AFError 是Alamofire定义的errortype，包含几个不同的错误类型

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

在 Swift 中，我们可以定义这样的枚举类型，它的每一个枚举项都有一个附加信息，来扩充这个枚举项的信息表示，这又叫做关联值。这里的每个case都有一个关联值

• URLConvertible 是一个协议定义在Alamofire.swift中，表示可以转换成URL的类型
  
• 另外四种都是枚举定义在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除了定义了AFError和上面的枚举，还包括继承Error的AdaptError

代码后面剩下的都是extension，分为三类

• Error Booleans ：AFError添加了五个属性都是Bool类型，用于判断错误的类型。

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添加了五个属性，返回case的关联值。

 /// 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:每种错误的说明

  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
        }
    }

这段代码中没有涉及到Alamofire的核心功能，看这段源码只是为了以后能更加简单的理解Alamofire中复杂的代码。