概述
Alamofire跟AFNetworking的功能差不多，都是对URLSession的封装，对上层提供易用的网络请求接口。Alamofire和AFNetworking分别是Swift和OC的实现版本。目前，这两个网络封装库的关注度和使用率非常高，代码质量也相当不错。本文想通过对
Alamofire源码的简单分析，了解其基本框架和设计思路。源码链接：Alamofire
一个GET请求的源码分析
- 从最简单的Get请求入手，分析
Alamofire的代码。一个请求流程，可以分为请求发送流程和请求响应流程，下文将从这两个流程展开分析。
// Get请求的调用方式
Alamofire.request("https://httpbin.org/get").responseJSON { response in
print(response.request) // original URL request
print(response.response) // HTTP URL response
print(response.data) // server data
print(response.result) // result of response serialization
if let JSON = response.result.value {
print("JSON: \(JSON)")
}
}
请求发送流程
-
Alamofire.Swift可以认为Alamofire一些对外接口的包装（Facade API）。
Alamofire.request实际上是调用了SessionManager.request。
// 调用request方法
/// Creates a `DataRequest` using the default `SessionManager` to retrieve the contents of a URL based on the specified `urlRequest`.
@discardableResult
public func request(_ urlRequest: URLRequestConvertible) -> DataRequest {
return SessionManager.default.request(urlRequest)
}
- 在
SessionManager.request，Request被组装创建，并加到发送队列中，然后等待一系列的响应事件。而SessionManager主要职责是管理发送队列，组装请求消息，设置Session相关的配置，设置工作线程等。
// 创建request对象，并开始发送
/// Creates a `DataRequest` to retrieve the contents of a URL based on the specified `urlRequest`.
open func request(_ urlRequest: URLRequestConvertible) -> DataRequest {
var originalRequest: URLRequest?
do {
originalRequest = try urlRequest.asURLRequest()
let originalTask = DataRequest.Requestable(urlRequest: originalRequest!)
let task = try originalTask.task(session: session, adapter: adapter, queue: queue)
let request = DataRequest(session: session, requestTask: .data(originalTask, task))
delegate[task] = request
if startRequestsImmediately { request.resume() }
return request
} catch {
return request(originalRequest, failedWith: error)
}
}
- 接着，通过
Request.responseJSON设置JSON响应回调的处理方法。
// 设置回调
/// Adds a handler to be called once the request has finished.
@discardableResult
public func responseJSON(
queue: DispatchQueue? = nil,
options: JSONSerialization.ReadingOptions = .allowFragments,
completionHandler: @escaping (DataResponse) -> Void)
-> Self
{
return response(
queue: queue,
responseSerializer: DataRequest.jsonResponseSerializer(options: options),
completionHandler: completionHandler
)
}
-
Request.responseJSON实际上是调用Request.response，将回调添加到Request.delegate.queue，然后等待响应事件。
/// Adds a handler to be called once the request has finished.
@discardableResult
public func response(
queue: DispatchQueue? = nil,
responseSerializer: T,
completionHandler: @escaping (DataResponse) -> Void)
-> Self
{
delegate.queue.addOperation {
let result = responseSerializer.serializeResponse(
self.request,
self.response,
self.delegate.data,
self.delegate.error
)
var dataResponse = DataResponse(
request: self.request,
response: self.response,
data: self.delegate.data,
result: result,
timeline: self.timeline
)
dataResponse.add(self.delegate.metrics)
(queue ?? DispatchQueue.main).async { completionHandler(dataResponse) }
}
return self
}
- 至此，发送流程完成，接着就等待响应事件。
请求响应流程
一个请求的响应事件会有多个，并按循序上报，例如以下几个主要事件，
HTTPS鉴权事件
func urlSession(_ session: URLSession,task: URLSessionTask, didReceive challenge: URLAuthenticationChallenge, completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void)收到Response响应头事件
func urlSession(_ session: URLSession,dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition) -> Void)收到Response Body数据事件
func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data)响应流程完成事件
func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?)
本文以最后一个响应流程完成事件为例，梳理下整个响应流程。
首先，
SessionDelegate会收到由URLSession.delegate上报的urlSession:task:didCompleteWithError，根据task找到URLSessionTask并通过其delegate上报事件给TaskDelegate。
/// Tells the delegate that the task finished transferring data.
open func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
/// Executed after it is determined that the request is not going to be retried
let completeTask: (URLSession, URLSessionTask, Error?) -> Void = { [weak self] session, task, error in
guard let strongSelf = self else { return }
if let taskDidComplete = strongSelf.taskDidComplete {
taskDidComplete(session, task, error)
} else if let delegate = strongSelf[task]?.delegate {
delegate.urlSession(session, task: task, didCompleteWithError: error)
}
NotificationCenter.default.post(
name: Notification.Name.Task.DidComplete,
object: strongSelf,
userInfo: [Notification.Key.Task: task]
)
strongSelf[task] = nil
}
guard let request = self[task], let sessionManager = sessionManager else {
completeTask(session, task, error)
return
}
// Run all validations on the request before checking if an error occurred
request.validations.forEach { $0() }
// Determine whether an error has occurred
var error: Error? = error
if let taskDelegate = self[task]?.delegate, taskDelegate.error != nil {
error = taskDelegate.error
}
/// If an error occurred and the retrier is set, asynchronously ask the retrier if the request
/// should be retried. Otherwise, complete the task by notifying the task delegate.
if let retrier = retrier, let error = error {
retrier.should(sessionManager, retry: request, with: error) { [weak self] shouldRetry, timeDelay in
guard shouldRetry else { completeTask(session, task, error) ; return }
DispatchQueue.utility.after(timeDelay) { [weak self] in
guard let strongSelf = self else { return }
let retrySucceeded = strongSelf.sessionManager?.retry(request) ?? false
if retrySucceeded, let task = request.task {
strongSelf[task] = request
return
} else {
completeTask(session, task, error)
}
}
}
} else {
completeTask(session, task, error)
}
}
- 接着，
TaskDelegate收到该事件后，恢复queue队列，按循序执行其中的回调，如ResponseJSON。
@objc(URLSession:task:didCompleteWithError:)
func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
if let taskDidCompleteWithError = taskDidCompleteWithError {
taskDidCompleteWithError(session, task, error)
} else {
if let error = error {
if self.error == nil { self.error = error }
if
let downloadDelegate = self as? DownloadTaskDelegate,
let resumeData = (error as NSError).userInfo[NSURLSessionDownloadTaskResumeData] as? Data
{
downloadDelegate.resumeData = resumeData
}
}
// queue队列中的operaion开始按循序执行，回调到上层。
queue.isSuspended = false
}
}
其他模块
除了发送，响应相关的代码，
Alamofire还有许多其他模块。例如，NetworkReachabilityManager管理网络状态。ParameterEncoding入参编解码方式。ResponseSerialization响应的反序列化方式。ServerTrustPolicyHTTPS的鉴权等等。
总结
- 分析得比较简单，抱砖引玉。