AFNetWorking(3.0)原始碼分析（五）——AFHTTPRequestSerializer & AFHTTPResponseSerializer
在前面的幾篇部落格中，我們分析了AFURLSessionMangerd以及它的子類AFHTTPSessionManager。我們對AF的主要兩個類，有了一個比較全面的瞭解。
對於AFHTTPSessionManager，當其在要傳送請求時，會呼叫AFHTTPRequestSerializer 來組裝請求。 而當請求獲得了響應，需要作出解析時，又會呼叫對應的response serializer來解析返回的data。
對於伺服器響應的解析過程，在AFHTTPSessionManager中，是通過AFHTTPResponseSerializer來實現的。
AFHTTPResponseSerializer
AFHTTPResponseSerializer 作為http response的解析類，其實是作為基類存在的，對於真正的資料解析，並沒有提供有用的程式碼，而是要讓子類實現。在AFHTTPResponseSerializer 中，僅提供了最基礎的配置資訊，如response接受何種語言型別，何種狀態碼等。以及子類的一些公共的功能函式validateResponse：
@interface AFHTTPResponseSerializer : NSObject <AFURLResponseSerialization> - (instancetype)init; @property (nonatomic, assign) NSStringEncoding stringEncoding DEPRECATED_MSG_ATTRIBUTE("The string encoding is never used. AFHTTPResponseSerializer only validates status codes and content types but does not try to decode the received data in any way."); /** Creates and returns a serializer with default configuration. */ + (instancetype)serializer; ///----------------------------------------- /// @name Configuring Response Serialization ///----------------------------------------- @property (nonatomic, copy, nullable) NSIndexSet *acceptableStatusCodes; @property (nonatomic, copy, nullable) NSSet <NSString *> *acceptableContentTypes; - (BOOL)validateResponse:(nullable NSHTTPURLResponse *)response data:(nullable NSData *)data error:(NSError * _Nullable __autoreleasing *)error; @end
在AFHTTPResponseSerializer的stringEncoding屬性的註釋中可以看到，AFHTTPResponseSerializer僅是指定了http response有效的status codes和content types，並沒有對response data做任何解析，因此，stringEncoding屬性在AFHTTPResponseSerializer中應當是從未被使用的。
The string encoding is never used. AFHTTPResponseSerializer only validates status codes and content types but does not try to decode the received data in any way.
AFHTTPResponseSerializer同時提供了一個父類方法validateResponse，用來判定當前的http response是否符合設定的status codes和content types，這個方法，在子類中是可以通用的：
- (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;
}
實現比較簡單，我們就不再分析。
在AFHTTPResponseSerializer的宣告中，我們可以發現，AFHTTPResponseSerializer是遵循AFURLResponseSerialization協議的。而AFURLResponseSerialization協議的定義如下：
@protocol AFURLResponseSerialization <NSObject, NSSecureCoding, NSCopying>
/**
The response object decoded from the data associated with a specified response.
@param response The response to be processed.
@param data The response data to be decoded.
@param error The error that occurred while attempting to decode the response data.
@return The object decoded from the specified response data.
*/
- (nullable id)responseObjectForResponse:(nullable NSURLResponse *)response
data:(nullable NSData *)data
error:(NSError * _Nullable __autoreleasing *)error NS_SWIFT_NOTHROW;
@end
僅定義了一個方法，responseObjectForResponse:data:error:。這個方法是用來真正的解析request請求返回的data的。其返回值為id型別，也就是解析後的mode物件，他可以是任何型別，也就為我們解析請求的擴充套件，提供了可能。
對於基類AFHTTPResponseSerializer，它responseObjectForResponse:data:error:方法的實現是：
- (id)responseObjectForResponse:(NSURLResponse *)response
data:(NSData *)data
error:(NSError *__autoreleasing *)error
{
[self validateResponse:(NSHTTPURLResponse *)response data:data error:error];
return data;
}
可以看到，其直接將data返回，沒有做任何解析。因此，需要我們繼承AFHTTPResponseSerializer，並重寫responseObjectForResponse:data:error:方法，才能夠做到對響應的真正解析。
我們可以自定義子類，而AF也預設給我們提供了幾個子類的實現：
/**
`AFJSONResponseSerializer` is a subclass of `AFHTTPResponseSerializer` that validates and decodes JSON responses.
By default, `AFJSONResponseSerializer` accepts the following MIME types, which includes the official standard, `application/json`, as well as other commonly-used types:
- `application/json`
- `text/json`
- `text/javascript`
In RFC 7159 - Section 8.1, it states that JSON text is required to be encoded in UTF-8, UTF-16, or UTF-32, and the default encoding is UTF-8. NSJSONSerialization provides support for all the encodings listed in the specification, and recommends UTF-8 for efficiency. Using an unsupported encoding will result in serialization error. See the `NSJSONSerialization` documentation for more details.
*/
@interface AFJSONResponseSerializer : AFHTTPResponseSerializer
/**
`AFXMLParserResponseSerializer` is a subclass of `AFHTTPResponseSerializer` that validates and decodes XML responses as an `NSXMLParser` objects.
By default, `AFXMLParserResponseSerializer` accepts the following MIME types, which includes the official standard, `application/xml`, as well as other commonly-used types:
- `application/xml`
- `text/xml`
*/
@interface AFXMLParserResponseSerializer : AFHTTPResponseSerializer
/**
`AFXMLDocumentResponseSerializer` is a subclass of `AFHTTPResponseSerializer` that validates and decodes XML responses as an `NSXMLDocument` objects.
By default, `AFXMLDocumentResponseSerializer` accepts the following MIME types, which includes the official standard, `application/xml`, as well as other commonly-used types:
- `application/xml`
- `text/xml`
*/
@interface AFXMLDocumentResponseSerializer : AFHTTPResponseSerializer
/**
`AFPropertyListResponseSerializer` is a subclass of `AFHTTPResponseSerializer` that validates and decodes XML responses as an `NSXMLDocument` objects.
By default, `AFPropertyListResponseSerializer` accepts the following MIME types:
- `application/x-plist`
*/
@interface AFPropertyListResponseSerializer : AFHTTPResponseSerializer
/**
`AFImageResponseSerializer` is a subclass of `AFHTTPResponseSerializer` that validates and decodes image responses.
By default, `AFImageResponseSerializer` accepts the following MIME types, which correspond to the image formats supported by UIImage or NSImage:
- `image/tiff`
- `image/jpeg`
- `image/gif`
- `image/png`
- `image/ico`
- `image/x-icon`
- `image/bmp`
- `image/x-bmp`
- `image/x-xbitmap`
- `image/x-win-bitmap`
*/
@interface AFImageResponseSerializer : AFHTTPResponseSerializer
/**
`AFCompoundSerializer` is a subclass of `AFHTTPResponseSerializer` that delegates the response serialization to the first `AFHTTPResponseSerializer` object that returns an object for `responseObjectForResponse:data:error:`, falling back on the default behavior of `AFHTTPResponseSerializer`. This is useful for supporting multiple potential types and structures of server responses with a single serializer.
*/
@interface AFCompoundResponseSerializer : AFHTTPResponseSerializer
注意一下，最後一個AFCompoundResponseSerializer，其實是一個集合，可以為其新增好幾種ResponseSerializer到它裡面。這是為了防止在response type未知的情況下，用組合式ResponseSerializer來嘗試是否有一種對應的解析器。
AFJSONResponseSerializer
對於response 解析器的實現，每個子類都有不同的實現。但它們的共同點是：
- 均繼承自AFHTTPResponseSerializer
- 都會重寫
responseObjectForResponse:data:error:方法
我們可以單獨拎出來AFJSONResponseSerializer，來看一下它是如何重寫responseObjectForResponse:data:error:方法的：
- (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;
}
}
// 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) {
return nil;
}
NSError *serializationError = nil;
id responseObject = [NSJSONSerialization JSONObjectWithData:data options:self.readingOptions error:&serializationError];
if (!responseObject)
{
if (error) {
*error = AFErrorWithUnderlyingError(serializationError, *error);
}
return nil;
}
if (self.removesKeysWithNullValues) {
return AFJSONObjectByRemovingKeysWithNullValues(responseObject, self.readingOptions);
}
return responseObject;
}
其核心也就是呼叫了NSJSONSerialization的方法，做JSON格式的解析。
上面的程式碼還是比較好理解的。那麼，AF又是在什麼時候呼叫Response Serializer的解析方法
- (id)responseObjectForResponse:(NSURLResponse *)response
data:(NSData *)data
error:(NSError *__autoreleasing *)error
的呢？自然，應該是在Server返回response的時候。
讓我們追根溯源，還記得在基類AFURLSessionManager中，我們曾提到過，對於每一個Session task的響應，AFURLSessionManager 都會分配一個AFURLSessionManagerTaskDelegate來處理。其中，就包含對於系統NSURLSessionTaskDelegate的處理。
對於NSURLSessionTaskDelegate的
- (void)URLSession:(__unused NSURLSession *)session
task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
響應裡面，AFURLSessionManagerTaskDelegate有這麼一段：
else {
dispatch_async(url_session_manager_processing_queue(), ^{
NSError *serializationError = nil;
responseObject = [manager.responseSerializer responseObjectForResponse:task.response data:data error:&serializationError];
。。。
}
在這裡會看到manager呼叫了responseSerializer對響應作出解析。而這裡的responseSerializer自然不是某個具體的類，而是符合AFURLResponseSerialization協議的一個id型別：
id <AFURLResponseSerialization> responseSerializer;
AF通過這種協議的抽象，使得我們可以在具體應用中任意替換解析物件，如，可以替換為上面提到的AFJSONResponseSerializer，因為它正符合AFURLResponseSerialization協議。
AFHTTPRequestSerializer
在AF中，對於request的序列化，同樣使用了抽象協議的方式：
@protocol AFURLRequestSerialization <NSObject, NSSecureCoding, NSCopying>
/**
Returns a request with the specified parameters encoded into a copy of the original request.
@param request The original request.
@param parameters The parameters to be encoded.
@param error The error that occurred while attempting to encode the request parameters.
@return A serialized request.
*/
- (nullable NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
withParameters:(nullable id)parameters
error:(NSError * _Nullable __autoreleasing *)error NS_SWIFT_NOTHROW;
@end
在上一章中，我們分析了AFHTTPRequestSerializer的部分實現，它是怎麼支援GET/PUT/HEAD/POST/DELETE等method的。
在這裡，我們就不再去分析其餘的實現了，而是關注一下在AFHTTPRequestSerializer中用到的一個小技巧。
在AF的AFHTTPRequestSerializer中，需要解決這麼一個問題，就是，對於NSMutableURLRequest來說，除了設定URL和http body，還有許多控制屬性可以讓使用者設定，如設定請求超時timeoutInterval，cache策略等。
對應的，在AFHTTPRequestSerializer中自然也是添加了這些對應的屬性來執行使用者設定NSMutableURLRequest：
@interface AFHTTPRequestSerializer : NSObject <AFURLRequestSerialization>
@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;
那麼，當AFHTTPRequestSerializer在組裝NSMutableURLRequest時，又是如何知道使用者設定了這些屬性，並設定這些屬性到NSMutableURLRequest呢？
一般的，我們是不是想這樣解決，給這每一個屬性都設定一個和系統預設值相等的初始值，在組裝request的時候，將這些值全部賦值給NSMutableURLRequest？ 如下面這樣做：
// 先初始化一些和系統預設值相等的初始值
_allowsCellularAccess = YES;
_cachePolicy = NSURLRequestUseProtocolCachePolicy;
...
// 將這些屬性賦值給request；
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
request.allowsCellularAccess = self.allowsCellularAccess;
request.cachePolicy = self.cachePolicy;
...
上面這種做法也沒錯，但是不是感覺有點挫？一大堆的屬性賦值，但使用者可能僅僅改變了其中一個或什麼屬性都沒有改。
比較好的做法是，我們需要知道使用者修改了那些屬性，然後我們僅僅設定使用者所修改的屬性。AF確實也是這麼做的，而且用了比較巧的方法。
首先，在AFHTTPRequestSerializer中，註冊了KVO監聽：
self.mutableObservedChangedKeyPaths = [NSMutableSet set];
for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
if ([self respondsToSelector:NSSelectorFromString(keyPath)]) { // 通過KVO的方式，檢測並記錄自身的屬性變換
[self addObserver:self forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:AFHTTPRequestSerializerObserverContext]; // AFHTTPRequestSerializerObserverContext 這到底是什麼context？
}
}
AFHTTPRequestSerializer要監聽的Key陣列定義為：
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;
}
對應的，重寫響應屬性的Set方法，並觸發KVO：
- (void)setAllowsCellularAccess:(BOOL)allowsCellularAccess {
[self willChangeValueForKey:NSStringFromSelector(@selector(allowsCellularAccess))];
_allowsCellularAccess = allowsCellularAccess;
[self didChangeValueForKey:NSStringFromSelector(@selector(allowsCellularAccess))];
}
我們來看KVO的響應函式：
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(__unused id)object
change:(NSDictionary *)change
context:(void *)context
{
if (context == AFHTTPRequestSerializerObserverContext) {
// 將為空的引數取出，將賦值的引數記錄下來，等到生成request的時候，使用這些值
if ([change[NSKeyValueChangeNewKey] isEqual:[NSNull null]]) {
[self.mutableObservedChangedKeyPaths removeObject:keyPath];
} else {
[self.mutableObservedChangedKeyPaths addObject:keyPath];
}
}
}
可以看到，當修改對應的屬性時，KVO會通報被修改的keyPath，AF中用self.mutableObservedChangedKeyPaths這個set來記錄修改的屬性的名稱。
當最後要配置request時，僅需要根據這個keyPath 集合，利用KVC取出對應的屬性並賦值即可：
// 3.設定mutableRequest 對應的屬性值。這是通過KVO使用者設定相關的屬性操作
for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
if ([self.mutableObservedChangedKeyPaths containsObject:keyPath]) {
[mutableRequest setValue:[self valueForKeyPath:keyPath] forKey:keyPath];
}
}
這是AF中關於KVO和KVC的一個很好的應用場景。
總結
這就是我們分析AFHTTPRequestSerializer和AFHTTPResponseSerializer的全部內容，關於這部分的程式碼，並不是特別複雜，只要靜下心來分析其呼叫路徑基本就能夠了解。關於AF中KVC和KVO的靈活應用，也是值得我們借鑑的地方。