AF的源碼主要分為以下五個(gè)部分,最近主要研讀了核心模塊--網(wǎng)絡(luò)會(huì)話的代碼,做一做筆記,方便以后查閱。

很多公司的網(wǎng)絡(luò)模塊都是基于AF封裝的,如果發(fā)HTTP請(qǐng)求的話,我們直接使用的是AFHTTPSessionManager。它是繼承AFURLSessionManager類的,只是對(duì)其做了一層HTTP請(qǐng)求的封裝,方便直接使用。先來(lái)一張示意圖,了解以下大概的流程,以免待會(huì)看代碼看到懵逼。

接下來(lái)直接看GET請(qǐng)求的API實(shí)現(xiàn):
- (NSURLSessionDataTask *)GET:(NSString *)URLString
parameters:(id)parameters
progress:(void (^)(NSProgress * _Nonnull))downloadProgress
success:(void (^)(NSURLSessionDataTask * _Nonnull, id _Nullable))success
failure:(void (^)(NSURLSessionDataTask * _Nullable, NSError * _Nonnull))failure
{
NSURLSessionDataTask *dataTask = [self dataTaskWithHTTPMethod:@"GET"
URLString:URLString
parameters:parameters
uploadProgress:nil
downloadProgress:downloadProgress
success:success
failure:failure];
[dataTask resume];
return dataTask;
}
這個(gè)方法里面沒(méi)有做什么實(shí)際的事情,直接進(jìn)到dataTaskWithHTTPMethod方法。這里留個(gè)標(biāo)記A
- 標(biāo)記A
- (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
{
NSError *serializationError = nil;
//調(diào)用self.requestSerializer生成requets
NSMutableURLRequest *request = [self.requestSerializer requestWithMethod:method URLString:[[NSURL URLWithString:URLString relativeToURL:self.baseURL] absoluteString] parameters:parameters error:&serializationError];
if (serializationError) {
if (failure) {
dispatch_async(self.completionQueue ?: dispatch_get_main_queue(), ^{
failure(nil, serializationError);
});
}
return nil;
}
__block NSURLSessionDataTask *dataTask = nil;
//根據(jù)request生成dataTask
dataTask = [self dataTaskWithRequest:request
uploadProgress:uploadProgress
downloadProgress:downloadProgress
completionHandler:^(NSURLResponse * __unused response, id responseObject, NSError *error) {
if (error) {
if (failure) {
failure(dataTask, error);
}
} else {
if (success) {
success(dataTask, responseObject);
}
}
}];
return dataTask;
}
可以很明顯的看到該方法的大體邏輯,就是使用Manager的requestSerializer根據(jù)傳進(jìn)來(lái)的url,parameters等參數(shù)生成request,然后使用該request生成NSURLSessionDataTask返回。這里主要是兩個(gè)方法的調(diào)用,即生成request的方法:
- (NSMutableURLRequest *)requestWithMethod:(NSString *)method
URLString:(NSString *)URLString
parameters:(nullable id)parameters
error:(NSError * _Nullable __autoreleasing *)error;
和生成NSURLSessionDataTask的方法:
- (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;
我們先來(lái)看看request的生成方法實(shí)現(xiàn),這里留個(gè)標(biāo)記1,待會(huì)會(huì)回過(guò)頭來(lái)講:
- 標(biāo)記1
- (NSMutableURLRequest *)requestWithMethod:(NSString *)method
URLString:(NSString *)URLString
parameters:(id)parameters
error:(NSError *__autoreleasing *)error
{
NSParameterAssert(method);
NSParameterAssert(URLString);
NSURL *url = [NSURL URLWithString:URLString];
NSParameterAssert(url);
NSMutableURLRequest *mutableRequest = [[NSMutableURLRequest alloc] initWithURL:url];
mutableRequest.HTTPMethod = method;
for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
if ([self.mutableObservedChangedKeyPaths containsObject:keyPath]) {
[mutableRequest setValue:[self valueForKeyPath:keyPath] forKey:keyPath];
}
}
//拼接HTTP頭和參數(shù)
mutableRequest = [[self requestBySerializingRequest:mutableRequest withParameters:parameters error:error] mutableCopy];
return mutableRequest;
}
除去斷言和基礎(chǔ)的url,request生成方法;比較有意思的是request根據(jù)keypath設(shè)置value的實(shí)現(xiàn)。AFHTTPRequestSerializerObservedKeyPaths是一個(gè)C函數(shù),返回一個(gè)字符串?dāng)?shù)組。具體實(shí)現(xiàn)如下:
static NSArray * AFHTTPRequestSerializerObservedKeyPaths() {
static NSArray *_AFHTTPRequestSerializerObservedKeyPaths = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_AFHTTPRequestSerializerObservedKeyPaths = @[NSStringFromSelector(@selector(allowsCellularAccess)), NSStringFromSelector(@selector(cachePolicy)), NSStringFromSelector(@selector(HTTPShouldHandleCookies)), NSStringFromSelector(@selector(HTTPShouldUsePipelining)), NSStringFromSelector(@selector(networkServiceType)), NSStringFromSelector(@selector(timeoutInterval))];
});
return _AFHTTPRequestSerializerObservedKeyPaths;
}
看起來(lái)是一些方法名轉(zhuǎn)成的字符串,實(shí)際上對(duì)應(yīng)的是AFHTTPRequestSerializer.h文件中聲明的6個(gè)屬性,如下圖:

全局搜一下這個(gè)函數(shù)名,可以看到其實(shí)在AFHTTPRequestSerializer的init方法中也有用到,就是KVO了自己的這幾個(gè)屬性,同時(shí)創(chuàng)建了后面需要用到的set容器mutableObservedChangedKeyPaths:
self.mutableObservedChangedKeyPaths = [NSMutableSet set];
for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
if ([self respondsToSelector:NSSelectorFromString(keyPath)]) {
[self addObserver:self forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:AFHTTPRequestSerializerObserverContext];
}
}
如果外部設(shè)置該屬性為有效值的話,就將對(duì)應(yīng)的屬性名加入到self.mutableObservedChangedKeyPaths中,反之則移除。
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(__unused id)object
change:(NSDictionary *)change
context:(void *)context
{
if (context == AFHTTPRequestSerializerObserverContext) {
if ([change[NSKeyValueChangeNewKey] isEqual:[NSNull null]]) {
[self.mutableObservedChangedKeyPaths removeObject:keyPath];
} else {
[self.mutableObservedChangedKeyPaths addObject:keyPath];
}
}
}
那么我們回到標(biāo)記1的地方,可以知道方法實(shí)現(xiàn)中的for循環(huán)部分,其實(shí)是將用戶可能設(shè)置了的AFHTTPRequestSerializer的6個(gè)屬性的值,映射到它生成的request中去,因?yàn)镹SMutableURLRequest有著相同的6個(gè)屬性。其實(shí)就是如果用戶設(shè)置了某個(gè)屬性的值,那么生成的request就使用相應(yīng)的值,沒(méi)有設(shè)置的話,就用系統(tǒng)默認(rèn)值的意思。剩下這個(gè)方法,就是拼接HTTP頭和參數(shù)生成request。即如下方法,這里留一個(gè)標(biāo)記2:
- 標(biāo)記2
- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
withParameters:(id)parameters
error:(NSError *__autoreleasing *)error
{
NSParameterAssert(request);
NSMutableURLRequest *mutableRequest = [request mutableCopy];
//拼接HTTP頭
[self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {
if (![request valueForHTTPHeaderField:field]) {
[mutableRequest setValue:value forHTTPHeaderField:field];
}
}];
NSString *query = nil;
if (parameters) {
//看看參數(shù)是否需要用戶自定義轉(zhuǎn)譯
if (self.queryStringSerialization) {
NSError *serializationError;
query = self.queryStringSerialization(request, parameters, &serializationError);
if (serializationError) {
if (error) {
*error = serializationError;
}
return nil;
}
} else {
//使用AFN的默認(rèn)轉(zhuǎn)譯方式
switch (self.queryStringSerializationStyle) {
case AFHTTPRequestQueryStringDefaultStyle:
query = AFQueryStringFromParameters(parameters);
break;
}
}
}
//如果請(qǐng)求方式是需要將查詢參數(shù)拼接到URL后面的(默認(rèn)包含`GET``HEAD``DELETE`)、則拼接
if ([self.HTTPMethodsEncodingParametersInURI containsObject:[[request HTTPMethod] uppercaseString]]) {
if (query && query.length > 0) {
mutableRequest.URL = [NSURL URLWithString:[[mutableRequest.URL absoluteString] stringByAppendingFormat:mutableRequest.URL.query ? @"&%@" : @"?%@", query]];
}
} else {
//否則、則放入請(qǐng)求體
// #2864: an empty string is a valid x-www-form-urlencoded payload
if (!query) {
query = @"";
}
if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {
[mutableRequest setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];
}
[mutableRequest setHTTPBody:[query dataUsingEncoding:self.stringEncoding]];
}
return mutableRequest;
}
先看拼接HTTP頭的部分,self.mutableHTTPRequestHeaders其實(shí)是在AFHTTPRequestSerializer的init方法里面初始化的,在這里為用戶添加了諸如Accept-Language,User-Agent的默認(rèn)頭信息。
設(shè)置頭的時(shí)候都是用的這個(gè)方法
[self setValue:userAgent forHTTPHeaderField:@"User-Agent"];
這里的一個(gè)知識(shí)點(diǎn)是巧用隊(duì)列,避免了頻繁加鎖。我們知道加鎖是比較重量級(jí)的操作,能不加鎖就盡量不加。
- (void)setValue:(NSString *)value
forHTTPHeaderField:(NSString *)field
{
dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
[self.mutableHTTPRequestHeaders setValue:value forKey:field];
});
}
- (NSString *)valueForHTTPHeaderField:(NSString *)field {
NSString __block *value;
dispatch_sync(self.requestHeaderModificationQueue, ^{
value = [self.mutableHTTPRequestHeaders valueForKey:field];
});
return value;
}
注意這里是set的時(shí)候用了異步柵欄,異步保證不需要等待就可以立即返回,柵欄保證了寫的唯一性;get的時(shí)候用的同步,保證肯定取到值以后才返回。其實(shí)這一小點(diǎn)在《Effective Objective-C 2.0編寫高質(zhì)量iOS與OS X代碼的52個(gè)有效方法》的第41條有詳細(xì)講解,我就不作摘抄了。
回到標(biāo)記2,設(shè)置完HTTP頭以后,就是參數(shù)拼接和設(shè)置。如果自定義留參數(shù)拼接,則會(huì)走自定義的;沒(méi)有設(shè)置就走AF默認(rèn)的拼接。拼接完以后,AF默認(rèn)GET``HEAD``DELETE三個(gè)方法都是將參數(shù)拼接到URL,其余方法會(huì)將拼接參數(shù)的放到請(qǐng)求體中。這里我就不細(xì)講了?,F(xiàn)在回到標(biāo)記A處,request生成好了,再看看根據(jù)request生成dataTask的方法都干了啥:
- (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 {
//生成dataTask
__block NSURLSessionDataTask *dataTask = nil;
url_session_manager_create_task_safely(^{
dataTask = [self.session dataTaskWithRequest:request];
});
//為dataTask設(shè)置處理的代理對(duì)象
[self addDelegateForDataTask:dataTask uploadProgress:uploadProgressBlock downloadProgress:downloadProgressBlock completionHandler:completionHandler];
return dataTask;
}
注意的是,這個(gè)方法的實(shí)現(xiàn)是放在父類AFURLSessionManager中的。為什么生成dataTask這里要采用提交到隊(duì)列這樣的實(shí)現(xiàn)呢?
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();
}
}
查看Issue才知道,原來(lái)是因?yàn)樵趇OS8以下的系統(tǒng)中,系統(tǒng)方法dataTaskWithRequest:生成dataTask時(shí),默認(rèn)是并發(fā)實(shí)現(xiàn)的,所以會(huì)導(dǎo)致taskIdentifier不唯一,那么dataTask的completionHandlers回調(diào)就會(huì)出現(xiàn)混亂??梢钥吹紸F是判斷系統(tǒng)版本,如果低于iOS8,則將生成dataTask的操作提交到到一個(gè)自己創(chuàng)建的串行隊(duì)列中去,由此保證taskIdentifier的唯一性。
self.session是在manager的init方法中初始化的:
- (instancetype)initWithSessionConfiguration:(NSURLSessionConfiguration *)configuration {
...
self.operationQueue = [[NSOperationQueue alloc] init];
self.operationQueue.maxConcurrentOperationCount = 1;
self.session = [NSURLSession sessionWithConfiguration:self.sessionConfiguration delegate:self delegateQueue:self.operationQueue];
...
}
代碼我沒(méi)有拷全,主要看session的初始化部分,將NSURLSession的代理設(shè)置為了manager,代理方法回調(diào)隊(duì)列為自定義的隊(duì)列,且最大并發(fā)操作數(shù)設(shè)置為了1,即為串行。關(guān)于為什么這樣做,在AF的Github的Issue中也有人提過(guò)類似問(wèn)題,看大神們的討論好像都是在說(shuō)這個(gè)屬性命名有問(wèn)題,不應(yīng)該叫operationQueue這個(gè)名字,它不是一個(gè)操作隊(duì)列,只是代理NSURLSession代理方法的回調(diào)隊(duì)列。反正我是沒(méi)有看到相關(guān)的正面回答。個(gè)人猜測(cè)是為了讓代理方法串行回調(diào),防止數(shù)據(jù)混亂。
- 這里我們注意一個(gè)小點(diǎn),所有的dataTask都是通過(guò)self.session生成的,說(shuō)明所有的dataTask共用了同一個(gè)NSURLSession,那么這么做的好處是什么?其實(shí)總結(jié)起來(lái)就是一句話:共享的Session將會(huì)復(fù)用TCP的連接,而如果每次都新建Session的操作將導(dǎo)致每次的網(wǎng)絡(luò)請(qǐng)求都開(kāi)啟一個(gè)TCP的三次握手。這里有篇文章說(shuō)的比較詳細(xì)。
接下來(lái)我們?cè)倏纯礊閐ataTask綁定deleagte對(duì)象的方法中做了什么:
- (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] initWithTask:dataTask];
delegate.manager = self;
delegate.completionHandler = completionHandler;
//設(shè)置每個(gè)task的Description
dataTask.taskDescription = self.taskDescriptionForSessionTasks;
//將delegate映射到字典中保存
[self setDelegate:delegate forTask:dataTask];
delegate.uploadProgressBlock = uploadProgressBlock;
delegate.downloadProgressBlock = downloadProgressBlock;
}
大體就是將每個(gè)task跟AFURLSessionManagerTaskDelegate對(duì)象綁定,將傳進(jìn)來(lái)的那些回調(diào)block都賦值給了delegate對(duì)象,后續(xù)的回調(diào)都由delegate來(lái)發(fā)起。這里有幾個(gè)小點(diǎn)需要留意一下:
- 設(shè)置task的taskDescription時(shí),實(shí)際上取的是該對(duì)象的地址。
- taskIdentifier作key,delegate作object,映射到了self.mutableTaskDelegatesKeyedByTaskIdentifier字典中。映射的操作加了鎖。(為什么映射的設(shè)置和獲取操作沒(méi)有使用上面說(shuō)到的隊(duì)列派發(fā),而使用了NSLock的鎖呢?這點(diǎn)暫時(shí)還沒(méi)有想明白。)
- delegate的manager屬性引用了AFURLSessionManager對(duì)象,方便后續(xù)能夠取到manager的responseSerializer處理響應(yīng)內(nèi)容。同時(shí)為了避免循環(huán)引用,delegate對(duì)manager的引用是弱引用。
至此,所有的正向操作都已經(jīng)完成了,dataTask已經(jīng)生成好返回了。接下來(lái)就是NSURLSession的代理方法回調(diào)處理。如果將NSURLSession的代理方式實(shí)現(xiàn)也寫在manager當(dāng)中的話,manager的職責(zé)就太繁雜了,沒(méi)有遵循單一職責(zé)的編程原則。所以才有了AFURLSessionManagerTaskDelegate這個(gè)類,manager只負(fù)責(zé)代理方法的轉(zhuǎn)發(fā),具體的實(shí)現(xiàn)都放在了AFURLSessionManagerTaskDelegate里。
- (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);
}
}
AFURLSessionManagerTaskDelegate中的代理方法實(shí)現(xiàn):
- (void)URLSession:(__unused NSURLSession *)session
task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
__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];
});
});
});
}
}
這里就用到了前面說(shuō)過(guò)的弱引用Manager,然后是一系列的data拷貝和錯(cuò)誤處理。最后的回調(diào)為什么要放在組里面,如果有知道的,還請(qǐng)留言指教一下。需要注意的是接收到通知后進(jìn)行的操作跟發(fā)通知的操作在同一線程。在AF中通知的發(fā)送都是在主線程,那么如果需要監(jiān)聽(tīng)通知做一些耗時(shí)操作的話,最好自己提交到其他線程。
至此,網(wǎng)絡(luò)會(huì)話的主體流程就梳理完了。這次只是研讀了核心模塊的主要流程,以后有時(shí)間還會(huì)細(xì)細(xì)研讀其他模塊或者其他第三方庫(kù)。溜了,溜了。。。

參考資料
http://www.itdecent.cn/p/dfd3ae145a68
http://www.itdecent.cn/p/1ce820dc4112