AFURLRequestSerialization 是 AFNetworking 用来序列化 HTTP 请求的模块，它本身是一个协议，声明如下：
@protocol AFURLRequestSerialization <NSObject, NSSecureCoding, NSCopying>
- (nullable NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
withParameters:(nullable id)parameters
error:(NSError * _Nullable __autoreleasing *)error NS_SWIFT_NOTHROW;
@end
AFNetworking 又通过了三个类 AFHTTPRequestSerializer、AFJSONRequestSerializer、AFPropertyListRequestSerializer 实现了这个协议，其中 AFHTTPRequestSerializer 是 AFJSONRequestSerializer、AFPropertyListRequestSerializer 两个类的父类，接下来将分别解析这三个类的代码。
AFHTTPRequestSerializer
先来看 AFHTTPRequestSerializer 的初始化方法：
- (instancetype)init {
self = [super init];
if (!self) {
return nil;
}
self.stringEncoding = NSUTF8StringEncoding;
self.mutableHTTPRequestHeaders = [NSMutableDictionary dictionary];
self.requestHeaderModificationQueue = dispatch_queue_create("requestHeaderModificationQueue", DISPATCH_QUEUE_CONCURRENT);
// 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;
#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
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;
}
初始化方法做了如下几件事情：
- 初始化 stringEncoding、mutableHTTPRequestHeaders、requestHeaderModificationQueue 和 HTTPMethodsEncodingParametersInURI。
- 获取设备中的前五种语言，根据一定的规则设置 header 的 Accept-Language 字段。
- 根据一定的规则设置 header 的 User-Agent 字段。
- 对自身的一些字段进行 KVO 监听。
其中，KVO 监听的 keyPath 是通过 AFHTTPRequestSerializerObservedKeyPaths() 函数获取的：
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;
}
这里有两个小技巧：
- 在我们使用 KVO 的时候，往往直接用字符串硬编码来代表要监听的 keyPath（如 @"allowsCellularAccess"），但这样就失去了编译器检查的特性。而 AFNetworking 则是通过诸如 NSStringFromSelector(@selector(allowsCellularAccess)) 这样的方式来获取 keyPath，不仅有代码上的自动提示，也可以获得编译器检查的优势。
- 由于该函数每次返回的结果都一定会是一样的，所以通过 static 变量和 dispatch_once 结合，减少函数调用成本。
通过 KVO 监听的属性如下所示：
@property (nonatomic, assign) BOOL allowsCellularAccess;
@property (nonatomic, assign) NSURLRequestCachePolicy cachePolicy;
@property (nonatomic, assign) BOOL HTTPShouldHandleCookies;
@property (nonatomic, assign) BOOL HTTPShouldUsePipelining;
@property (nonatomic, assign) NSURLRequestNetworkServiceType networkServiceType;
@property (nonatomic, assign) NSTimeInterval timeoutInterval;
它们都是标量值，所以 KVO 需要自己手动触发，如下所示：
- (void)setAllowsCellularAccess:(BOOL)allowsCellularAccess {
[self willChangeValueForKey:NSStringFromSelector(@selector(allowsCellularAccess))];
_allowsCellularAccess = allowsCellularAccess;
[self didChangeValueForKey:NSStringFromSelector(@selector(allowsCellularAccess))];
}
同时还需要实现 NSObject 协议中的方法，告诉系统这些属性的 KVO 由我自己处理：
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key {
if ([AFHTTPRequestSerializerObservedKeyPaths() containsObject:key]) {
return NO;
}
return [super automaticallyNotifiesObserversForKey:key];
}
在这些属性值发生改变的时候，会通过 KVO 的回调告知我们，处理如下：
- (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];
}
}
}
即当 KVO 监听的这些值变为空的时候，就把它们从 mutableObservedChangedKeyPaths 中移除出去，反之则添加进来。
代理方法的实现
AFHTTPRequestSerializer 实现了 AFURLRequestSerialization 协议中的代理方法，如下所示：
- (NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
withParameters:(id)parameters
error:(NSError *__autoreleasing *)error
{
NSParameterAssert(request);
NSMutableURLRequest *mutableRequest = [request mutableCopy];
[self.HTTPRequestHeaders enumerateKeysAndObjectsUsingBlock:^(id field, id value, BOOL * __unused stop) {
if (![request valueForHTTPHeaderField:field]) {
[mutableRequest setValue:value forHTTPHeaderField:field];
}
}];
NSString *query = nil;
if (parameters) {
if (self.queryStringSerialization) {
NSError *serializationError;
query = self.queryStringSerialization(request, parameters, &serializationError);
if (serializationError) {
if (error) {
*error = serializationError;
}
return nil;
}
} else {
switch (self.queryStringSerializationStyle) {
case AFHTTPRequestQueryStringDefaultStyle:
query = AFQueryStringFromParameters(parameters);
break;
}
}
}
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 {
// #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;
}
实现的代理方法做了如下几件事情：
- 把 AFHTTPRequestSerializer 对象中的 HTTPRequestHeaders 中的键值对进行遍历，如果传入的 NSURLRequest 对象的 header 信息中不存在对应的值，则设置进去。
- 如果 parameters 存在，如果 queryStringSerialization 存在，则交由 queryStringSerialization 进行序列化，反之则通过 AFQueryStringFromParameters 函数进行序列化。
- 如果是 HTTPMethodsEncodingParametersInURI 中的 HTTPMethod，则将参数按规则拼接在后面，反之则放入 body 中。
AFQueryStringFromParameters 实现如下：
NSString * AFQueryStringFromParameters(NSDictionary *parameters) {
NSMutableArray *mutablePairs = [NSMutableArray array];
for (AFQueryStringPair *pair in AFQueryStringPairsFromDictionary(parameters)) {
[mutablePairs addObject:[pair URLEncodedStringValue]];
}
return [mutablePairs componentsJoinedByString:@"&"];
}
通过 AFQueryStringPairsFromDictionary 函数把 parameters 拆解成一个个的 AFQueryStringPair 对象。AFQueryStringPairsFromDictionary 实现如下所示：
NSArray * AFQueryStringPairsFromDictionary(NSDictionary *dictionary) {
return AFQueryStringPairsFromKeyAndValue(nil, dictionary);
}
NSArray * AFQueryStringPairsFromKeyAndValue(NSString *key, id value) {
NSMutableArray *mutableQueryStringComponents = [NSMutableArray array];
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"description" ascending:YES selector:@selector(compare:)];
if ([value isKindOfClass:[NSDictionary class]]) {
NSDictionary *dictionary = value;
// Sort dictionary keys to ensure consistent ordering in query string, which is important when deserializing potentially ambiguous sequences, such as an array of dictionaries
for (id nestedKey in [dictionary.allKeys sortedArrayUsingDescriptors:@[ sortDescriptor ]]) {
id nestedValue = dictionary[nestedKey];
if (nestedValue) {
[mutableQueryStringComponents addObjectsFromArray:AFQueryStringPairsFromKeyAndValue((key ? [NSString stringWithFormat:@"%@[%@]", key, nestedKey] : nestedKey), nestedValue)];
}
}
} else if ([value isKindOfClass:[NSArray class]]) {
NSArray *array = value;
for (id nestedValue in array) {
[mutableQueryStringComponents addObjectsFromArray:AFQueryStringPairsFromKeyAndValue([NSString stringWithFormat:@"%@[]", key], nestedValue)];
}
} else if ([value isKindOfClass:[NSSet class]]) {
NSSet *set = value;
for (id obj in [set sortedArrayUsingDescriptors:@[ sortDescriptor ]]) {
[mutableQueryStringComponents addObjectsFromArray:AFQueryStringPairsFromKeyAndValue(key, obj)];
}
} else {
[mutableQueryStringComponents addObject:[[AFQueryStringPair alloc] initWithField:key value:value]];
}
return mutableQueryStringComponents;
}
最后，通过调用 AFQueryStringPair 的对象方法 URLEncodedStringValue 把组装好的模型对象转化为字符串然后拼接起来。
- (NSString *)URLEncodedStringValue {
if (!self.value || [self.value isEqual:[NSNull null]]) {
return AFPercentEscapedStringFromString([self.field description]);
} else {
return [NSString stringWithFormat:@"%@=%@", AFPercentEscapedStringFromString([self.field description]), AFPercentEscapedStringFromString([self.value description])];
}
}
创建新的请求
AFHTTPRequestSerializer 提供了方法用于创建一个新的请求，如下所示：
- (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];
}
}
mutableRequest = [[self requestBySerializingRequest:mutableRequest withParameters:parameters error:error] mutableCopy];
return mutableRequest;
}
该方法做了这几年事情：
- 根据 method 和 URLString 创建新的请求。
- 把 AFHTTPRequestSerializerObservedKeyPaths() 存储的值放入 header 中。
- 序列化和拼接请求参数。
AFJSONRequestSerializer
AFJSONRequestSerializer 是 AFHTTPRequestSerializer 的一个子类，用于序列化 JSON 数据作为网络请求的参数。AFJSONRequestSerializer 的初始化方法如下：
+ (instancetype)serializer {
return [self serializerWithWritingOptions:(NSJSONWritingOptions)0];
}
+ (instancetype)serializerWithWritingOptions:(NSJSONWritingOptions)writingOptions
{
AFJSONRequestSerializer *serializer = [[self alloc] init];
serializer.writingOptions = writingOptions;
return serializer;
}
默认的初始化方法初始化了初始化了 writingOptions，默认是 0。
代理方法的实现
-
(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"];
}if (![NSJSONSerialization isValidJSONObject:parameters]) { if (error) { NSDictionary *userInfo = @{NSLocalizedFailureReasonErrorKey: NSLocalizedStringFromTable(@"The `parameters` argument is not valid JSON.", @"AFNetworking", nil)}; *error = [[NSError alloc] initWithDomain:AFURLRequestSerializationErrorDomain code:NSURLErrorCannotDecodeContentData userInfo:userInfo]; } return nil; } NSData *jsonData = [NSJSONSerialization dataWithJSONObject:parameters options:self.writingOptions error:error]; if (!jsonData) { return nil; } [mutableRequest setHTTPBody:jsonData];}
return mutableRequest;
}
代理方法做了如下几件事情：
- 如果请求的 HTTPMethod 是把参数拼接在 URL 之后的类型，则直接返回调用父类方法。
- 将 HTTPRequestHeaders 中的内容设置到请求的 header 中。
- 如果参数不为 nil，设置 header 的 Content-Type 为 application/json，并将 JSON 编码后放入请求的 body 中。
AFPropertyListRequestSerializer
AFPropertyListRequestSerializer 是 AFHTTPRequestSerializer 的一个子类，用于序列化 PropertyList 数据作为网络请求的参数。AFPropertyListRequestSerializer 的初始化方法如下：
+ (instancetype)serializer {
return [self serializerWithFormat:NSPropertyListXMLFormat_v1_0 writeOptions:0];
}
+ (instancetype)serializerWithFormat:(NSPropertyListFormat)format
writeOptions:(NSPropertyListWriteOptions)writeOptions
{
AFPropertyListRequestSerializer *serializer = [[self alloc] init];
serializer.format = format;
serializer.writeOptions = writeOptions;
return serializer;
}
默认的初始化方法初始化了 format 和 writeOptions。
代理方法的实现
- (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/x-plist" forHTTPHeaderField:@"Content-Type"];
}
NSData *plistData = [NSPropertyListSerialization dataWithPropertyList:parameters format:self.format options:self.writeOptions error:error];
if (!plistData) {
return nil;
}
[mutableRequest setHTTPBody:plistData];
}
return mutableRequest;
}
代理方法做了如下几件事情：
- 如果请求的 HTTPMethod 是把参数拼接在 URL 之后的类型，则直接返回调用父类方法。
- 将 HTTPRequestHeaders 中的内容设置到请求的 header 中。
- 如果参数不为 nil，设置 header 的 Content-Type 为 application/x-plist，并将 PropertyList 编码后放入请求的 body 中。
PS: 关于 appendData 的部分过于复杂，等我实际开发中用到这个功能的时候，再来补全博文。
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