AFNetworking的post请求过程解析

AFNetworking是基于NSURLSession搭建的网络请求框架，发起请求流程如下：

1、AFHTTPSessionManager对象的requestSerializer生成NSMutableURLRequest对象

这一步为设置请求参数，我们的parameters参数为此步设置，若请求参数出现问题，我们可以在这一步中进行问题排查。

NSMutableURLRequest *request = [self.requestSerializer requestWithMethod:method URLString:[[NSURL URLWithString:URLString relativeToURL:self.baseURL] absoluteString] parameters:parameters error:&serializationError];

2、设置NSMutableURLRequest对象的headerField

for (NSString *headerField in headers.keyEnumerator) {
    [request setValue:headers[headerField] forHTTPHeaderField:headerField];
}

4、AFHTTPSessionManager对象的NSURLSession属性生成NSURLSessionDataTask对象，NSURLSession回调代理为AFHTTPSessionManager对象

- (NSURLSession *)session {
    
    @synchronized (self) {
        if (!_session) {
            _session = [NSURLSession sessionWithConfiguration:self.sessionConfiguration delegate:self delegateQueue:self.operationQueue];
        }
    }
    return _session;
}

    NSURLSessionDataTask *dataTask = [self.session dataTaskWithRequest:request];

5、创建AFURLSessionManagerTaskDelegate对象，设置回调代理和回调block，以NSURLSessionDataTask.taskIdentifier为key，delegate为value保存在mutableTaskDelegatesKeyedByTaskIdentifier字典中

AFURLSessionManagerTaskDelegate *delegate = [[AFURLSessionManagerTaskDelegate alloc] initWithTask:dataTask];
delegate.manager = self;
delegate.completionHandler = completionHandler;

dataTask.taskDescription = self.taskDescriptionForSessionTasks;
[self setDelegate:delegate forTask:dataTask];

delegate.uploadProgressBlock = uploadProgressBlock;
delegate.downloadProgressBlock = downloadProgressBlock;

- (void)setDelegate:(AFURLSessionManagerTaskDelegate *)delegate
            forTask:(NSURLSessionTask *)task
{
    NSParameterAssert(task);
    NSParameterAssert(delegate);

    [self.lock lock];
    self.mutableTaskDelegatesKeyedByTaskIdentifier[@(task.taskIdentifier)] = delegate;
    [self addNotificationObserverForTask:task];
    [self.lock unlock];
}

6、AFHTTPSessionManager接收NSURLSession回调

AFHTTPSessionManager对象会从mutableTaskDelegatesKeyedByTaskIdentifier字典中根据NSURLSessionDataTask.taskIdentifier为key，取出代理delegate，进行数据拼接，进度回调调用等。
若接收数据或发送数据出现问题，则可以从这些代理中进行问题排查。

- (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);
    }
}

- (void)URLSession:(__unused NSURLSession *)session
          dataTask:(__unused NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data
{
    self.downloadProgress.totalUnitCount = dataTask.countOfBytesExpectedToReceive;
    self.downloadProgress.completedUnitCount = dataTask.countOfBytesReceived;

    [self.mutableData appendData:data];
}

7、数据接收完毕，调用AFHTTPSessionManager对象的responseSerializer的方法解析接收到的数据

若解析数据出现问题，我们可以在这一步里进行问题排查。

 responseObject = [manager.responseSerializer responseObjectForResponse:task.response data:data error:&serializationError];

8、把解析的数据通过completionHandler block回调给使用者

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

9、从mutableTaskDelegatesKeyedByTaskIdentifier移除NSURLSessionDataTask.taskIdentifier为key的对象信息

[self removeDelegateForTask:task];

- (void)removeDelegateForTask:(NSURLSessionTask *)task {
    NSParameterAssert(task);

    [self.lock lock];
    [self removeNotificationObserverForTask:task];
    [self.mutableTaskDelegatesKeyedByTaskIdentifier removeObjectForKey:@(task.taskIdentifier)];
    [self.lock unlock];
}

在此过程中，我们发现AFHTTPSessionManager、AFURLSessionManager、AFURLSessionManagerTaskDelegate、AFHTTPRequestSerializer、AFHTTPResponseSerializer五个类极为重要，AFHTTPSessionManager、AFURLSessionManager为管理整个流程的类，AFURLSessionManagerTaskDelegate为保存回调的类，AFHTTPRequestSerializer为对请求头进行数据组装的类，AFHTTPResponseSerializer为接收数据后进行解析的类。
其中还有一个AFSecurityPolicy类，设置安全政策的管理类，https的ssl链接使用，可以设置三种不同的模式。

typedef NS_ENUM(NSUInteger, AFSSLPinningMode) {
    AFSSLPinningModeNone,  //Do not used pinned certificates to validate servers.  无条件信任返回的证书
    AFSSLPinningModePublicKey, //Validate host certificates against public keys of pinned certificates.  与本地证书部分校验
    AFSSLPinningModeCertificate, //Validate host certificates against pinned certificates.  与本地证书全部校验
};