AFNetworking詳解(2)

源代碼剖析

AFSecurityPolicy

AFSecurityPolicy 類用於服務器 SSL 證書安全鏈接，不講解，自行觀看其實現。

AFURLRequestSerialization

這裏包含了三個類和兩個函數：

• AFHTTPRequestSerializer

• AFJSONRequestSerializer

• AFPropertyListRequestSerializer

• NSString AFPercentEscapedStringFromString(NSString string)

• NSString AFQueryStringFromParameters(NSString string)

後面兩個函數一個用於請求字符串轉義，另外一個是生成編碼 URL 請求參數工具方法

AFHTTPRequestSerializer

AFHTTPRequestSerializer 自己繼承自 NSObject，實現了 AFURLRequestSerialization 協議，此協議是框架定義的，代碼以下

@protocol AFURLRequestSerialization <NSObject, NSSecureCoding, NSCopying>

- (nullable NSURLRequest *)requestBySerializingRequest:(NSURLRequest *)request
                               withParameters:(nullable id)parameters
                                        error:(NSError * _Nullable __autoreleasing *)error NS_SWIFT_NOTHROW;

@end

就是根據 request 和請求參數構造 NSURLRequest 對象。
而後其中就是一系列成員變量用於存儲 HTTP 請求所需的各項參數，看到這裏你們應該也看出來了，這些類實際上不是真正存儲 HTTPRequest 的對象，而是構造出一個模型類，模型類實現了特定生成方法，而後生成方法根據類成員變量構造實際的 HTTPRequest。明白了這一點，代碼就很容易看懂了，由於徹底是根據 HTTP 協議所需構造的類模型。有 HTTPHeader，有 RequestBody。先來看初始化方法

{
    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;
}

首先設置編碼類型爲 NSUTF8StringEncoding，而後填充 Accept-Language 請求頭，設置 UserAgent，設置須要在 URI 中編碼的 HTTP 方法名稱。

AFJSONRequestSerializer

AFJSONRequestSerializer 繼承自 AFHTTPRequestSerializer，相對於 AFHTTPRequestSerializer 只是將參數所有設置到 HTTPBody 了而已。其中 JSON 轉化使用的就是蘋果自家的 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 差很少，只不過使用 NSPropertyListSerialization 將其轉化爲了 PropertyList 的 XML 格式罷了。

AFURLResponseSerialization

AFURLResponseSerialization 包含了

• AFHTTPResponseSerializer

• AFXMLParserResponseSerializer

• AFPropertyListResponseSerializer

• AFJSONResponseSerializer

• AFImageResponseSerializer

• AFCompoundResponseSerializer

其實和 AFURLRequestSerialization 的流程是同樣的，都是經過存儲屬性，使用協議方法編碼解碼相應的 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;
}

確實比請求對象的初始化方法簡單太多了，只有編碼類型、有效響應狀態碼（200 - 299）、可接受內容類型等成員變量初始化。
而後再看實現的協議方法

- (id)responseObjectForResponse:(NSURLResponse *)response
                           data:(NSData *)data
                          error:(NSError *__autoreleasing *)error
{
    [self validateResponse:(NSHTTPURLResponse *)response data:data error:error];

    return data;
}

實際上就是調用了另外一個實例方法 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;
}

看起來代碼雖然不少，可是不要慌，仔細看下去，實際上就只有兩項檢查：檢查 Content-Type、檢查 HTTPStatusCode。
AFJSONResponseSerializer 繼承於 AFHTTPResponseSerializer。相比於父類，這個類纔是真正開始使用返回的數據，首先在初始化函數中添加以下代碼

self.acceptableContentTypes = [NSSet setWithObjects:@"application/json", @"text/json", @"text/javascript", nil];

也就是說接受 JSON 數據而且轉義。而後就是協議方法

- (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;
}

除了驗證返回的數據外，還使用 NSJSONSerialization 將數據轉義成字典或者數組。除此之外，還根據 removesKeysWithNullValues 的值決定是否移除 [NSNull null] 類型，AFJSONObjectByRemovingKeysWithNullValues 函數使用遞歸的方法，最終構造出無 null 的對象。這點確實能夠借鑑一二。

AFURLSessionManager

這個類是最大的類，AFURLSessionManager 繼承自 NSObject，實現 <NSURLSessionDelegate, NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate, NSSecureCoding, NSCopying> 協議，用過蘋果網絡通訊的朋友應該知道，NSURLSession 使用 block 或者 delegate 傳遞各種狀態，這裏其實是使用了 delegate。
每一個 AFURLSessionManager 都持有 NSURLSession、NSOperationQueue、id<AFURLResponseSerialization>、AFSecurityPolicy、AFNetworkReachabilityManager、NSArray<NSURLSessionTask >、NSArray<NSURLSessionDataTask >、NSArray<NSURLSessionUploadTask >、NSArray<NSURLSessionDownloadTask > 等成員變量，這裏基本上也能看出 AFURLSessionManager 到底作了什麼，先來看初始化函數

- (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;
}

這裏傳入了一個參數 NSURLSessionConfiguration，熟悉網絡編程的朋友應該知道，NSURLSession 有三種配置類型 defaultSessionConfiguration、ephemeralSessionConfiguration、backgroundSessionConfiguration，三者區別以下：

1. Default session 使用持久磁盤 cache，用戶憑證存儲在鑰匙串中。

2. Ephemeral session 不存儲任何數據到磁盤中。

3. Background session 和 Default session 相似，可是在單獨進程中運行

通常狀況下是 Default session configuration 類型。operationQueue 用於異步回調使用 self.session = [NSURLSession sessionWithConfiguration:self.sessionConfiguration delegate:self delegateQueue:self.operationQueue]; 則很顯而易見的看出來 session 建立的過程。同時建立 AFJSONResponseSerializer 做爲響應序列化器，建立 securityPolicy 和 reachabilityManager。
最後使用了很巧妙的方法，getTasksWithCompletionHandler 函數返回全部的 task，而後存儲到各自的數組中。
對於子類調用的 dataTaskWithRequest:uploadProgress:downloadProgress:completionHandler: 其實是調用了 url_session_manager_create_task_safely block，實際上調用的是 [NSURLSession dataTaskWithRequest]，而後調用 - [AFURLSessionManager addDelegateForDataTask:uploadProgress:downloadProgress:completionHandler:] 將 task 的回調設置爲自身，你們能夠注意到，代碼中有不少地方使用了鎖機制，由於回調自己是放到 GCD 多線程中的，因此須要注意競爭問題致使的資源搶佔。

總結

AFNetworking 實際上只是對蘋果自己的 NSURLSession 作了封裝，幫助開發者更容易得到各種信息，好比進度條、信息編碼解碼，而真正想要使用好網絡通訊，蘋果自身提供的 NSURLSession 纔是須要仔細研究的。筆者認爲，AFNetworking 其實是開源項目中可貴的容易閱讀，代碼也很少的項目，很是值得做爲第一個入手的源碼閱讀項目。