AFNetworking作为iOS开发中最常用的网络请求框架之一，其内部实现究竟是怎么一步一步实现的呢？现在我们来大致分析下其内部源码，由于本人水平有限，说错了的地方请大家积极指正，谢谢!

以3.0.0的版本为例,其源码结构如下:

源码结构.png

1.NSURLSession文件夹中的类专门用于网络通信,其中最核心的类是AFURLSessionManager,其中的AFHTTPSessionManager这个类是前者的子类，顾名思义，AFHTTPSessionManager这个类是专门用于Http请求,

2.Reachability中的AFNetworkRechabilityManager专门用于检测网络变化

3.Security用于网络安全策略

4.Serilization中的两个类专门用于请求头、响应头的处理
其类与类之间的关系如下图所示:

类与类之间关联.png

AF工作流程 .png

可以看到AFSessionManager中实现的最核心的类是Apple提供的系统类NSURLSession和NSURLSessionDataTask,这两个类提供了网络请求所需要的全部,由于我们最常使用的类是AFHTTPSessionManager,在进行网络请求时，我们一般使用一个AFHTTPSessionManager的实例变量来进行请求,如下代码所示:

   AFHTTPSessionManager *httpClient = [[AFHTTPSessionManager alloc] initWithBaseURL:[NSURL URLWithString:@"www.baidu.com"]];
    httpClient.requestSerializer = [AFHTTPRequestSerializer serializer];
    httpClient.responseSerializer = [AFHTTPResponseSerializer serializer];
    [httpClient POST:@""
          parameters:@{}
             success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
                 //获取responseObject的数据，在主线程做UI刷新操作
             } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {   
  }];

首先设置基地址:

- (instancetype)initWithBaseURL:(NSURL *)url
           sessionConfiguration:(NSURLSessionConfiguration *)configuration
{
    self = [super initWithSessionConfiguration:configuration];
    if (!self) {
        return nil;
    }
    //http://www.baidu.com 确保完整路径的基地址
    // Ensure terminal slash for baseURL path, so that NSURL +URLWithString:relativeToURL: works as expected
    if ([[url path] length] > 0 && ![[url absoluteString] hasSuffix:@"/"]) {
        url = [url URLByAppendingPathComponent:@""];
    }
    self.baseURL = url;
    //请求和响应序列化工具
    self.requestSerializer = [AFHTTPRequestSerializer serializer];
    self.responseSerializer = [AFJSONResponseSerializer serializer];
    return self;
}

这里设置了一个requestSerializer和responseSerializer,这两个类的作用是设置请求头、响应头相关的信息，主要用于传输不同类型的文件并设置不同的contentType等等,我们进入requestSerializer看下源码看具体做了什么:

- (instancetype)init {
    self = [super init];
    if (!self) {
        return nil;
    }
    //编码
    self.stringEncoding = NSUTF8StringEncoding;
    //设置请求头
    self.mutableHTTPRequestHeaders = [NSMutableDictionary dictionary];
    // Accept-Language HTTP Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4
    NSMutableArray *acceptLanguagesComponents = [NSMutableArray array];
    [[NSLocale preferredLanguages] enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        float q = 1.0f - (idx * 0.1f);
        [acceptLanguagesComponents addObject:[NSString stringWithFormat:@"%@;q=%0.1g", obj, q]];
        *stop = q <= 0.5f;
    }];
    //服务器接收语言
    [self setValue:[acceptLanguagesComponents componentsJoinedByString:@", "] forHTTPHeaderField:@"Accept-Language"];
    //用户代理
    NSString *userAgent = nil;
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wgnu"
#if TARGET_OS_IOS
    //获取请求设备的相关信息 用户代理
    // User-Agent Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43
    userAgent = [NSString stringWithFormat:@"%@/%@ (%@; iOS %@; Scale/%0.2f)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[UIDevice currentDevice] model], [[UIDevice currentDevice] systemVersion], [[UIScreen mainScreen] scale]];
#elif TARGET_OS_WATCH
    // User-Agent Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43
    userAgent = [NSString stringWithFormat:@"%@/%@ (%@; watchOS %@; Scale/%0.2f)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[WKInterfaceDevice currentDevice] model], [[WKInterfaceDevice currentDevice] systemVersion], [[WKInterfaceDevice currentDevice] screenScale]];
#elif defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
    userAgent = [NSString stringWithFormat:@"%@/%@ (Mac OS X %@)", [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleExecutableKey] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleIdentifierKey], [[NSBundle mainBundle] infoDictionary][@"CFBundleShortVersionString"] ?: [[NSBundle mainBundle] infoDictionary][(__bridge NSString *)kCFBundleVersionKey], [[NSProcessInfo processInfo] operatingSystemVersionString]];
#endif
#pragma clang diagnostic pop
    if (userAgent) {
        if (![userAgent canBeConvertedToEncoding:NSASCIIStringEncoding]) {
            NSMutableString *mutableUserAgent = [userAgent mutableCopy];
            if (CFStringTransform((__bridge CFMutableStringRef)(mutableUserAgent), NULL, (__bridge CFStringRef)@"Any-Latin; Latin-ASCII; [:^ASCII:] Remove", false)) {
                userAgent = mutableUserAgent;
            }
        }
        [self setValue:userAgent forHTTPHeaderField:@"User-Agent"];
    }

    // HTTP Method Definitions; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html
    self.HTTPMethodsEncodingParametersInURI = [NSSet setWithObjects:@"GET", @"HEAD", @"DELETE", nil];

    self.mutableObservedChangedKeyPaths = [NSMutableSet set];
    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self respondsToSelector:NSSelectorFromString(keyPath)]) {
            [self addObserver:self forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:AFHTTPRequestSerializerObserverContext];
        }
    }
    return self;
}

这些信息主要是和后台人员沟通确定的，比如一般的后台请求，我们要设置AcceptableContentType为applicaiont/json，注明接受类型是已经被后台人员转好的json，在回调中我们就可以直接获取到json数据，而不用去进行转换, 有的时候，我们在勤奋求的时候，报错"Request failed: unacceptable content-type",就需要在请求头的contentype列表里面添加一些指定的contentType.
其次就是主体的请求代码：

[httpClient POST:@""
          parameters:@{}
             success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
                 //获取responseObject的数据，在主线程做UI刷新操作
             } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
                 
             }];

我们点进去看看,可以看到所有的请求方法都是基于下面这个方法来构建的：

/**
 构建网络请求:所有请求(POST、GET、DELETE、PATCH、HEAD)都基于这个方法来构建起来的
 @param method POST、GET、DELETE、PATCH、HEAD
 @param URLString 请求地址
 @param parameters 请求参数
 @param uploadProgress 上传进度回调
 @param downloadProgress 下载进度列表
 @param success 成功回调
 @param failure 失败回调
 @return NSURLSessionDataTask网络请求
 */
- (NSURLSessionDataTask *)dataTaskWithHTTPMethod:(NSString *)method
                                       URLString:(NSString *)URLString
                                      parameters:(id)parameters
                                  uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgress
                                downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgress
                                         success:(void (^)(NSURLSessionDataTask *, id))success
                                         failure:(void (^)(NSURLSessionDataTask *, NSError *))failure

我们重点来分析下这个方法里面的代码:

// 根据请求地址、方法、参数构建网络请求NSMutableURLRequest
    NSMutableURLRequest *request = [self.requestSerializer requestWithMethod:method URLString:[[NSURL URLWithString:URLString relativeToURL:self.baseURL] absoluteString] parameters:parameters error:&serializationError];

这里使用了requestSerializer 的私有方法 method、params来构建了一个NSMutableURLRequest,并设置了请求体，请求头信息等等:代码如下:

- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
                               withParameters:(id)parameters
                                        error:(NSError *__autoreleasing *)error
{
    NSParameterAssert(request);

    NSMutableURLRequest *mutableRequest = [request mutableCopy];

    ///设置请求头信息
    [self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {
        if (![request valueForHTTPHeaderField:field]) {
            [mutableRequest setValue:value forHTTPHeaderField:field];
        }
    }];
    NSString *query = nil;
    if (parameters) {
        if (self.queryStringSerialization) {
            NSError *serializationError;
            query = self.queryStringSerialization(request, parameters, &serializationError);
            if (serializationError) {
                if (error) {
                    *error = serializationError;
                }
                return nil;
            }
        } else {
            switch (self.queryStringSerializationStyle) {
                case AFHTTPRequestQueryStringDefaultStyle:
                    query = AFQueryStringFromParameters(parameters);
                    break;
            }
        }
    }
   //判断是否是GET/HEAD/DELETE方法， 对于GET/HEAD/DELETE方法，把参数加到URL后面,这也就是GET请求里面的参数会暴露在请求地址里面
    if ([self.HTTPMethodsEncodingParametersInURI containsObject:[[request HTTPMethod] uppercaseString]]) {
        if (query) {
            mutableRequest.URL = [NSURL URLWithString:[[mutableRequest.URL absoluteString] stringByAppendingFormat:mutableRequest.URL.query ? @"&%@" : @"?%@", query]];
        }
    } else {
        // #2864: an empty string is a valid x-www-form-urlencoded payload
       //一般用于POST请求
        if (!query) {
            query = @"";
        }
        ///设置Content-Type HTTP头，告诉服务端body的参数编码类型
        if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {
            [mutableRequest setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];
        }
        //设置请求体信息 将json字符串转化为NSData类型
        [mutableRequest setHTTPBody:[query dataUsingEncoding:self.stringEncoding]];
    }
    return mutableRequest;
}

接下来的就是构架一个NSURLSessionDataTask:

__block NSURLSessionDataTask *dataTask = nil;
    dataTask = [self dataTaskWithRequest:request
                          uploadProgress:uploadProgress
                        downloadProgress:downloadProgress
                       completionHandler:^(NSURLResponse * __unused response, id responseObject, NSError *error) {
//成功失败回调
        if (error) {
            if (failure) {
                failure(dataTask, error);
            }
        } else {
            if (success) {
                success(dataTask, responseObject);
            }
        }
    }];  //使用这个方法构建的task 需要手动resume 才会启动任务
  ///使用NSURLSession和NSURLSessionDataTask请求数据
- (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(^{
    //这里使用了NSURLSession来实例化一个dataTask
        dataTask = [self.session dataTaskWithRequest:request];
    });
    //为task添加代理来传输数据等相关信息
    [self addDelegateForDataTask:dataTask uploadProgress:uploadProgressBlock downloadProgress:downloadProgressBlock completionHandler:completionHandler];
    return dataTask;
}

///给NSURLSessionDataTask对象实例设置Delegate。用于实时了解网络请求
- (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
{
    AFURLSessionManagerTaskDelegate *delegate = [[AFURLSessionManagerTaskDelegate alloc] init];
    delegate.manager = self;
    delegate.completionHandler = completionHandler;
    dataTask.taskDescription = self.taskDescriptionForSessionTasks;
//为dataTask设置代理来接受并处理数据
    [self setDelegate:delegate forTask:dataTask];
    delegate.uploadProgressBlock = uploadProgressBlock;
    delegate.downloadProgressBlock = downloadProgressBlock;
}

- (void)setDelegate:(AFURLSessionManagerTaskDelegate *)delegate
            forTask:(NSURLSessionTask *)task
{
    NSParameterAssert(task);
    NSParameterAssert(delegate);
   //加锁 避免同时修改数据，造成线程安全问题
    [self.lock lock];
    self.mutableTaskDelegatesKeyedByTaskIdentifier[@(task.taskIdentifier)] = delegate;
    [delegate setupProgressForTask:task];
    [self addNotificationObserverForTask:task];
    [self.lock unlock];
}

///这里使用KVO监测task的请求过程
- (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];
        }];
    }
    [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];
}

//监听下载和上传任务的数据并回调
- (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]) {
        //下载回调
        if (self.downloadProgressBlock) {
            self.downloadProgressBlock(object);
        }
    }
    else if ([object isEqual:self.uploadProgress]) {
        if (self.uploadProgressBlock) {
        //上传回调
            self.uploadProgressBlock(object);
        }
    }
}

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data {
  //获取NSURLSessionDataTask代理对象
    AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:dataTask];
    //试用delegate追加数据
    [delegate URLSession:session dataTask:dataTask didReceiveData:data];
    //数据接受回调
    if (self.dataTaskDidReceiveData) {
        self.dataTaskDidReceiveData(session, dataTask, data);
    }
}

//接收数据
- (void)URLSession:(__unused NSURLSession *)session
          dataTask:(__unused NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data
{
    [self.mutableData appendData:data];
}

@interface AFURLSessionManagerTaskDelegate : NSObject <NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate>

AFURLSessionManagerTaskDelegate接管了NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate的各种回调，然后做内部数据处理，。这也是第三方网络请求框架的重点，让网络请求更加易用，好用。
当然，由于本人技术有限，很多东西没理解透彻，如果有错误之处，欢迎大家指正，共同进步,后续的深入理解会继续写文章大家一起谈论,如果亲觉着这篇文章对于理解AFN有帮助的话，请下方点赞，谢谢!

-------作者:mrChan1234