AFError 是Alamofire定義的errortype，包含幾個不同的錯誤類型

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

在 Swift 中，我們可以定義這樣的枚舉類型，它的每一個枚舉項都有一個附加信息，來擴充這個枚舉項的信息表示，這又叫做關聯值。這里的每個case都有一個關聯值

• URLConvertible 是一個協議定義在Alamofire.swift中，表示可以轉換成URL的類型
  
• 另外四種都是枚舉定義在AFError中

    public enum ParameterEncodingFailureReason {
        case missingURL
        case jsonEncodingFailed(error: Error)
        case propertyListEncodingFailed(error: Error)
    }

    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)
    }

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

    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)
    }

另外AFError除了定義了AFError和上面的枚舉，還包括繼承Error的AdaptError

代碼后面剩下的都是extension，分為三類

• Error Booleans ：AFError添加了五個屬性都是Bool類型，用于判斷錯誤的類型。

public var isInvalidURLError: Bool {
        if case .invalidURL = self { return true }
        return false
    } 
public var isParameterEncodingError: Bool 
public var isMultipartEncodingError: Bool 
public var isResponseValidationError: Bool
public var isResponseSerializationError: Bool

• Convenience Properties ： AFError添加了五個屬性，返回case的關聯值。

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

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

    /// 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
        }
    }

• Error Descriptions:每種錯誤的說明

  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
        }
    }

這段代碼中沒有涉及到Alamofire的核心功能，看這段源碼只是為了以后能更加簡單的理解Alamofire中復雜的代碼。