Swift 3.0封装 URLSession 的GET/SET方法代替 Alamofire

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：

public func request(
_ url: URLConvertible,
method: HTTPMethod = .get,
parameters: Parameters? = nil,
encoding: ParameterEncoding = URLEncoding.default,
headers: HTTPHeaders? = nil)
-> DataRequest

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：

public func request(
_ url: URLConvertible,
method: HTTPMethod = .get,
parameters: Parameters? = nil,
encoding: ParameterEncoding = URLEncoding.default,
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：

@discardableResult
open func request(
_ url: URLConvertible,
method: HTTPMethod = .get,
parameters: Parameters? = nil,
encoding: ParameterEncoding = URLEncoding.default,
headers: HTTPHeaders? = nil)
-> DataRequest
{
do {
let urlRequest = try URLRequest(url: url, method: method, 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：

func httpRequest(url:String,method:HTTPMethod,parameters:[String:Any]?,completion:@escaping (_ json:JSON?,_ error:Error?)-> Void) {
//HTTP头部需要传入的信息，如果没有可以省略
var head:[String:String]?
//生成session
let config = URLSessionConfiguration.default
let session = URLSession(configuration: config)
let trueURL = URL(url)!
//请求成功时需要调用的代码封装为一个嵌套的方法，以便复用
func success(json:JSON){
completion(json,nil)
}
//同理请求失败需要执行的代码
func fail(error:Error,json:JSON){
completion(json,error)
}
do {
//自己封装一个request
let request = try URLRequest(url: trueURL, method: method, headers: head)
//这里我没有设置参数，使用了默认的编码方式
let encodedURLRequest = try URLEncoding.default.encode(request, with: parameters)
//生成一个dataTask
let dataTask = session.dataTask(with: encodedURLRequest) { (data, response, 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)
}

}

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：

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：

func httpRequest(url:String,method:HTTPMethod,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, method: method, headers: head)
let encodedURLRequest = try URLEncoding.default.encode(request, with: parameters)
let dataTask = session.dataTask(with: encodedURLRequest) { (data, response, 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

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：

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：

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：

let encodedURLRequest = try URLEncoding.default.encode(request, with: parameters)

这个方法中非常多依赖的方法，罗列如下：

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：

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)
}

现在报错的代码部分修改为：

func httpRequest(url:String,method:HTTPMethod,parameters:[String:Any]?,completion:@escaping (_ json:JSON?,_ error:Error?)-> Void) {
//HTTP头部需要传入的信息，如果没有可以省略
var head:[String:String]?
//生成session
let config = URLSessionConfiguration.default
let session = URLSession(configuration: config)
let trueURL = URL(url)!
//请求成功时需要调用的代码封装为一个嵌套的方法，以便复用
func success(json:JSON){
completion(json,nil)
}
//同理请求失败需要执行的代码
func fail(error:Error,json:JSON){
completion(json,error)
}
do {
//自己封装一个request
let request = try URLRequest(url: trueURL, method: method, 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, with: parameters)
//生成一个dataTask
let dataTask = session.dataTask(with: encodedURLRequest) { (data, response, 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)
}

}