源代码剖析
AFSecurityPolicy
AFSecurityPolicy 类用于服务器 SSL 证书安全连接，不讲解，自行观看其实现。
AFURLRequestSerialization
这里包含了三个类和两个函数：
AFHTTPRequestSerializer
AFJSONRequestSerializer
AFPropertyListRequestSerializer
NSString AFPercentEscapedStringFromString(NSString string)
NSString AFQueryStringFromParameters(NSString string)
后面两个函数一个用于请求字符串转义，另一个是生成编码 URL 请求参数工具方法
AFHTTPRequestSerializer
AFHTTPRequestSerializer 本身继承自 NSObject，实现了 AFURLRequestSerialization 协议，此协议是框架定义的，代码如下
@protocol AFURLRequestSerialization
- (nullable NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
withParameters:(nullable id)parameters
error:(NSError * _Nullable __autoreleasing *)error NS_SWIFT_NOTHROW;
@end 就是根据 request 和请求参数构造 NSURLRequest 对象。
然后其中就是一系列成员变量用于存储 HTTP 请求所需的各项参数，看到这里大家应该也看出来了，这些类实际上不是真正存储 HTTPRequest 的对象，而是构造出一个模型类，模型类实现了特定生成方法，然后生成方法根据类成员变量构造实际的 HTTPRequest。明白了这一点，代码就很容易看懂了，因为完全是根据 HTTP 协议所需构造的类模型。有 HTTPHeader，有 RequestBody。先来看初始化方法
{
self = [super init];
if (!self) {
return nil;
}
self.stringEncoding = NSUTF8StringEncoding;
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;
}];
[self setValue:[acceptLanguagesComponents componentsJoinedByString:@", "] forHTTPHeaderField:@"Accept-Language"];
NSString *userAgent = nil;
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wgnu"
#if TARGET_OS_IOS
// 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;
}首先设置编码类型为 NSUTF8StringEncoding，然后填充 Accept-Language 请求头，设置 UserAgent，设置需要在 URI 中编码的 HTTP 方法名称。
AFJSONRequestSerializer
AFJSONRequestSerializer 继承自 AFHTTPRequestSerializer，相对于 AFHTTPRequestSerializer 只是将参数全部设置到 HTTPBody 了而已。其中 JSON 转化使用的就是苹果自家的 NSJSONSerialization。
- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
withParameters:(id)parameters
error:(NSError *__autoreleasing *)error
{
NSParameterAssert(request);
if ([self.HTTPMethodsEncodingParametersInURI containsObject:[[request HTTPMethod] uppercaseString]]) {
return [super requestBySerializingRequest:request withParameters:parameters error:error];
}
NSMutableURLRequest *mutableRequest = [request mutableCopy];
[self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {
if (![request valueForHTTPHeaderField:field]) {
[mutableRequest setValue:value forHTTPHeaderField:field];
}
}];
if (parameters) {
if (![mutableRequest valueForHTTPHeaderField:@"Content-Type"]) {
[mutableRequest setValue:@"application/json" forHTTPHeaderField:@"Content-Type"];
}
[mutableRequest setHTTPBody:[NSJSONSerialization dataWithJSONObject:parameters options:self.writingOptions error:error]];
}
return mutableRequest;
}AFPropertyListRequestSerializer
AFPropertyListRequestSerializer 基本和 AFJSONRequestSerializer 差不多，只不过使用 NSPropertyListSerialization 将其转化为了 PropertyList 的 XML 格式罢了。
AFURLResponseSerialization
AFURLResponseSerialization 包含了
AFHTTPResponseSerializer
AFXMLParserResponseSerializer
AFPropertyListResponseSerializer
AFJSONResponseSerializer
AFImageResponseSerializer
AFCompoundResponseSerializer
其实和 AFURLRequestSerialization 的流程是一样的，都是通过存储属性，使用协议方法编码解码相应的 Request 或者 Response。
下面是 AFHTTPRequestSerialization 的初始化方法
- (instancetype)init {
self = [super init];
if (!self) {
return nil;
}
self.stringEncoding = NSUTF8StringEncoding;
self.acceptableStatusCodes = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(200, 100)];
self.acceptableContentTypes = nil;
return self;
}确实比请求对象的初始化方法简单太多了，只有编码类型、有效响应状态码（200 - 299）、可接受内容类型等成员变量初始化。
然后再看实现的协议方法
- (id)responseObjectForResponse:(NSURLResponse *)response
data:(NSData *)data
error:(NSError *__autoreleasing *)error
{
[self validateResponse:(NSHTTPURLResponse *)response data:data error:error];
return data;
}实际上就是调用了另一个实例方法 validateResponse:data:。
- (BOOL)validateResponse:(NSHTTPURLResponse *)response
data:(NSData *)data
error:(NSError * __autoreleasing *)error
{
BOOL responseIsValid = YES;
NSError *validationError = nil;
if (response && [response isKindOfClass:[NSHTTPURLResponse class]]) {
if (self.acceptableContentTypes && ![self.acceptableContentTypes containsObject:[response MIMEType]] &&
!([response MIMEType] == nil && [data length] == 0)) {
if ([data length] > 0 && [response URL]) {
NSMutableDictionary *mutableUserInfo = [@{
NSLocalizedDescriptionKey: [NSString stringWithFormat:NSLocalizedStringFromTable(@"Request failed: unacceptable content-type: %@", @"AFNetworking", nil), [response MIMEType]],
NSURLErrorFailingURLErrorKey:[response URL],
AFNetworkingOperationFailingURLResponseErrorKey: response,
} mutableCopy];
if (data) {
mutableUserInfo[AFNetworkingOperationFailingURLResponseDataErrorKey] = data;
}
validationError = AFErrorWithUnderlyingError([NSError errorWithDomain:AFURLResponseSerializationErrorDomain code:NSURLErrorCannotDecodeContentData userInfo:mutableUserInfo], validationError);
}
responseIsValid = NO;
}
if (self.acceptableStatusCodes && ![self.acceptableStatusCodes containsIndex:(NSUInteger)response.statusCode] && [response URL]) {
NSMutableDictionary *mutableUserInfo = [@{
NSLocalizedDescriptionKey: [NSString stringWithFormat:NSLocalizedStringFromTable(@"Request failed: %@ (%ld)", @"AFNetworking", nil), [NSHTTPURLResponse localizedStringForStatusCode:response.statusCode], (long)response.statusCode],
NSURLErrorFailingURLErrorKey:[response URL],
AFNetworkingOperationFailingURLResponseErrorKey: response,
} mutableCopy];
if (data) {
mutableUserInfo[AFNetworkingOperationFailingURLResponseDataErrorKey] = data;
}
validationError = AFErrorWithUnderlyingError([NSError errorWithDomain:AFURLResponseSerializationErrorDomain code:NSURLErrorBadServerResponse userInfo:mutableUserInfo], validationError);
responseIsValid = NO;
}
}
if (error && !responseIsValid) {
*error = validationError;
}
return responseIsValid;
}看起来代码虽然很多，但是不要慌，仔细看下去，实际上就只有两项检查：检查 Content-Type、检查 HTTPStatusCode。
AFJSONResponseSerializer 继承于 AFHTTPResponseSerializer。相比于父类，这个类才是真正开始使用返回的数据，首先在初始化函数中添加如下代码
self.acceptableContentTypes = [NSSet setWithObjects:@"application/json", @"text/json", @"text/javascript", nil];也就是说接受 JSON 数据并且转义。然后就是协议方法
- (id)responseObjectForResponse:(NSURLResponse *)response
data:(NSData *)data
error:(NSError *__autoreleasing *)error
{
if (![self validateResponse:(NSHTTPURLResponse *)response data:data error:error]) {
if (!error || AFErrorOrUnderlyingErrorHasCodeInDomain(*error, NSURLErrorCannotDecodeContentData, AFURLResponseSerializationErrorDomain)) {
return nil;
}
}
id responseObject = nil;
NSError *serializationError = nil;
// Workaround for behavior of Rails to return a single space for `head :ok` (a workaround for a bug in Safari), which is not interpreted as valid input by NSJSONSerialization.
// See https://github.com/rails/rails/issues/1742
BOOL isSpace = [data isEqualToData:[NSData dataWithBytes:" " length:1]];
if (data.length > 0 && !isSpace) {
responseObject = [NSJSONSerialization JSONObjectWithData:data options:self.readingOptions error:&serializationError];
} else {
return nil;
}
if (self.removesKeysWithNullValues && responseObject) {
responseObject = AFJSONObjectByRemovingKeysWithNullValues(responseObject, self.readingOptions);
}
if (error) {
*error = AFErrorWithUnderlyingError(serializationError, *error);
}
return responseObject;
}除了验证返回的数据外，还使用 NSJSONSerialization 将数据转义成字典或者数组。除此以外，还根据 removesKeysWithNullValues 的值决定是否移除 [NSNull null] 类型，AFJSONObjectByRemovingKeysWithNullValues 函数使用递归的方法，最终构造出无 null 的对象。这点确实可以借鉴一二。
AFURLSessionManager
这个类是最大的类，AFURLSessionManager 继承自 NSObject，实现
每个 AFURLSessionManager 都持有 NSURLSession、NSOperationQueue、id
- (instancetype)initWithSessionConfiguration:(NSURLSessionConfiguration *)configuration {
self = [super init];
if (!self) {
return nil;
}
if (!configuration) {
configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
}
self.sessionConfiguration = configuration;
self.operationQueue = [[NSOperationQueue alloc] init];
self.operationQueue.maxConcurrentOperationCount = 1;
self.session = [NSURLSession sessionWithConfiguration:self.sessionConfiguration delegate:self delegateQueue:self.operationQueue];
self.responseSerializer = [AFJSONResponseSerializer serializer];
self.securityPolicy = [AFSecurityPolicy defaultPolicy];
#if !TARGET_OS_WATCH
self.reachabilityManager = [AFNetworkReachabilityManager sharedManager];
#endif
self.mutableTaskDelegatesKeyedByTaskIdentifier = [[NSMutableDictionary alloc] init];
self.lock = [[NSLock alloc] init];
self.lock.name = AFURLSessionManagerLockName;
[self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
for (NSURLSessionDataTask *task in dataTasks) {
[self addDelegateForDataTask:task uploadProgress:nil downloadProgress:nil completionHandler:nil];
}
for (NSURLSessionUploadTask *uploadTask in uploadTasks) {
[self addDelegateForUploadTask:uploadTask progress:nil completionHandler:nil];
}
for (NSURLSessionDownloadTask *downloadTask in downloadTasks) {
[self addDelegateForDownloadTask:downloadTask progress:nil destination:nil completionHandler:nil];
}
}];
return self;
}这里传入了一个参数 NSURLSessionConfiguration，熟悉网络编程的朋友应该知道，NSURLSession 有三种配置类型 defaultSessionConfiguration、ephemeralSessionConfiguration、backgroundSessionConfiguration，三者区别如下：
Default session 使用持久磁盘 cache，用户凭证存储在钥匙串中。
Ephemeral session 不存储任何数据到磁盘中。
Background session 和 Default session 类似，但是在单独进程中运行
一般情况下是 Default session configuration 类型。operationQueue 用于异步回调使用 self.session = [NSURLSession sessionWithConfiguration:self.sessionConfiguration delegate:self delegateQueue:self.operationQueue]; 则很显而易见的看出来 session 创建的过程。同时创建 AFJSONResponseSerializer 作为响应序列化器，创建 securityPolicy 和 reachabilityManager。
最后使用了很巧妙的方法，getTasksWithCompletionHandler 函数返回所有的 task，然后存储到各自的数组中。
对于子类调用的 dataTaskWithRequest:uploadProgress:downloadProgress:completionHandler: 实际上是调用了 url_session_manager_create_task_safely block，实际上调用的是 [NSURLSession dataTaskWithRequest]，然后调用 - [AFURLSessionManager addDelegateForDataTask:uploadProgress:downloadProgress:completionHandler:] 将 task 的回调设置为自身，大家可以注意到，代码中有很多地方使用了锁机制，因为回调本身是放到 GCD 多线程中的，所以需要注意竞争问题导致的资源抢占。
总结
AFNetworking 实际上只是对苹果本身的 NSURLSession 做了封装，帮助开发者更容易获得各类信息，比如进度条、信息编码解码，而真正想要使用好网络通信，苹果自身提供的 NSURLSession 才是需要仔细研究的。笔者认为，AFNetworking 实际上是开源项目中难得的容易阅读，代码也不多的项目，非常值得作为第一个入手的源码阅读项目。