先看一下afnetworking的目录结构：

图1.0

大家都看见了网络请求其实有两种方式。一种是用AFHTTPRequestOperationManager ，另一种是用AFHTTPSessionManager。那么这两种有什么区别尼？

可能有人有所不知，AFNetworking最初的版本使用的就是AFHTTPRequestOperationManager 以就是自己自定义NSOperation的封装实现的。AFHTTPSessionManager是在2.0以后才引入进来的。以就是说在2.0之前，都是使用AFHTTPRequestOperationManager。

AFHTTPSessionManager 继承自 AFURLSessionManager，而AFURLSessionManager主要是使用系统提供的 NSURLSession和NSURLSessionTask进行网络操作的。我们看一下官方文档对这两个类的描述：

NS_CLASS_AVAILABLE(NSURLSESSION_AVAILABLE, 7_0)
@interface NSURLSession : NSObject

/*
 * NSURLSessionTask - a cancelable object that refers to the lifetime
 * of processing a given request.
 */
NS_CLASS_AVAILABLE(NSURLSESSION_AVAILABLE, 7_0)
@interface NSURLSessionTask : NSObject <NSCopying>

是的，你没有看错，NSURLSession和NSURLSessionTask是iOS7以后才出现的，所以如果你想要适配到iOS6，那么请你乖乖的使用前者 进行网络请求。这就解释了为什么作者会把两种请求方式都放在这儿。

下面分两部分进行网络AFNetworking网络请求的分析：
AFHTTPRequestOperationManager 部分的网络请求原理：

AFHTTPRequestOperationManager的init方法是这样的：

- (instancetype)initWithBaseURL:(NSURL *)url {
    self = [super init];
    if (!self) {
        return nil;
    }

    // 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;

    self.requestSerializer = [AFHTTPRequestSerializer serializer];
    self.responseSerializer = [AFJSONResponseSerializer serializer];

    self.securityPolicy = [AFSecurityPolicy defaultPolicy];

    self.reachabilityManager = [AFNetworkReachabilityManager sharedManager];

    self.operationQueue = [[NSOperationQueue alloc] init];

    self.shouldUseCredentialStorage = YES;

    return self;
}

其中默认的请求方式和解析方式被设置了默认值：

self.requestSerializer = [AFHTTPRequestSerializer serializer];
self.responseSerializer = [AFJSONResponseSerializer serializer];

当然用户可以修改这两个参数，指定自己的请求方式和解析方式。

下面以GET请求为例来说。
当用户发起一个GET请求，下面的方法会被掉用：

- (AFHTTPRequestOperation *)GET:(NSString *)URLString
                     parameters:(id)parameters
                        success:(void (^)(AFHTTPRequestOperation *operation, id responseObject))success
                        failure:(void (^)(AFHTTPRequestOperation *operation, NSError *error))failure
{
    AFHTTPRequestOperation *operation = [self HTTPRequestOperationWithHTTPMethod:@"GET" URLString:URLString parameters:parameters success:success failure:failure];

    [self.operationQueue addOperation:operation];

    return operation;
}

其中[self.operationQueue addOperation:operation];就是将当前的任务放进操作队列。
关键我们看看AFHTTPRequestOperation 里面都做了什么。

AFHTTPRequestOperation 是继承自AFURLConnectionOperation，AFURLConnectionOperation实现了各种代理：

@interface AFURLConnectionOperation : NSOperation <NSURLConnectionDelegate, NSURLConnectionDataDelegate, NSSecureCoding, NSCopying>

我们知道当一个operation任务被启动的时候start方法就会被调用：

- (void)start {
    [self.lock lock];
    if ([self isCancelled]) {
        [self performSelector:@selector(cancelConnection) onThread:[[self class] networkRequestThread] withObject:nil waitUntilDone:NO modes:[self.runLoopModes allObjects]];
    } else if ([self isReady]) {
        self.state = AFOperationExecutingState;

        [self performSelector:@selector(operationDidStart) onThread:[[self class] networkRequestThread] withObject:nil waitUntilDone:NO modes:[self.runLoopModes allObjects]];
    }
    [self.lock unlock];
}

- (void)operationDidStart {
    [self.lock lock];
    if (![self isCancelled]) {
        self.connection = [[NSURLConnection alloc] initWithRequest:self.request delegate:self startImmediately:NO];

        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
        for (NSString *runLoopMode in self.runLoopModes) {
            [self.connection scheduleInRunLoop:runLoop forMode:runLoopMode];
            [self.outputStream scheduleInRunLoop:runLoop forMode:runLoopMode];
        }

        [self.outputStream open];
        [self.connection start];
    }
    [self.lock unlock];

    dispatch_async(dispatch_get_main_queue(), ^{
        [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingOperationDidStartNotification object:self];
    });
}

我们分析一下以上代码块，当AFURLConnectionOperation任务被正常启动的时候，下面的方法会被调用：

[self performSelector:@selector(operationDidStart) onThread:[[self class] networkRequestThread] withObject:nil waitUntilDone:NO modes:[self.runLoopModes allObjects]];

operationDidStart方法会在[[self class] networkRequestThread]返回的线程中被调用。我们看看这是一个什么样的线程：
+ (void)networkRequestThreadEntryPoint:(id)__unused object {
    @autoreleasepool {
        [[NSThread currentThread] setName:@"AFNetworking"];

        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
        [runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
        [runLoop run];
    }
}

+ (NSThread *)networkRequestThread {
    static NSThread *_networkRequestThread = nil;
    static dispatch_once_t oncePredicate;
    dispatch_once(&oncePredicate, ^{
        _networkRequestThread = [[NSThread alloc] initWithTarget:self selector:@selector(networkRequestThreadEntryPoint:) object:nil];
        [_networkRequestThread start];
    });

    return _networkRequestThread;
}

是的这个新的小线程命名为"AFNetworking", 它在创建的时候就启动了一个人runloop事件循环，并添加了一个NSMachPort 空端口，改NSMachPort 仅仅只是一个空的端口其目的是用来维护runloop的执行不被退出。

operationDidStart 方法中的：

        self.connection = [[NSURLConnection alloc] initWithRequest:self.request delegate:self startImmediately:NO];
        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
        for (NSString *runLoopMode in self.runLoopModes) {
            [self.connection scheduleInRunLoop:runLoop forMode:runLoopMode];
            [self.outputStream scheduleInRunLoop:runLoop forMode:runLoopMode];
        }

        [self.outputStream open];
        [self.connection start];

说明self.connection和self.outputStream周期性任务被绑定在当期的runloop的self.runLoopModes 模式中。（self.runLoopModes在初始化的时候被赋值为[NSSet setWithObject:NSRunLoopCommonModes]）

行了，那么当网络请求数据到达的时候，数据是如何被接收到的尼，我想这一点才是大家最关心的。
网络请求是一个异步的过程，当网络请求数据流到达的时候，runloop监听到该事件源__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__方法会被调用。之后CFNetwork的_NSURLConnectionDidReceiveData(_CFURLConnection*, __CFData const*, long, void const*)放会被调用获取到网络数据。与此同时NSURLConnectionInternal的_withActiveConnectionAndDelegate方法会被调用，本地代理被激活。AFURLConnectionOperation中的代理方法：

- (void)connection:(NSURLConnection __unused *)connection
    didReceiveData:(NSData *)data
{
    NSUInteger length = [data length];
    while (YES) {
        NSInteger totalNumberOfBytesWritten = 0;
        if ([self.outputStream hasSpaceAvailable]) {
            const uint8_t *dataBuffer = (uint8_t *)[data bytes];

            NSInteger numberOfBytesWritten = 0;
            while (totalNumberOfBytesWritten < (NSInteger)length) {
                numberOfBytesWritten = [self.outputStream write:&dataBuffer[(NSUInteger)totalNumberOfBytesWritten] maxLength:(length - (NSUInteger)totalNumberOfBytesWritten)];
                if (numberOfBytesWritten == -1) {
                    break;
                }

                totalNumberOfBytesWritten += numberOfBytesWritten;
            }

            break;
        } else {
            [self.connection cancel];
            if (self.outputStream.streamError) {
                [self performSelector:@selector(connection:didFailWithError:) withObject:self.connection withObject:self.outputStream.streamError];
            }
            return;
        }
    }

    dispatch_async(dispatch_get_main_queue(), ^{
        self.totalBytesRead += (long long)length;

        if (self.downloadProgress) {
            self.downloadProgress(length, self.totalBytesRead, self.response.expectedContentLength);
        }
    });
}

被调用，通过以上代码中的：

           NSInteger numberOfBytesWritten = 0;
            while (totalNumberOfBytesWritten < (NSInteger)length) {
                numberOfBytesWritten = [self.outputStream write:&dataBuffer[(NSUInteger)totalNumberOfBytesWritten] maxLength:(length - (NSUInteger)totalNumberOfBytesWritten)];
                if (numberOfBytesWritten == -1) {
                    break;
                }

                totalNumberOfBytesWritten += numberOfBytesWritten;
            }
            

网络数据流被写入缓存。数据被写入缓存完成后，代理方法：

- (void)connectionDidFinishLoading:(NSURLConnection __unused *)connection {
    self.responseData = [self.outputStream propertyForKey:NSStreamDataWrittenToMemoryStreamKey];

    [self.outputStream close];
    if (self.responseData) {
       self.outputStream = nil;
    }

    self.connection = nil;

    [self finish];
}

会被调用后。我们追踪[self finish]; 看看它里面的实现：

- (void)finish {
    [self.lock lock];
    self.state = AFOperationFinishedState;
    [self.lock unlock];

    dispatch_async(dispatch_get_main_queue(), ^{
        [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingOperationDidFinishNotification object:self];
    });
}

self.state = AFOperationFinishedState这句代码是重点，它标示了该请求任务已经结束。而这句赋值还做了一个KVO的操作，如下代码：

- (void)setState:(AFOperationState)state {
    if (!AFStateTransitionIsValid(self.state, state, [self isCancelled])) {
        return;
    }

    [self.lock lock];
    NSString *oldStateKey = AFKeyPathFromOperationState(self.state);
    NSString *newStateKey = AFKeyPathFromOperationState(state);

    [self willChangeValueForKey:newStateKey];
    [self willChangeValueForKey:oldStateKey];
    _state = state;
    [self didChangeValueForKey:oldStateKey];
    [self didChangeValueForKey:newStateKey];
    [self.lock unlock];
}

NSOperationInternal中的_observeValueForKeyPath:ofObject:changeKind:oldValue:newValue:indexes:context:方法会监听state状态的改变。然后回归到主线程，AFURLConnectionOperation中的setCompletionBlock方法被回调：

- (void)setCompletionBlock:(void (^)(void))block {
    [self.lock lock];
    if (!block) {
        [super setCompletionBlock:nil];
    } else {
        __weak __typeof(self)weakSelf = self;
        [super setCompletionBlock:^ {
            __strong __typeof(weakSelf)strongSelf = weakSelf;

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wgnu"
            dispatch_group_t group = strongSelf.completionGroup ?: url_request_operation_completion_group();
            dispatch_queue_t queue = strongSelf.completionQueue ?: dispatch_get_main_queue();
#pragma clang diagnostic pop

            dispatch_group_async(group, queue, ^{
                block();
            });

            dispatch_group_notify(group, url_request_operation_completion_queue(), ^{
                [strongSelf setCompletionBlock:nil];
            });
        }];
    }
    [self.lock unlock];
}

关键看这句代码：

dispatch_group_async(group, queue, ^{
                block();
            });

block()一调用就调用到了AFHTTPRequestOperation的方法：

#pragma mark - AFHTTPRequestOperation
- (void)setCompletionBlockWithSuccess:(void (^)(AFHTTPRequestOperation *operation, id responseObject))success
                              failure:(void (^)(AFHTTPRequestOperation *operation, NSError *error))failure
{
    // completionBlock is manually nilled out in AFURLConnectionOperation to break the retain cycle.
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-retain-cycles"
#pragma clang diagnostic ignored "-Wgnu"
    self.completionBlock = ^{
        if (self.completionGroup) {
            dispatch_group_enter(self.completionGroup);
        }

        dispatch_async(http_request_operation_processing_queue(), ^{
            if (self.error) {
                if (failure) {
                    dispatch_group_async(self.completionGroup ?: http_request_operation_completion_group(), self.completionQueue ?: dispatch_get_main_queue(), ^{
                        failure(self, self.error);
                    });
                }
            } else {
                id responseObject = self.responseObject;
                if (self.error) {
                    if (failure) {
                        dispatch_group_async(self.completionGroup ?: http_request_operation_completion_group(), self.completionQueue ?: dispatch_get_main_queue(), ^{
                            failure(self, self.error);
                        });
                    }
                } else {
                    if (success) {
                        dispatch_group_async(self.completionGroup ?: http_request_operation_completion_group(), self.completionQueue ?: dispatch_get_main_queue(), ^{
                            success(self, responseObject);
                        });
                    }
                }
            }

            if (self.completionGroup) {
                dispatch_group_leave(self.completionGroup);
            }
        });
    };
#pragma clang diagnostic pop
}

self.completionBlock中的内容被执行，这里面最关键的一句id responseObject = self.responseObject 获得了解析数据，我们看看self.responseObject的实现：

- (id)responseObject {
    [self.lock lock];
    if (!_responseObject && [self isFinished] && !self.error) {
        NSError *error = nil;
        self.responseObject = [self.responseSerializer responseObjectForResponse:self.response data:self.responseData error:&error];
        if (error) {
            self.responseSerializationError = error;
        }
    }
    [self.lock unlock];

    return _responseObject;
}

我靠！春天来了，这句代码：

self.responseObject = [self.responseSerializer responseObjectForResponse:self.response data:self.responseData error:&error];

将stream中的数据解析成了我们想要数据比如json、xml、plist等等。然后AFHTTPRequestOperation方法setCompletionBlockWithSuccess中代码success(self, responseObject)一回调。好了，现在就不用说了，我们通常就是在这个success block做我们自己的处理的。

下面我们看一下AFHTTPSessionManager 网络请求是怎么样进行的
苹果已经对NSURLSessionDataTask做了高度的封装，类似于AFURLConnectionOperation这样的复杂封装已经看不见了。但是原理跟AFURLConnectionOperation的差不多这里就不在赘述了。

到一个网络数据流到达的时候，NSURLSession的URLSession:dataTask:didReceiveData:方法就会被激活。AFURLSessionManager实现了NSURLSession的代理，于是乎AFURLSessionManager中的代理方法就会被调用：

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data
{
    AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:dataTask];
    [delegate URLSession:session dataTask:dataTask didReceiveData:data];

    if (self.dataTaskDidReceiveData) {
        self.dataTaskDidReceiveData(session, dataTask, data);
    }
}

[delegate URLSession:session dataTask:dataTask didReceiveData:data]里面的实现四这样的：

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

这样网络数据就被写入了self.mutableData。
当数据获取完成之后URLSession:task:didCompleteWithError:方法就会被调用：

- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
    AFURLSessionManagerTaskDelegate *delegate = [self delegateForTask:task];

    // delegate may be nil when completing a task in the background
    if (delegate) {
        [delegate URLSession:session task:task didCompleteWithError:error];

        [self removeDelegateForTask:task];
    }

    if (self.taskDidComplete) {
        self.taskDidComplete(session, task, error);
    }

}

我们看看 [delegate URLSession:session task:task didCompleteWithError:error] 方法中的实现：

- (void)URLSession:(__unused NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wgnu"
    __strong AFURLSessionManager *manager = self.manager;

    __block id responseObject = nil;

    __block NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
    userInfo[AFNetworkingTaskDidCompleteResponseSerializerKey] = manager.responseSerializer;

    //Performance Improvement from #2672
    NSData *data = nil;
    if (self.mutableData) {
        data = [self.mutableData copy];
        //We no longer need the reference, so nil it out to gain back some memory.
        self.mutableData = nil;
    }

    if (self.downloadFileURL) {
        userInfo[AFNetworkingTaskDidCompleteAssetPathKey] = self.downloadFileURL;
    } else if (data) {
        userInfo[AFNetworkingTaskDidCompleteResponseDataKey] = data;
    }

    if (error) {
        userInfo[AFNetworkingTaskDidCompleteErrorKey] = error;

        dispatch_group_async(manager.completionGroup ?: url_session_manager_completion_group(), manager.completionQueue ?: dispatch_get_main_queue(), ^{
            if (self.completionHandler) {
                self.completionHandler(task.response, responseObject, error);
            }

            dispatch_async(dispatch_get_main_queue(), ^{
                [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidCompleteNotification object:task userInfo:userInfo];
            });
        });
    } else {
        dispatch_async(url_session_manager_processing_queue(), ^{
            NSError *serializationError = nil;
            responseObject = [manager.responseSerializer responseObjectForResponse:task.response data:data error:&serializationError];

            if (self.downloadFileURL) {
                responseObject = self.downloadFileURL;
            }

            if (responseObject) {
                userInfo[AFNetworkingTaskDidCompleteSerializedResponseKey] = responseObject;
            }

            if (serializationError) {
                userInfo[AFNetworkingTaskDidCompleteErrorKey] = serializationError;
            }

            dispatch_group_async(manager.completionGroup ?: url_session_manager_completion_group(), manager.completionQueue ?: dispatch_get_main_queue(), ^{
                if (self.completionHandler) {
                    self.completionHandler(task.response, responseObject, serializationError);
                }

                dispatch_async(dispatch_get_main_queue(), ^{
                    [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidCompleteNotification object:task userInfo:userInfo];
                });
            });
        });
    }
#pragma clang diagnostic pop
}

以上代码中 data = [self.mutableData copy] 可知网络数据流被copy到了 data之中，关键代码：

responseObject = [manager.responseSerializer responseObjectForResponse:task.response data:data error:&serializationError]
网络数据被解析成了我们最终获取到的数据。self.completionHandler一执行如下代码块：

if (self.completionHandler) {
                    self.completionHandler(task.response, responseObject, serializationError);
                }
                

就回到了AFHTTPSessionManager的方法：

- (NSURLSessionDataTask *)dataTaskWithHTTPMethod:(NSString *)method
                                       URLString:(NSString *)URLString
                                      parameters:(id)parameters
                                         success:(void (^)(NSURLSessionDataTask *, id))success
                                         failure:(void (^)(NSURLSessionDataTask *, NSError *))failure
{
    NSError *serializationError = nil;
    NSMutableURLRequest *request = [self.requestSerializer requestWithMethod:method URLString:[[NSURL URLWithString:URLString relativeToURL:self.baseURL] absoluteString] parameters:parameters error:&serializationError];
    if (serializationError) {
        if (failure) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wgnu"
            dispatch_async(self.completionQueue ?: dispatch_get_main_queue(), ^{
                failure(nil, serializationError);
            });
#pragma clang diagnostic pop
        }

        return nil;
    }

    __block NSURLSessionDataTask *dataTask = nil;
    dataTask = [self dataTaskWithRequest:request completionHandler:^(NSURLResponse * __unused response, id responseObject, NSError *error) {
        if (error) {
            if (failure) {
                failure(dataTask, error);
            }
        } else {
            if (success) {
                success(dataTask, responseObject);
            }
        }
    }];

    return dataTask;
}

这block就会被调用：

    __block NSURLSessionDataTask *dataTask = nil;
    dataTask = [self dataTaskWithRequest:request completionHandler:^(NSURLResponse * __unused response, id responseObject, NSError *error) {
        if (error) {
            if (failure) {
                failure(dataTask, error);
            }
        } else {
            if (success) {
                success(dataTask, responseObject);
            }
        }
    }];

这里的success和 failure 就是我们经常看见的回调方法。