在iOS开发中AFNetworking占据了不小的分量，因此，对AFN进行详细的剖析。先看下AFN的目录结构：
Snip20161205_1.png
网络请求有两种方式。一种就是用 AFHTTPRequestOperationManager，另一种就是 AFHTTPSessionManager。那么这两种有什么区别呢？
AFNetworking最初的版本使用的是AFHTTPRequestOperationManager，以自己定义的NSOperation的封装实现的。AFHTTPSessionManager是2.0以后才引进的。
AFHTTPSessionManager继承自AFURLSessionManager，而AFURLSessionManager主要是使用系统提供的NSURLSessionManager和NSURLSessionTask进行网络操作的。这两个类是iOS7以后才出现的，所以，如果想适配到iOS6，那么就要使用AFHTTPRequestOperationManager进行网络请求。
下面就分两部分对AFN网络请求进行分析：
AFHTTPRequestOperationManager的网络请求原理：
AFHTTPRequestOperationManager的init方法:
- (instancetype)initWithBaseURL:(NSURL *)url {
if (!(self = [super init])) {
return nil;
}
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请求时，下面的方法会被调用：
- (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
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]）
那么当网络请求数据到达的时候，数据是如何被接收到的?
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;
}
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。
AFHTTPSessionManager的网络请求原理:
苹果对NSURLSeesionDataTask做了封装，类似于AFURLConnectOperation这样的封装已经不见了，但原理跟AFURLConnectionOperation差不多。
当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]里面的实现是这样的：
- (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。