable(at: URL)
        case bodyPartFileNotReachableWithError(atURL: URL, error: Error)
        case bodyPartFileIsDirectory(at: URL)
        case bodyPartFileSizeNotAvailable(at: URL)
        case bodyPartFileSizeQueryFailedWithError(forURL: URL, error: Error)
        case bodyPartInputStreamCreationFailed(for: URL)

        case outputStreamCreationFailed(for: URL)
        case outputStreamFileAlreadyExists(at: URL)
        case outputStreamURLInvalid(url: URL)
        case outputStreamWriteFailed(error: Error)

        case inputStreamReadFailed(error: Error)
    }

多部分编码错误一般发生在上传或下载请求中对数据的处理过程中，这里边最重要的是对上传数据的处理过程，会在后续的MultipartFormData.swift这一篇文章中给出详细的解释，我们就简单的分析下MultipartEncodingFailureReason子选项错误出现的原因：

bodyPartURLInvalid(url: URL) 上传数据时，可以通过fileURL的方式，读取本地文件数据，如果fileURL不可用，就会抛出这个错误
bodyPartFilenameInvalid(in: URL) 如果使用fileURL的lastPathComponent或者pathExtension获取filename为空抛出的错误
bodyPartFileNotReachable(at: URL) 通过fileURL不能访问数据，也就是不可达的
bodyPartFileNotReachableWithError(atURL: URL, error: Error) 这个不同于bodyPartFileNotReachable(at: URL),当尝试检测fileURL是不是可达的情况下抛出的错误
bodyPartFileIsDirectory(at: URL) 当fileURL是一个文件夹时抛出错误
bodyPartFileSizeNotAvailable(at: URL) 当使用系统Api获取fileURL指定文件的size出现错误
bodyPartFileSizeQueryFailedWithError(forURL: URL, error: Error) 查询fileURL指定文件size出现错误
bodyPartInputStreamCreationFailed(for: URL) 通过fileURL创建inputStream出现错误
outputStreamCreationFailed(for: URL) 当尝试把编码后的数据写入到硬盘时，创建outputStream出现错误
outputStreamFileAlreadyExists(at: URL) 数据不能被写入，因为指定的fileURL已经存在
outputStreamURLInvalid(url: URL) fileURL不是一个file URL
outputStreamWriteFailed(error: Error) 数据流写入错误
inputStreamReadFailed(error: Error) 数据流读入错误
综上所述，这些错误基本上都跟数据的操作相关，这个在后续会做出很详细的说明。

ResponseValidationFailureReason

public enum ResponseValidationFailureReason {
        case dataFileNil
        case dataFileReadFailed(at: URL)
        case missingContentType(acceptableContentTypes: [String])
        case unacceptableContentType(acceptableContentTypes: [String], responseContentType: String)
        case unacceptableStatusCode(code: Int)
    }

Alamofire不管请求是否成功，都会返回response。它提供了验证ContentType和StatusCode的功能，关于验证，再后续的文章中会有详细的解答，我们先看看这些原因：

dataFileNil 保存数据的URL不存在，这种情况一般出现在下载任务中，指的是下载代理中的fileURL缺失
dataFileReadFailed(at: URL) 保存数据的URL无法读取数据，同上
missingContentType(acceptableContentTypes: [String]) 服务器返回的response不包含ContentType且提供的acceptableContentTypes不包含通配符(通配符表示可以接受任何类型)
unacceptableContentType(acceptableContentTypes: [String], responseContentType: String) ContentTypes不匹配
unacceptableStatusCode(code: Int) StatusCode不匹配

ResponseSerializationFailureReason

public enum ResponseSerializationFailureReason {
    case inputDataNil
    case inputDataNilOrZeroLength
    case inputFileNil
    case inputFileReadFailed(at: URL)
    case stringSerializationFailed(encoding: String.Encoding)
    case jsonSerializationFailed(error: Error)
    case propertyListSerializationFailed(error: Error)
}

我们在上篇中已经提到，Alamofire支持把服务器的response序列成几种数据格式。

response 直接返回HTTPResponse，未序列化
responseData 序列化为Data
responseJSON 序列化为Json
responseString 序列化为字符串
responsePropertyList 序列化为Any
那么在序列化的过程中，很可能会发生下边的错误：

inputDataNil 服务器返回的response没有数据
inputDataNilOrZeroLength 服务器返回的response没有数据或者数据的长度是0
inputFileNil 指向数据的URL不存在
inputFileReadFailed(at: URL) 指向数据的URL无法读取数据

AFError

上边内容中介绍的ParameterEncodingFailureReason MultipartEncodingFailureReason ResponseValidationFailureReason和 ResponseSerializationFailureReason，他们是定义在AFError中独立的枚举，他们之间是包含和被包含的关系，理解这一点很重要，因为有了这种包含的管理，在使用中就需要通过AFError.ParameterEncodingFailureReason这种方式进行操作。

那么最重要的问题就是，如何把上边4个独立的枚举进行串联呢？Alamofire巧妙的地方就在这里，有4个独立的枚举，分别代表4大错误。也就是说这个网络框架肯定有这4大错误模块，我们只需要给AFError设计4个子选项，每个子选项关联上上边4个独立枚举的值就ok了。

这个设计真的很巧妙，试想，如果把所有的错误都放到AFError中，就显得非常冗余。那么下边的代码就呼之欲出了，大家好好体会体会在swift下这么设计的妙用：

case invalidURL(url: URLConvertible)
case parameterEncodingFailed(reason: ParameterEncodingFailureReason)
case multipartEncodingFailed(reason: MultipartEncodingFailureReason)
case responseValidationFailed(reason: ResponseValidationFailureReason)
case responseSerializationFailed(reason: ResponseSerializationFailureReason)

AFError的扩展

也许在开发中，我们完成了上边的代码就认为够用了，但对于一个开源框架而言，远远是不够的。我们一点点进行剖析：

现在给定一条数据：

 func findErrorType(error: AFError) {

  }

我只需要知道这个error是不是参数编码错误，应该怎么办？因此为AFError提供5个布尔类型的属性，专门用来获取当前的错误是不是某个指定的类型。这个功能的实现比较简单，代码如下：

extension AFError {
    /// Returns whether the AFError is an invalid URL error.
    public var isInvalidURLError: Bool {
        if case .invalidURL = self { return true }
        return false
    }

    /// Returns whether the AFError is a parameter encoding error. When `true`, the `underlyingError` property will
    /// contain the associated value.
    public var isParameterEncodingError: Bool {
        if case .parameterEncodingFailed = self { return true }
        return false
    }

    /// Returns whether the AFError is a multipart encoding error. When `true`, the `url` and `underlyingError` properties
    /// will contain the associated values.
    public var isMultipartEncodingError: Bool {
        if case .multipartEncodingFailed = self { return true }
        return false
    }

    /// Returns whether the `AFError` is a response validation error. When `true`, the `acceptableContentTypes`,
    /// `responseContentType`, and `responseCode` properties will contain the associated values.
    public var isResponseValidationError: Bool {
        if case .responseValidationFailed = self { return true }
        return false
    }

    /// Returns whether the `AFError` is a response serialization error. When `true`, the `failedStringEncoding` and
    /// `underlyingError` properties will contain the associated values.
    public var isResponseSerializationError: Bool {
        if case .responseSerializationFailed = self { return true }
        return false
    }
}

总而言之，这些都是给AFError这个枚举扩展的属性，还包含下边这些属性
urlConvertible: URLConvertible? 获取某个属性，这个属性实现了URLConvertible协议，在AFError中只有case invalidURL(url: URLConvertible)这个选项符合要求

/// The `URLConvertible` associated with the error.
      public var urlConvertible: URLConvertible? {
          switch self {
          case .invalidURL(let url):
              return url
          default:
              return nil
          }
      }

url: URL? 获取AFError中的URL，当然这个URL只跟MultipartEncodingFailureReason这个子选项有关

/// The `URL` associated with the error.
      public var url: URL? {
          switch self {
          case .multipartEncodingFailed(let reason):
              return reason.url
          default:
              return nil
          }
      }

underlyingError: Error? AFError中封装的所有的可能出现的错误中，并不是每种可能都会返回Error这个错误信息，因此这个属性是可选的

/// The `Error` returned by a system framework associated with a `.parameterEncodingFailed`,
      /// `.multipartEncodingFailed` or `.responseSerializationFailed` error.
      public var underlyingError: Error? {
          switch self {
          case .parameterEncodingFailed(let reason):
              return reason.underlyingError
          case .multipartEncodingFailed(let reason):
              return reason.underlyingError
          case .responseSerializationFailed(let reason):
              return reason.underlyingError
          default:
              return nil
          }
      }

acceptableContentTypes: [String]? 可接受的ContentType

/// The response `Content-Type` of a `.responseValidationFailed` error.
      public var responseContentType: String? {
          switch self {
          case .responseValidationFailed(let reason):
              return reason.responseContentType
          default:
              return nil
          }
      }

responseCode: Int? 响应码

/// The response code of a `.responseValidationFailed` error.
  public var responseCode: Int? {
      switch self {
      case .responseValidationFailed(let reason):
          return reason.responseCode
      default:
          return nil
      }
  }

failedStringEncoding: String.Encoding? 错误的字符串编码

/// The `String.Encoding` associated with a failed `.stringResponse()` call.
      public var failedStringEncoding: String.Encoding? {
          switch self {
          case .responseSerializationFailed(let reason):
              return reason.failedStringEncoding
          default:
              return nil
          }
      }

这里是一个小的分割线，在上边属性的获取中，也是用到了下边代码中的扩展功能：

extension AFError.ParameterEncodingFailureReason {
      var underlyingError: Error? {
          switch self {
          case .jsonEncodingFailed(let error), .propertyListEncodingFailed(let error):
              return error
          default:
              return nil
          }
      }
  }

  extension AFError.MultipartEncodingFailureReason {
      var url: URL? {
          switch self {
          case .bodyPartURLInvalid(let url), .bodyPartFilenameInvalid(let url), .bodyPartFileNotReachable(let url),
               .bodyPartFileIsDirectory(let url), .bodyPartFileSizeNotAvailable(let url),
               .bodyPartInputStreamCreationFailed(let url), .outputStreamCreationFailed(let url),
               .outputStreamFileAlreadyExists(let url), .outputStreamURLInvalid(let url),
               .bodyPartFileNotReachableWithError(let url, _), .bodyPartFileSizeQueryFailedWithError(let url, _):
              return url
          default:
              return nil
          }
      }

      var underlyingError: Error? {
          switch self {
          case .bodyPartFileNotReachableWithError(_, let error), .bodyPartFileSizeQueryFailedWithError(_, let error),
               .outputStreamWriteFailed(let error), .inputStreamReadFailed(let error):
              return error
          default:
              return nil
          }
      }
  }

  extension AFError.ResponseValidationFailureReason {
      var acceptableContentTypes: [String]? {
          switch self {
          case .missingContentType(let types), .unacceptableContentType(let types, _):
              return types
          default:
              return nil
          }
      }

      var responseContentType: String? {
          switch self {
          case .unacceptableContentType(_, let responseType):
              return responseType
          default:
              return nil
          }
      }

      var responseCode: Int? {
          switch self {
          case .unacceptableStatusCode(let code):
              return code
          default:
              return nil
          }
      }
  }

  extension AFError.ResponseSerializationFailureReason {
      var failedStringEncoding: String.Encoding? {
          switch self {
          case .stringSerializationFailed(let encoding):
              return encoding
          default:
              return nil
          }
      }

      var underlyingError: Error? {
          switch self {
          case .jsonSerializationFailed(let error), .propertyListSerializationFailed(let error):
              return error
          default:
              return nil
          }
      }
  }

错误描述

在开发中，如果程序遇到错误，我们往往会给用户展示更加直观的信息，这就要求我们把错误信息转换成易于理解的内容。因此我们只要实现LocalizedError协议就好了。这里边的内容很简单，在这里就直接把代码写上了，不做分析

extension AFError: LocalizedError {
    public var errorDescription: String? {
        switch self {
        case .invalidURL(let url):
            return "URL is not valid: \(url)"
        case .parameterEncodingFailed(let reason):
            return reason.localizedDescription
        case .multipartEncodingFailed(let reason):
            return reason.localizedDescription
        case .responseValidationFailed(let reason):
            return reason.localizedDescription
        case .responseSerializationFailed(let reason):
            return reason.localizedDescription
        }
    }
}

extension AFError.ParameterEncodingFailureReason {
    var localizedDescription: String {
        switch self {
        case .missingURL:
            return "URL request to encode was missing a URL"
        case .jsonEncodingFailed(let error):
            return "JSON could not be encoded because of error:\n\(error.localizedDescription)"
        case .propertyListEncodingFailed(let error):
            return "PropertyList could not be encoded because of error:\n\(error.localizedDescription)"
        }
    }
}

extension AFError.MultipartEncodingFailureReason {
    var localizedDescription: String {
        switch self {
        case .bodyPartURLInvalid(let url):
            return "The URL provided is not a file URL: \(url)"
        case .bodyPartFilenameInvalid(let url):
            return "The URL provided does not have a valid filename: \(url)"
        case .bodyPartFileNotReachable(let url):
            return "The URL provided is not reachable: \(url)"
        case .bodyPartFileNotReachableWithError(let url, let error):
            return (
                "The system returned an error while checking the provided URL for " +
                "reachability.\nURL: \(url)\nError: \(error)"
            )
        case .bodyPartFileIsDirectory(let url):
            return "The URL provided is a directory: \(url)"
        case .bodyPartFileSizeNotAvailable(let url):
            return "Could not fetch the file size from the provided URL: \(url)"
        case .bodyPartFileSizeQueryFailedWithError(let url, let error):
            return (
                "The system returned an error while attempting to fetch the file size from the " +
                "provided URL.\nURL: \(url)\nError: \(error)"
            )
        case .bodyPartInputStreamCreationFailed(let url):
            return "Failed to create an InputStream for the provided URL: \(url)"
        case .outputStreamCreationFailed(let url):
            return "Failed to create an OutputStream for URL: \(url)"
        case .outputStreamFileAlreadyExists(let url):
            return "A file already exists at the provided URL: \(url)"
        case .outputStreamURLInvalid(let url):
            return "The provided OutputStream URL is invalid: \(url)"
        case .outputStreamWriteFailed(let error):
            return "OutputStream write failed with error: \(error)"
        case .inputStreamReadFailed(let error):
            return "InputStream read failed with error: \(error)"
        }
    }
}

extension AFError.ResponseSerializationFailureReason {
    var localizedDescription: String {
        switch self {
        case .inputDataNil:
            return "Response could not be serialized, input data was nil."
        case .inputDataNilOrZeroLength:
            return "Response could not be serialized, input data was nil or zero length."
        case .inputFileNil:
            return "Response could not be serialized, input file was nil."
        case .inputFileReadFailed(let url):
            return "Response could not be serialized, input file could not be read: \(url)."
        case .stringSerializationFailed(let encoding):
            return "String could not be serialized with encoding: \(encoding)."
        case .jsonSerializationFailed(let error):
            return "JSON could not be serialized because of error:\n\(error.localizedDescription)"
        case .propertyListSerializationFailed(let error):
            return "PropertyList could not be serialized because of error:\n\(error.localizedDescription)"
        }
    }
}

extension AFError.ResponseValidationFailureReason {
    var localizedDescription: String {
        switch self {
        case .dataFileNil:
            return "Response could not be validated, data file was nil."
        case .dataFileReadFailed(let url):
            return "Response could not be validated, data file could not be read: \(url)."
        case .missingContentType(let types):
            return (
                "Response Content-Type was missing and acceptable content types " +
                "(\(types.joined(separator: ","))) do not match \"*/*\"."
            )
        case .unacceptableContentType(let acceptableTypes, let responseType):
            return (
                "Response Content-Type \"\(responseType)\" does not match any acceptable types: " +
                "\(acceptableTypes.joined(separator: ","))."
            )
        case .unacceptableStatusCode(let code):
            return "Response status code was unacceptable: \(code)."
        }
    }
}

总结

通过阅读AFError这篇代码，给了我很大的震撼，在代码的设计上，可以参考这种设计方式。
由于知识水平有限，如有错误，还望指出