你好，我是Emma，今天研究的课题是Alamofire中SessionManager和DataRequest以及task之间的关系。本篇文档以
SessionManager.default.request(urlString)
.response { (response) in debugPrint(response) }为切入点。
1. SessionManager类
open class SessionManager {
public static let `default`: SessionManager = {
let configuration = URLSessionConfiguration.default
configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders
return SessionManager(configuration: configuration)
}()
}
其中的SessionManager.defaultHTTPHeaders指的是什么？
public static let defaultHTTPHeaders: HTTPHeaders = {
let acceptEncoding: String = "gzip;q=1.0, compress;q=0.5"
let acceptLanguage = Locale.preferredLanguages.prefix(6).enumerated().map { index, languageCode in
let quality = 1.0 - (Double(index) * 0.1)
return "\(languageCode);q=\(quality)"
}.joined(separator: ", ")
let userAgent: String = {...}()
return [
"Accept-Encoding": acceptEncoding,
"Accept-Language": acceptLanguage,
"User-Agent": userAgent
]
}()
初始化方法：
open class SessionManager {
public init(
configuration: URLSessionConfiguration = URLSessionConfiguration.default,
//原生和Alomofire之间的代理移交
delegate: SessionDelegate = SessionDelegate(),
serverTrustPolicyManager: ServerTrustPolicyManager? = nil)
{
//设置代理这个类的delegate 就是 SessionDelegate = SessionDelegate()，做的是移交工作。
//这里是如何实现将UIApplicationDelegate后台的方法桥接到URLSessionDelegate中的初始化方法。这点体现了SessionDelegate这个类的强大之处。
self.delegate = delegate
//初始化这个类的URLSession
self.session = URLSession(configuration: configuration, delegate: delegate, delegateQueue: nil)
//提供给SessionDelegate的一些变量初始化
commonInit(serverTrustPolicyManager: serverTrustPolicyManager)
}
}
commonInit方法的具体实现：
//好吧，被SessionDelegate 的set方法附身之感
private func commonInit(serverTrustPolicyManager: ServerTrustPolicyManager?) {
session.serverTrustPolicyManager = serverTrustPolicyManager
//初始化中有self.delegate = delegate,这里有delegate.sessionManager持有self不会造成循环引用吗？不会。原因 `weak var sessionManager: SessionManager?`
delegate.sessionManager = self
//这个主要是给这个类提供backgroundCompletionHandler?()属性的作用。
delegate.sessionDidFinishEventsForBackgroundURLSession = { [weak self] session in
guard let strongSelf = self else { return }
DispatchQueue.main.async { strongSelf.backgroundCompletionHandler?() }
}
}
SessionManager与SessionDelegate的关联小节：
open class SessionManager {
public let delegate: SessionDelegate
public init?(delegate: SessionDelegate)
{
self.delegate = delegate
commonInit()
}
private func commonInit() {
delegate.sessionManager = self
}
}
open class SessionDelegate: NSObject {
weak var sessionManager: SessionManager?
}
//这样双方互相持有但是是打破循环链的持有。可以在SessionDelegate中做一些关于SessionManager的容错处理。
2.Request之DataRequest
下面进入今天的正题Request之DataRequest，首先看request方法：
@discardableResult
open func request(
_ url: URLConvertible,
method: HTTPMethod = .get,//指定是请求的方法类型
parameters: Parameters? = nil,//参数
encoding: ParameterEncoding = URLEncoding.default,//编码，此处默认的是URLEncoding.default
headers: HTTPHeaders? = nil)
-> DataRequest
{
var originalRequest: URLRequest?
do {
originalRequest = try URLRequest(url: url, method: method, headers: headers)
//重点重点，编码重点
let encodedURLRequest = try encoding.encode(originalRequest!, with: parameters)
return request(encodedURLRequest)
} catch {
return request(originalRequest, failedWith: error)
}
}
方法的百分号编码和区分方法的具体实现：
public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
var urlRequest = try urlRequest.asURLRequest()
//判断是否有参数
guard let parameters = parameters else { return urlRequest }
//根据HTTPMethod这个枚举进行派分
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 {
//进行百分号编码和添加&符号。
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
}
private func query(_ parameters: [String: Any]) -> String {
//创建元祖
var components: [(String, String)] = []
//遍历
for key in parameters.keys.sorted(by: <) {
let value = parameters[key]!
//进行递归操作去除key，value
components += queryComponents(fromKey: key, value: value)
}
//joined添加&分隔符，实例：A&B&C
return components.map { "\($0)=\($1)" }.joined(separator: "&")
}
从request方法闭包中的return request(encodedURLRequest)方法可以看出走的是下面这步
@discardableResult
open func request(_ urlRequest: URLRequestConvertible) -> DataRequest {
var originalRequest: URLRequest?
do {
//保存的请求
originalRequest = try urlRequest.asURLRequest()
//保存的任务
let originalTask = DataRequest.Requestable(urlRequest: originalRequest!)
let task = try originalTask.task(session: session, adapter: adapter, queue: queue)
//重点点击进入下面代码，RequestTask是枚举，区分了.data,.download,.stream,.upload
let request = DataRequest(session: session, requestTask: .data(originalTask, task))
//task和Request是意义对应的关联关系。
delegate[task] = request
if startRequestsImmediately { request.resume() }
return request
} catch {
return request(originalRequest, failedWith: error)
}
}
//保存了DataRequest的task适配器请求序列
struct Requestable: TaskConvertible {
let urlRequest: URLRequest
func task(session: URLSession, adapter: RequestAdapter?, queue: DispatchQueue) throws -> URLSessionTask {
do {
let urlRequest = try self.urlRequest.adapt(using: adapter)
//重点是通过请求找到dataTask
return queue.sync { session.dataTask(with: urlRequest) }
} catch {
throw AdaptError(error: error)
}
}
}
这个Request的初始化主要是一些与task的代理之间的持有关系和保存一些值。
open class Request{
init(session: URLSession, requestTask: RequestTask, error: Error? = nil) {
//session这是是用来进行保存session的。
self.session = session
// requestTask是一个枚举，进行任务和代理的分发
switch requestTask {
case .data(let originalTask, let task):
taskDelegate = DataTaskDelegate(task: task)
self.originalTask = originalTask
case .download(let originalTask, let task):
taskDelegate = DownloadTaskDelegate(task: task)
self.originalTask = originalTask
case .upload(let originalTask, let task):
taskDelegate = UploadTaskDelegate(task: task)
self.originalTask = originalTask
case .stream(let originalTask, let task):
taskDelegate = TaskDelegate(task: task)
self.originalTask = originalTask
}
delegate.error = error
//此处这个线程添加了addOperation ，其初始化见下面代码。
delegate.queue.addOperation { self.endTime = CFAbsoluteTimeGetCurrent() }
}
}
open class TaskDelegate: NSObject {
init(task: URLSessionTask?) {
//关于task代理中线程的初始化
self.queue = {
let operationQueue = OperationQueue()
operationQueue.maxConcurrentOperationCount = 1
operationQueue.isSuspended = true
operationQueue.qualityOfService = .utility
return operationQueue
}()
}
}
总结：
1)delegate[task] = request这样设计的原因是什么？
原因是实现SessionManager和Request之间的解耦。
2)那么SessionManager，Request，Task之间各自负责什么职能又是如何联系的？
SessionManager通过SessionDelegate来做请求的主体序列，通过这个来塞入一些头信息。请求体就有DataRequest等Request来负责，他负责的是一些URL，params，Method，Ecode，header等的打包，请求得开线程啊，线程这部分就分离出来由task负责，并且是Request和task是一对一的对练关系。
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