AFNetworking是基于NSURLSession實(shí)現(xiàn)的。回想一下NSURLSession的使用方式：

創(chuàng)建NSURLSessionConfig對(duì)象
用之前創(chuàng)建的NSURLSessionConfig對(duì)象創(chuàng)建配置NSURLSession對(duì)象。
用NSURLSession對(duì)象創(chuàng)建對(duì)應(yīng)的task對(duì)象 并用resume方法執(zhí)行之。
用delegate方法或completion block 響應(yīng)網(wǎng)絡(luò)事件及數(shù)據(jù)。
對(duì)應(yīng)于每次網(wǎng)絡(luò)會(huì)話（這里可以簡(jiǎn)單理解為一個(gè)網(wǎng)頁），對(duì)應(yīng)一個(gè)NSURLSession對(duì)象，而每個(gè)會(huì)話，可以生成若干task對(duì)象用于數(shù)據(jù)的交互。

而在AFNet中， AFURLSessionManager作為核心類，封裝并提供了上述網(wǎng)絡(luò)交互功能。

AFURLSessionManager組成

AFNet運(yùn)用了組合的設(shè)計(jì)模式，將不同功能搭建成AFURLSessionManager的功能。
AFURLSessionManager的主要屬性：

/**
The managed session.
*/
@property (readonly, nonatomic, strong) NSURLSession *session;

/**
The operation queue on which delegate callbacks are run.
*/
@property (readonly, nonatomic, strong) NSOperationQueue *operationQueue;

/**
Responses sent from the server in data tasks created with dataTaskWithRequest:success:failure: and run using the GET / POST / et al. convenience methods are automatically validated and serialized by the response serializer. By default, this property is set to an instance of AFJSONResponseSerializer.

@warning responseSerializer must not be nil.
*/
@property (nonatomic, strong) id <AFURLResponseSerialization> responseSerializer;

///-------------------------------
/// @name Managing Security Policy
///-------------------------------

/**
The security policy used by created session to evaluate server trust for secure connections. AFURLSessionManager uses the defaultPolicy unless otherwise specified.
*/
@property (nonatomic, strong) AFSecurityPolicy *securityPolicy;

if !TARGET_OS_WATCH

///--------------------------------------
/// @name Monitoring Network Reachability
///--------------------------------------

/**
The network reachability manager. AFURLSessionManager uses the sharedManager by default.
*/
@property (readwrite, nonatomic, strong) AFNetworkReachabilityManager *reachabilityManager;

endif

///----------------------------
/// @name Getting Session Tasks
///----------------------------

/**
The data, upload, and download tasks currently run by the managed session.
*/
@property (readonly, nonatomic, strong) NSArray <NSURLSessionTask *> *tasks;

/**
The data tasks currently run by the managed session.
*/
@property (readonly, nonatomic, strong) NSArray <NSURLSessionDataTask *> *dataTasks;

/**
The upload tasks currently run by the managed session.
*/
@property (readonly, nonatomic, strong) NSArray <NSURLSessionUploadTask *> *uploadTasks;

/**
The download tasks currently run by the managed session.
*/
@property (readonly, nonatomic, strong) NSArray <NSURLSessionDownloadTask *> *downloadTasks;

///-------------------------------
/// @name Managing Callback Queues
///-------------------------------

/**
The dispatch queue for completionBlock. If NULL (default), the main queue is used.
*/
@property (nonatomic, strong, nullable) dispatch_queue_t completionQueue;

/**
The dispatch group for completionBlock. If NULL (default), a private dispatch group is used.
*/
@property (nonatomic, strong, nullable) dispatch_group_t completionGroup;

AFNet的注釋很詳細(xì)，其屬性可以作如下分類

AFURLSessionManager所管理的Session對(duì)象
@property (readonly, nonatomic, strong) NSURLSession *session;

delegate所返回的NSOperationQueue
@property (readonly, nonatomic, strong) NSOperationQueue *operationQueue;

解析網(wǎng)絡(luò)返回?cái)?shù)據(jù)的對(duì)象（遵循AFURLResponseSerialization協(xié)議）
@property (nonatomic, strong) id responseSerializer;

用于處理網(wǎng)絡(luò)連接安全處理策略的AFSecurityPolicy對(duì)象
@property (nonatomic, strong) AFSecurityPolicy *securityPolicy;

用于檢測(cè)網(wǎng)絡(luò)數(shù)據(jù)的AFNetworkReachabilityManager對(duì)象
@property (readwrite, nonatomic, strong) AFNetworkReachabilityManager *reachabilityManager;

當(dāng)前的Session task
*tasks;

*dataTasks;

*uploadTasks;

*downloadTasks;

設(shè)置Call back隊(duì)列
在這里除了能設(shè)置Session的completion block外（默認(rèn)為main block），還可以設(shè)置completion group。

看過這些分類，是不是覺得AFURLSessionManager也不是多么復(fù)雜了？確實(shí)，AFNet本就是一套簡(jiǎn)潔的網(wǎng)絡(luò)框架，功能強(qiáng)大而結(jié)構(gòu)清晰。
下面，我們就一起看一看，利用AFURLSessionManager是如何完成一次簡(jiǎn)單的網(wǎng)絡(luò)請(qǐng)求的。

運(yùn)用AFURLSessionManager完成網(wǎng)絡(luò)請(qǐng)求

運(yùn)用AFRULSessionManager獲取網(wǎng)絡(luò)數(shù)據(jù)，僅需如下幾個(gè)步驟：

// step1. 初始化AFURLSessionManager對(duì)象
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:configuration];

// step2. 獲取AFURLSessionManager的task對(duì)象
NSURLSessionDataTask *dataTask = [manager dataTaskWithRequest:request completionHandler:^(NSURLResponse *response, id responseObject, NSError *error) {
if (error) {
NSLog(@"Error: %@", error);
} else {
NSLog(@"Get Net data success!");
}
}];

// step3. 發(fā)動(dòng)task
[dataTask resume];

OK，簡(jiǎn)單的三步，就完成了對(duì)網(wǎng)絡(luò)數(shù)據(jù)的請(qǐng)求。

下面我們就一步步分析，這三步背后AFNet的源代碼是如何實(shí)現(xiàn)的。

初始化AFURLSessionManager

要進(jìn)行網(wǎng)絡(luò)請(qǐng)求，第一步就是初始化AFURLSessionManager，調(diào)用函數(shù)

• (instancetype)initWithSessionConfiguration:(NSURLSessionConfiguration *)configuration
  1
  該函數(shù)很簡(jiǎn)單，只有一個(gè)NSURLSessionConfiguration用來配置AFURLSessionManager所管理的NSURLSession對(duì)象。

它的實(shí)現(xiàn)代碼如下：

(instancetype)initWithSessionConfiguration:(NSURLSessionConfiguration *)configuration {
self = [super init];
if (!self) {
return nil;
}
// =============== 設(shè)置NSURLSession ===============
// 若configuration為nil，則采用默認(rèn)defaultSessionConfiguration

if (!configuration) {
    configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
}

self.sessionConfiguration = configuration;
// session 的delegate callback queue， 最大并發(fā)為1 
self.operationQueue = [[NSOperationQueue alloc] init];
self.operationQueue.maxConcurrentOperationCount = 1;
// 初始化所管理的session
self.session = [NSURLSession sessionWithConfiguration:self.sessionConfiguration delegate:self delegateQueue:self.operationQueue];

// =============== 設(shè)置response 數(shù)據(jù)序列化 ===============
self.responseSerializer = [AFJSONResponseSerializer serializer];

// =============== 設(shè)置網(wǎng)絡(luò)安全策略 ===============
self.securityPolicy = [AFSecurityPolicy defaultPolicy];

// =============== 設(shè)置網(wǎng)絡(luò)狀態(tài)監(jiān)控Manager（注意這里是一個(gè)全局單例實(shí)現(xiàn)） ===============

if !TARGET_OS_WATCH

self.reachabilityManager = [AFNetworkReachabilityManager sharedManager];

endif

// =============== 設(shè)置存儲(chǔ)NSURL task與AFURLSessionManagerTaskDelegate的詞典（重點(diǎn)，在AFNet中，每一個(gè)task都會(huì)被匹配一個(gè)AFURLSessionManagerTaskDelegate 來做task的delegate事件處理） ===============
self.mutableTaskDelegatesKeyedByTaskIdentifier = [[NSMutableDictionary alloc] init];

// =============== 設(shè)置AFURLSessionManagerTaskDelegate 詞典的鎖，確保詞典在多線程訪問時(shí)的線程安全===============
self.lock = [[NSLock alloc] init];
self.lock.name = AFURLSessionManagerLockName;

// =============== 為所管理的session的所有task設(shè)置完成塊（這里不是很明白，既然是初始化函數(shù)，那么應(yīng)該沒有任何task在運(yùn)行才是，怎么需要在這里設(shè)置完成塊呢？？？）===============
[self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
for (NSURLSessionDataTask *task in dataTasks) {
[self addDelegateForDataTask:task uploadProgress:nil downloadProgress:nil completionHandler:nil];
}

    for (NSURLSessionUploadTask *uploadTask in uploadTasks) {
        [self addDelegateForUploadTask:uploadTask progress:nil completionHandler:nil];
    }

    for (NSURLSessionDownloadTask *downloadTask in downloadTasks) {
        [self addDelegateForDownloadTask:downloadTask progress:nil destination:nil completionHandler:nil];
    }
}];

// =============== 返回初始化好的self ===============
return self;
}
這就是AFURLSessionManager的初始化函數(shù)，主要是對(duì)其屬性進(jìn)行初始化，要注意的是它的私有屬性

@property (readwrite, nonatomic, strong) NSMutableDictionary *mutableTaskDelegatesKeyedByTaskIdentifier;

@property (readwrite, nonatomic, strong) NSLock *lock;

AFURLSessionManager會(huì)為每一個(gè)所管理的task對(duì)應(yīng)創(chuàng)建一個(gè)AFURLSessionManagerTaskDelegate對(duì)象，Manager會(huì)交ManagerTaskDelegate對(duì)象由來具體處理各個(gè)task事物，從而實(shí)現(xiàn)了同一個(gè)AFURLSessionManager對(duì)多個(gè)task的管理。

生成AFURLSessionManager的task對(duì)象

當(dāng)初始化好AFURLSessionManager后，就需要獲取一個(gè)代表我們當(dāng)前網(wǎng)絡(luò)請(qǐng)求的task了：

/**
Creates an NSURLSessionDataTask with the specified request.

@param request The HTTP request for the request.
@param completionHandler A block object to be executed when the task finishes. This block has no return value and takes three arguments: the server response, the response object created by that serializer, and the error that occurred, if any.
*/

• (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request
  completionHandler:(nullable void (^)(NSURLResponse *response, id _Nullable responseObject, NSError * _Nullable error))completionHandler;

當(dāng)然還可以獲取download task 和upload task， 這里我們僅分析最簡(jiǎn)單的data task。

其實(shí)現(xiàn)如下：

return [self dataTaskWithRequest:request uploadProgress:nil downloadProgress:nil completionHandler:completionHandler];
1
僅一行代碼，調(diào)用自身另一方法：

• (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request
  uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
  downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
  completionHandler:(nullable void (^)(NSURLResponse *response, id _Nullable responseObject, NSError * _Nullable error))completionHandler {

  __block NSURLSessionDataTask *dataTask = nil;
  url_session_manager_create_task_safely(^{
  dataTask = [self.session dataTaskWithRequest:request];
  });

  [self addDelegateForDataTask:dataTask uploadProgress:uploadProgressBlock downloadProgress:downloadProgressBlock completionHandler:completionHandler];

  return dataTask;
  }

上述方法完成了兩件事情：

1. 生成一個(gè)data task對(duì)象，并返回。
2. 為該data task對(duì)象生成一個(gè)匹配的AFURLSessionManagerTaskDelegate對(duì)象，并關(guān)聯(lián)之。

對(duì)于功能1，沒什么多說的，就是這里AFNet為了避免iOS 8.0以下版本中偶發(fā)的taskIdentifiers不唯一的bug，AFNet使用了

static void url_session_manager_create_task_safely(dispatch_block_t block) {
if (NSFoundationVersionNumber < NSFoundationVersionNumber_With_Fixed_5871104061079552_bug) {
// Fix of bug
// Open Radar:http://openradar.appspot.com/radar?id=5871104061079552 (status: Fixed in iOS8)
// Issue about:https://github.com/AFNetworking/AFNetworking/issues/2093
dispatch_sync(url_session_manager_creation_queue(), block);
} else {
block();
}
}

函數(shù)創(chuàng)建task對(duì)象。

其實(shí)現(xiàn)應(yīng)該是因?yàn)閕OS 8.0以下版本中會(huì)并發(fā)地創(chuàng)建多個(gè)task對(duì)象，而同步有沒有做好，導(dǎo)致taskIdentifiers 不唯一…

對(duì)于功能二，AFNet為每一個(gè)task生成對(duì)應(yīng)的task delegate對(duì)象，我們則應(yīng)該重點(diǎn)了解一下。

• (void)addDelegateForDataTask:(NSURLSessionDataTask *)dataTask
  uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
  downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
  completionHandler:(void (^)(NSURLResponse *response, id responseObject, NSError *error))completionHandler
  {
  // 創(chuàng)建AFURLSessionManagerTaskDelegate對(duì)象
  AFURLSessionManagerTaskDelegate *delegate = [[AFURLSessionManagerTaskDelegate alloc] init];
  // AFURLSessionManagerTaskDelegate與AFURLSessionManager建立相互關(guān)系
  delegate.manager = self;
  // 設(shè)置AF delegate的完成塊為用戶傳入的完成塊
  delegate.completionHandler = completionHandler;
  // 設(shè)置dataTask的taskDescription
  dataTask.taskDescription = self.taskDescriptionForSessionTasks;
  // ***** 將AF delegate對(duì)象與 dataTask建立關(guān)系
  [self setDelegate:delegate forTask:dataTask];
  // 設(shè)置AF delegate的上傳進(jìn)度，下載進(jìn)度塊。
  delegate.uploadProgressBlock = uploadProgressBlock;
  delegate.downloadProgressBlock = downloadProgressBlock;
  }

AFNet通過封裝AFURLSessionManagerTaskDelegate對(duì)象，對(duì)每個(gè)task 進(jìn)行管理，而AFURLSessionManager僅需要管理建立task 與AF delegate的詞典即可，實(shí)現(xiàn)了AFURLSessionManager功能的下放。
那么我們這里重點(diǎn)看一下，AF delegate與task是如何建立關(guān)系的：

[self setDelegate:delegate forTask:dataTask];

實(shí)現(xiàn)：

• (void)setDelegate:(AFURLSessionManagerTaskDelegate *)delegate
  forTask:(NSURLSessionTask *)task
  {
  NSParameterAssert(task);
  NSParameterAssert(delegate);
  // 加鎖，確保詞典的線程安全
  [self.lock lock];
  // 將AF delegate放入以taskIdentifier標(biāo)記的詞典中（同一個(gè)NSURLSession中的taskIdentifier是唯一的）
  self.mutableTaskDelegatesKeyedByTaskIdentifier[@(task.taskIdentifier)] = delegate;
  // 為AF delegate 設(shè)置task 的progress監(jiān)聽
  [delegate setupProgressForTask:task];
  [self addNotificationObserverForTask:task];
  // 詞典操作完畢，解鎖
  [self.lock unlock];
  }

AFURLSessionManager會(huì)將每個(gè)AF delegate放入其詞典中，同時(shí)，AF delegate會(huì)監(jiān)聽每個(gè)task的進(jìn)度，這是通過

[delegate setupProgressForTask:task];

實(shí)現(xiàn)的。我們繼續(xù)看 AF delegate 是如何監(jiān)聽task的各種進(jìn)度的：

pragma mark - NSProgress Tracking

• (void)setupProgressForTask:(NSURLSessionTask *)task {
  __weak typeof(task) weakTask = task;

  self.uploadProgress.totalUnitCount = task.countOfBytesExpectedToSend;
  self.downloadProgress.totalUnitCount = task.countOfBytesExpectedToReceive;
  [self.uploadProgress setCancellable:YES];
  [self.uploadProgress setCancellationHandler:^{
  typeof(weakTask) strongTask = weakTask;
  [strongTask cancel];
  }];
  [self.uploadProgress setPausable:YES];
  [self.uploadProgress setPausingHandler:^{
  typeof(weakTask) strongTask = weakTask;
  [strongTask suspend];
  }];
  if ([self.uploadProgress respondsToSelector:@selector(setResumingHandler:)]) {
  [self.uploadProgress setResumingHandler:^{
  typeof(weakTask) strongTask = weakTask;
  [strongTask resume];
  }];
  }

  [self.downloadProgress setCancellable:YES];
  [self.downloadProgress setCancellationHandler:^{
  typeof(weakTask) strongTask = weakTask;
  [strongTask cancel];
  }];
  [self.downloadProgress setPausable:YES];
  [self.downloadProgress setPausingHandler:^{
  typeof(weakTask) strongTask = weakTask;
  [strongTask suspend];
  }];

  if ([self.downloadProgress respondsToSelector:@selector(setResumingHandler:)]) {
  [self.downloadProgress setResumingHandler:^{
  typeof(weakTask) strongTask = weakTask;
  [strongTask resume];
  }];
  }

  // 這里為什么要用NSStringFromSelector 而不直接使用NSString???
  [task addObserver:self
  forKeyPath:NSStringFromSelector(@selector(countOfBytesReceived))
  options:NSKeyValueObservingOptionNew
  context:NULL];
  [task addObserver:self
  forKeyPath:NSStringFromSelector(@selector(countOfBytesExpectedToReceive))
  options:NSKeyValueObservingOptionNew
  context:NULL];

  [task addObserver:self
  forKeyPath:NSStringFromSelector(@selector(countOfBytesSent))
  options:NSKeyValueObservingOptionNew
  context:NULL];
  [task addObserver:self
  forKeyPath:NSStringFromSelector(@selector(countOfBytesExpectedToSend))
  options:NSKeyValueObservingOptionNew
  context:NULL];

  [self.downloadProgress addObserver:self
  forKeyPath:NSStringFromSelector(@selector(fractionCompleted))
  options:NSKeyValueObservingOptionNew
  context:NULL];
  [self.uploadProgress addObserver:self
  forKeyPath:NSStringFromSelector(@selector(fractionCompleted))
  options:NSKeyValueObservingOptionNew
  context:NULL];
  }

上面的代碼可以分為兩個(gè)部分，

1. 設(shè)置AF delegate的uploadProgress 和 downloadProgress。（注意為了防止對(duì)block對(duì)象截取產(chǎn)生的循環(huán)引用，將傳入的task設(shè)置為了weak。但是這里的循環(huán)引用在哪里呢??? 其實(shí)這里鑒于block所造成的一系列循環(huán)引用的問題，AFNet采取了一種防御式編程的方法，對(duì)于沒有必要在block中進(jìn)行強(qiáng)引用的變量，一律采用弱引用。并且，當(dāng)task置為nil后，block也沒有理由繼續(xù)strong引用task變量）
2. 利用KVO， 監(jiān)聽task屬性及uploadProgress，downloadProgress屬性的變化，進(jìn)一步監(jiān)聽task的數(shù)據(jù)傳輸進(jìn)程。

看到了KVO，那我們就直接看其對(duì)應(yīng)的響應(yīng)函數(shù)：

• (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
  if ([object isKindOfClass:[NSURLSessionTask class]]) {
  if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesReceived))]) {
  self.downloadProgress.completedUnitCount = [change[@"new"] longLongValue];
  } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesExpectedToReceive))]) {
  self.downloadProgress.totalUnitCount = [change[@"new"] longLongValue];
  } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesSent))]) {
  self.uploadProgress.completedUnitCount = [change[@"new"] longLongValue];
  } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesExpectedToSend))]) {
  self.uploadProgress.totalUnitCount = [change[@"new"] longLongValue];
  }
  }
  else if ([object isEqual:self.downloadProgress]) {
  if (self.downloadProgressBlock) {
  self.downloadProgressBlock(object);
  }
  }
  else if ([object isEqual:self.uploadProgress]) {
  if (self.uploadProgressBlock) {
  self.uploadProgressBlock(object);
  }
  }
  }

很簡(jiǎn)單，當(dāng)監(jiān)控的Task屬性改變時(shí)，同時(shí)改變由AF delegate所管理持有的Progress對(duì)象的屬性。
同時(shí)，當(dāng)AF delegate的Progress對(duì)象屬性改變時(shí)，調(diào)用對(duì)應(yīng)的progress block。

NSURLSessionDelegate 的響應(yīng)

因?yàn)锳FURLSessionManager所管理的NSURLSession對(duì)象的delegate被設(shè)置為AFURLSessionManager自身，因此所有的NSURLSessionDelegate回調(diào)方法的目的地都是AFURLSessionManager，而AFURLSessionManager又會(huì)根據(jù)是否需要具體處理，會(huì)將屬于AF delegate要響應(yīng)的delegate，傳遞到對(duì)應(yīng)的AF delegate去。
AFURLSessionManager 和 AFURLSessionManagerTaskDelegate 相應(yīng)的delegate回調(diào)方法如下：

AFURLSessionManager 需要處理的NSURLSessionDelegate：
這里寫圖片描述

AFURLSessionManagerTaskDelegate 需要處理的NSURLSessionDelegate：
這里寫圖片描述

當(dāng)AFURLSessionManager 覺得應(yīng)該有AF delegate來處理該事件時(shí)，會(huì)取出對(duì)應(yīng)task的delegate，并將該事件原封不動(dòng)的傳遞到delegate。
其實(shí)細(xì)看AF delegate所實(shí)現(xiàn)的NSURLSessionDelegate也不多，僅三條：

-URLSession:task:didCompleteWithError:
-URLSession:dataTask:didReceiveData:
-URLSession:downloadTask:didFinishDownloadingToURL:

可以看出，屬于AF task delegate處理的回調(diào)，兩條是對(duì)于completion的處理，另一條則是對(duì)于每個(gè)task，分別接受并記錄其receive data。

而AFURLSessionManager對(duì)于NSSession的回調(diào)處理相對(duì)簡(jiǎn)單，不外乎兩步：

1. 如果有對(duì)應(yīng)的user block，則將當(dāng)前NSSession block傳遞至user block（此處可能會(huì)利用user block的返回值，來繼續(xù)下一步操作）。
2. 如果需要AF task delegate處理，則將該回調(diào)事件傳給相應(yīng)的AF task delegate。

我們可以看下面幾個(gè)例子：

• (void)URLSession:(NSURLSession *)session
  task:(NSURLSessionTask *)task
  willPerformHTTPRedirection:(NSHTTPURLResponse *)response
  newRequest:(NSURLRequest *)request
  completionHandler:(void (^)(NSURLRequest *))completionHandler
  {
  NSURLRequest *redirectRequest = request;
  // step1. 看是否有對(duì)應(yīng)的user block
  if (self.taskWillPerformHTTPRedirection) {
  redirectRequest = self.taskWillPerformHTTPRedirection(session, task, response, request);
  }

  if (completionHandler) {
  // step2. 運(yùn)用user block返回值或是原始值，使NSSession事件繼續(xù)
  completionHandler(redirectRequest);
  }
  }

• (void)URLSession:(NSURLSession *)session
  task:(NSURLSessionTask *)task
  didCompleteWithError:(NSError *)error
  {
  // step1. 應(yīng)交由AF task delegate處理的事件，取出對(duì)應(yīng)AF task delegate，
  AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:task];

  // delegate may be nil when completing a task in the background
  if (delegate) {
  // step2. 將事件交由AF task delegate處理
  [delegate URLSession:session task:task didCompleteWithError:error];
  // NOTE: 此處是session task最終的回調(diào)函數(shù)，task不會(huì)再返回任何信息。因此刪除對(duì)應(yīng)的AF task delegate
  [self removeDelegateForTask:task];
  }
  // step3. 若有對(duì)應(yīng)的user block，則調(diào)用之
  if (self.taskDidComplete) {
  self.taskDidComplete(session, task, error);
  }
  }

AFURLSessionManager 與 AFURLSessionManagerTaskDelegate 的分工

到這里我們可以想一想，既然NSURLSession的delegate是AFURLSessionManager對(duì)象，那么為什么不在AFURLSessionManager中處理所有的事件回調(diào)，搞出來一個(gè)AFURLSessionManagerTaskDelegate干什么？

我們知道，NSURLSession的回調(diào)有很多，而當(dāng)我們啟動(dòng)一個(gè)task，真正想要獲取的信息是什么呢？就是網(wǎng)絡(luò)請(qǐng)求最終所返回的數(shù)據(jù)(我所進(jìn)行的網(wǎng)絡(luò)操作成功或是失敗，服務(wù)器為我返回的數(shù)據(jù))唄！ 其它的回調(diào)，什么認(rèn)證質(zhì)詢，task需要新的body stream，什么request即將重定向, 統(tǒng)統(tǒng)都是浮云，都是為了我們能夠最終獲取到服務(wù)器返回的數(shù)據(jù)所服務(wù)的。

另外我們也想要知道我們的網(wǎng)絡(luò)請(qǐng)求的進(jìn)度。
總結(jié)為兩點(diǎn)：

1. 獲取最終返回?cái)?shù)據(jù)
2. 獲知當(dāng)前請(qǐng)求的進(jìn)度

于是，AFNetWorking 便在紛繁復(fù)雜的回調(diào)處理中，特意抽象出AFURLSessionManagerTaskDelegate，用于應(yīng)付網(wǎng)絡(luò)返回?cái)?shù)據(jù)的處理（包括保存中間值，序列化返回值，錯(cuò)誤處理）和網(wǎng)絡(luò)請(qǐng)求進(jìn)度。
這樣做可以使代碼結(jié)構(gòu)更分明，邏輯更清晰。
這在AFURLSessionManagerTaskDelegate的聲明中也可以看出一二， 其屬性按功能分類就兩種，一種用來保存服務(wù)器返回?cái)?shù)據(jù)及completion回調(diào)，另一種就是反應(yīng)當(dāng)前網(wǎng)絡(luò)處理進(jìn)度：

@interface AFURLSessionManagerTaskDelegate : NSObject <NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate>
@property (nonatomic, weak) AFURLSessionManager *manager;
@property (nonatomic, strong) NSMutableData *mutableData; // 保存服務(wù)器返回的數(shù)據(jù)
@property (nonatomic, strong) NSProgress *uploadProgress; // 上傳進(jìn)度
@property (nonatomic, strong) NSProgress *downloadProgress; // 下載進(jìn)度
@property (nonatomic, copy) NSURL *downloadFileURL; // 下載文件目的存儲(chǔ)地址
@property (nonatomic, copy) AFURLSessionDownloadTaskDidFinishDownloadingBlock downloadTaskDidFinishDownloading;
@property (nonatomic, copy) AFURLSessionTaskProgressBlock uploadProgressBlock;
@property (nonatomic, copy) AFURLSessionTaskProgressBlock downloadProgressBlock;
@property (nonatomic, copy) AFURLSessionTaskCompletionHandler completionHandler;
@end