• 这段代码就是对ParameterEncoding协议的三种不同实现，分别是URLEncoding，JSONEncoding，PropertyListEncoding。以及定义了Parameters类型

/// A dictionary of parameters to apply to a `URLRequest`.
public typealias Parameters = [String: Any]

/// A type used to define how a set of parameters are applied to a `URLRequest`.
public protocol ParameterEncoding {
    /// Creates a URL request by encoding parameters and applying them onto an existing request.
    ///
    /// - parameter urlRequest: The request to have parameters applied.
    /// - parameter parameters: The parameters to apply.
    ///
    /// - throws: An `AFError.parameterEncodingFailed` error if encoding fails.
    ///
    /// - returns: The encoded request.
    func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest
}

• ParameterEncoding协议定义了一个encode方法，返回encoded request

URLEncoding

定义了四个静态参数，用于快速获取对应的类型。default和methodDependent都返回默认值

    public enum Destination {
        case methodDependent, queryString, httpBody
    }

    // MARK: Properties

    /// Returns a default `URLEncoding` instance.
    public static var `default`: URLEncoding { return URLEncoding() }

    /// Returns a `URLEncoding` instance with a `.methodDependent` destination.
    public static var methodDependent: URLEncoding { return URLEncoding() }

    /// Returns a `URLEncoding` instance with a `.queryString` destination.
    public static var queryString: URLEncoding { return URLEncoding(destination: .queryString) }

    /// Returns a `URLEncoding` instance with an `.httpBody` destination.
    public static var httpBody: URLEncoding { return URLEncoding(destination: .httpBody) }

URLEncoding的encode方法：

    public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
        var urlRequest = try urlRequest.asURLRequest()

        guard let parameters = parameters else { return urlRequest }

        if let method = HTTPMethod(rawValue: urlRequest.httpMethod ?? "GET"), encodesParametersInURL(with: method) {
            guard let url = urlRequest.url else {
                throw AFError.parameterEncodingFailed(reason: .missingURL)
            }

            if var urlComponents = URLComponents(url: url, resolvingAgainstBaseURL: false), !parameters.isEmpty {
                let percentEncodedQuery = (urlComponents.percentEncodedQuery.map { $0 + "&" } ?? "") + query(parameters)
                urlComponents.percentEncodedQuery = percentEncodedQuery
                urlRequest.url = urlComponents.url
            }
        } else {
            if urlRequest.value(forHTTPHeaderField: "Content-Type") == nil {
                urlRequest.setValue("application/x-www-form-urlencoded; charset=utf-8", forHTTPHeaderField: "Content-Type")
            }

            urlRequest.httpBody = query(parameters).data(using: .utf8, allowLossyConversion: false)
        }

        return urlRequest
    }

• 检查urlRequest能否正常生成，parameters不能为空。
• 当destination为.queryString，或者method是.get, .head, .delete类型时，如果parameter不为空把里面的键值对传化为“xx=xx&yy=yy"的格式然后赋值给URLComponents.percentEncodedQuery最后urlRequest.url = urlComponents.url。另一种情况直接把query返回的字符串赋给httpBody

["a":"apple","b":"book","c":["conent","capacity","contact"],
"d":["4":"dose","5":"dairy"],"e":true,"f":12]
应该转化成：
"a=apple&b=book&c[]=conent&c[]=capacity
&c[]=contact&d[4]=dose&d[5]=dairy&e=1&f=12"
但是实际转化过程中使用了
public func escape(_ string: String) -> String 方法
所以最终结果是如下：
"a=apple&b=book&c%5B%5D=conent&c%5B%5D=capacity
&c%5B%5D=contact&d%5B4%5D=dose&d%5B5%5D=dairy&e=1&f=12"
其中的“[”,"]"变成了“%5B”,"%5D"

JSONEncoding

JSONEncoding有两个encode方法，除了ParameterEncoding的添加了一个encode(_ urlRequest:,withJSONObject jsonObject:Any? = nil) -> URLRequest,两个方法的实现基本相似并且很简单

  public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
        var urlRequest = try urlRequest.asURLRequest()

        guard let parameters = parameters else { return urlRequest }

        do {
            let data = try JSONSerialization.data(withJSONObject: parameters, options: options)

            if urlRequest.value(forHTTPHeaderField: "Content-Type") == nil {
                urlRequest.setValue("application/json", forHTTPHeaderField: "Content-Type")
            }

            urlRequest.httpBody = data
        } catch {
            throw AFError.parameterEncodingFailed(reason: .jsonEncodingFailed(error: error))
        }

        return urlRequest
    }

PropertyListEncoding

很少用到

    public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
        var urlRequest = try urlRequest.asURLRequest()

        guard let parameters = parameters else { return urlRequest }

        do {
            let data = try PropertyListSerialization.data(
                fromPropertyList: parameters,
                format: format,
                options: options
            )

            if urlRequest.value(forHTTPHeaderField: "Content-Type") == nil {
                urlRequest.setValue("application/x-plist", forHTTPHeaderField: "Content-Type")
            }

            urlRequest.httpBody = data
        } catch {
            throw AFError.parameterEncodingFailed(reason: .propertyListEncodingFailed(error: error))
        }

        return urlRequest
    }
}