升级到 Swift3.0 之后，新版本的 Alamofire 只支持 iOS 9.0 以上的系统，如果要适配 iOS 8，需要自己封装 URLSession，下面是笔者的方案：
这里使用的是 Swift 自己的原生类型 URLSession，而不是NSURLSession。
Alamofire 4.0 中的request方法的参数列表如下：
method参数的类型是HTTPMethod，这个是系统类型，可以从外部传值，默认值是get。在request方法的方法体中，调用SessionManager.default.request方法，接受了全部的外部参数，并返回一个组装好的 DataRequest对象：
@H_301_5@public func request( _ url: URLConvertible,headers: HTTPHeaders? = nil) -> DataRequest { return SessionManager.default.request( url,method: method,parameters: parameters,encoding: encoding,headers: headers ) }SessionManager.default.request方法的实现如下：
@H_301_5@@discardableResult open func request( _ url: URLConvertible,headers: HTTPHeaders? = nil) -> DataRequest { do { let urlRequest = try URLRequest(url: url,headers: headers) let encodedURLRequest = try encoding.encode(urlRequest,with: parameters) return request(encodedURLRequest) } catch { return request(FailedWith: error) } }这个方法主要完成的工作是加工request：使用url、method和headers三个参数创建一个URLRequest对象，然后把参数parameters中保存的HTTP请求携带的参数按照encoding所指定的编码方式进行编码得到最终的URLRequest对象，只有这两步都顺利完成了编码才算成功。成功后调用另一个重载的request方法，这个方法接受request字面量，可以直接传入生成的URLRequest对象。失败的情况下调用的是另一个重载的request方法，接受一个Error类型，实际上所有失败的情况下都会调用这个request方法。
所有重载版本的request方法最后都会返回一个 DataRequest类型，这个DataRequest是Alamofire封装的request对象，绕的有点晕。如果你准备自己封装，需要创建一个URLRequest对象代替DataRequest，这里我用了SwiftyJSON库，用来序列化网络返回的结果：
Alamofire的调用是函数式的，使用Alamofire请求返回一个son格式的数据的时候使用的是 responseJSON 方法，原来的格式大致如下：
@H_301_5@Alamofire.request(URL,method: .get,parameters:parameters,encoding:URLEncoding.default,headers:head).validate().responseJSON { response in switch response.result { case .success: //成功的操作 //调用completion(json,error) case .failure(let error): //失败的操作 //调用completion(json,error) } }responseJSON方法的回调是基于result的状态的，但是原生的URLResponse对象没有这个状态，所以你需要自己去判断成功与失败的状态：
@H_301_5@func httpRequest(url:String,parameters:[String:Any]?,completion:@escaping (_ json:JSON?,_ error:Error?)-> Void) { var head:[String:String]? //自定义HTTP头 let config = URLSessionConfiguration.default let session = URLSession(configuration: config) let trueURL = URL(string: baseURL + url)! func success(json:JSON){ completion(json,nil) } func fail(error:Error,json:JSON){ //错误处理 completion(json,error) } do { let request = try URLRequest(url: trueURL,headers: head) let encodedURLRequest = try URLEncoding.default.encode(request,with: parameters) let dataTask = session.dataTask(with: encodedURLRequest) { (data,error) in DispatchQueue.main.async { //下面的几种情况参照了responseJSON方法的实现 guard error == nil else { fail(error: error!,json:JSON(NSNull())) return } if let response = response as? HTTPURLResponse,[204,205].contains(response.statusCode) { success(json: JSON(NSNull())) return } guard let validData = data,validData.count > 0 else { fail(error:AFError.responseSerializationFailed(reason: .inputDatanil),json: JSON(NSNull())) return } //使用了SwiftyJSON的构造器 let js = JSON(data: validData) success(json: js) } } defer{ dataTask.resume() } } catch { print(error) } }此时如果删掉
@H_301_5@import Alamofire会发现有几处报错的地方，这是因为我们仍旧在使用Alamofire中的代码，首先HTTPMethod这个枚举类型是定义在Alamofire中的，因为原生API中指定HTTP方法使用的是字符串格式，编译器不会帮你检查错误，你可以把HTTPMethod的定义拷贝出来：
@H_301_5@public enum HTTPMethod: String { case options = "OPTIONS" case get = "GET" case head = "HEAD" case post = "POST" case put = "PUT" case patch = "PATCH" case delete = "DELETE" case trace = "TRACE" case connect = "CONNECT" }然后我们创建的URLRequest使用的实际上是Alamofire扩展的URLRequest，你需要自己动手写一个扩展，实现一个相同的构造器：
@H_301_5@extension URLRequest { public init(url: URL,method: HTTPMethod,headers: [String: String]? = nil) { self.init(url: url) httpMethod = method.rawValue if let headers = headers { for (headerField,headerValue) in headers { setValue(headerValue,forHTTPHeaderField: headerField) } } } }下一个问题是，Alamofire封装了一套把参数写进HTTP请求的编码方法，也就是你调用的：
@H_301_5@let encodedURLRequest = try URLEncoding.default.encode(request,with: parameters) @H_301_5@public typealias Parameters = [String: Any] public func escape(_ string: String) -> String { let generalDelimitersToEncode = ":#[]@" // does not include "?" or "/" due to RFC 3986 - Section 3.4 let subDelimitersToEncode = "!$&'()*+,;=" var allowedCharacterSet = CharacterSet.urlQueryAllowed allowedCharacterSet.remove(charactersIn: "\(generalDelimitersToEncode)\(subDelimitersToEncode)") return string.addingPercentEncoding(withAllowedCharacters: allowedCharacterSet) ?? string } public func queryComponents(fromKey key: String,value: Any) -> [(String,String)] { var components: [(String,String)] = [] if let dictionary = value as? [String: Any] { for (nestedKey,value) in dictionary { components += queryComponents(fromKey: "\(key)[\(nestedKey)]",value: value) } } else if let array = value as? [Any] { for value in array { components += queryComponents(fromKey: "\(key)[]",value: value) } } else if let value = value as? NSNumber { if value.isBool { components.append((escape(key),escape((value.boolValue ? "1" : "0")))) } else { components.append((escape(key),escape("\(value)"))) } } else if let bool = value as? Bool { components.append((escape(key),escape((bool ? "1" : "0")))) } else { components.append((escape(key),escape("\(value)"))) } return components } func query(_ parameters: [String: Any]) -> String { var components: [(String,String)] = [] for key in parameters.keys.sorted(by: <) { let value = parameters[key]! components += queryComponents(fromKey: key,value: value) } return components.map { "\($0)=\($1)" }.joined(separator: "&") } func encodesParametersInURL(with method: HTTPMethod) -> Bool { switch method { case .get,.head,.delete: return true default: return false } } public func encode(_ urlRequest: URLRequest,with parameters: Parameters?) throws-> URLRequest { var urlRequest = urlRequest 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 } extension NSNumber { fileprivate var isBool: Bool { return CFBooleanGetTypeID() == CFGetTypeID(self) } }这一系列方法抛出的错误也是Alamofire自己定义的，拷贝出来：
@H_301_5@public enum AFError: Error { 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) } case invalidURL(url: URL) case parameterEncodingFailed(reason: ParameterEncodingFailureReason) case multipartEncodingFailed(reason: MultipartEncodingFailureReason) case responseValidationFailed(reason: ResponseValidationFailureReason) case responseSerializationFailed(reason: ResponseSerializationFailureReason) } @H_301_5@func httpRequest(url:String,headers: head) //这里我没有设置参数，使用了默认的编码方式 let encodedURLRequest = try encode(request,error) in //下面是回调部分，需要手动切换线程 DispatchQueue.main.async { //处理回调 } } defer{ dataTask.resume() } } catch { print(error) } }