AFNetworking 最新版本是3.1.0，相对于2.x版本，删除了基于NSURLConnection的AFURLConnnectionOperation的API支持，整个网络请求的代码完全基于NSURLSession.剩余架构和2.X版本基本一样，由网络请求管理(AFURLSessionManager)，安全协议(AFSecurityPolicy)，网络状态管理(AFNetworkReachabilityManager)和序列化(AFHTTPRequestSerializer和AFHTTPResponseSerializer)组成.

AFNetworking.png

AFURLSessionManager

AFURLSessionManager定义如下:

@interface AFURLSessionManager : NSObject <NSURLSessionDelegate, NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate, NSSecureCoding, NSCopying>

实现部分的url_session_manager_create_task_safely的函数解决iOS8下使用同步队列会出现问题:

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();
    }
}

交换xNSURLSessionTask的resume和suspend方法的辅助函数:

static inline void af_swizzleSelector(Class theClass, SEL originalSelector, SEL swizzledSelector) {
    Method originalMethod = class_getInstanceMethod(theClass, originalSelector);
    Method swizzledMethod = class_getInstanceMethod(theClass, swizzledSelector);
    method_exchangeImplementations(originalMethod, swizzledMethod);
}

static inline BOOL af_addMethod(Class theClass, SEL selector, Method method) {
    return class_addMethod(theClass, selector,  method_getImplementation(method),  method_getTypeEncoding(method));
}

AFURLSessionManger实现了NSURLSessionTaskDelegate, NSURLSessionDataDelegate和 NSURLSessionDownloadDelegate是三个协议，但是内部继续封装了AFURLSessionManagerTaskDelegate.设置Delegate的过程.

NSParameterAssert(task);

    AFURLSessionManagerTaskDelegate *delegate = nil;
    [self.lock lock];
    delegate = self.mutableTaskDelegatesKeyedByTaskIdentifier[@(task.taskIdentifier)];
    [self.lock unlock];

    return delegate;
}

- (void)setDelegate:(AFURLSessionManagerTaskDelegate *)delegate
            forTask:(NSURLSessionTask *)task
{
    NSParameterAssert(task);
    NSParameterAssert(delegate);

    [self.lock lock];
    self.mutableTaskDelegatesKeyedByTaskIdentifier[@(task.taskIdentifier)] = delegate;
    [delegate setupProgressForTask:task];
    [self addNotificationObserverForTask:task];
    [self.lock unlock];
}

- (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] init];
    delegate.manager = self;
    delegate.completionHandler = completionHandler;

    dataTask.taskDescription = self.taskDescriptionForSessionTasks;
    [self setDelegate:delegate forTask:dataTask];

    delegate.uploadProgressBlock = uploadProgressBlock;
    delegate.downloadProgressBlock = downloadProgressBlock;
}

需要注意的AFURLSessionManagerTaskDelegate的修饰符是weak:

@interface AFURLSessionManagerTaskDelegate : NSObject <NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate>
@property (nonatomic, weak) AFURLSessionManager *manager;

AFHTTPSessionManager

AFHTTPSessionManager 是AFURLSessionManger的子类，提供了GET，HEAD，POST，PUT，PATCH和DELETE六个调用的方法，GET调用如下:

- (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;
}

AFNetworkReachabilityManager

AFNetworkReachabilityManager中监控网络的四种状态，枚举如下:

typedef NS_ENUM(NSInteger, AFNetworkReachabilityStatus) {
    AFNetworkReachabilityStatusUnknown          = -1,
    AFNetworkReachabilityStatusNotReachable     = 0,
    AFNetworkReachabilityStatusReachableViaWWAN = 1,
    AFNetworkReachabilityStatusReachableViaWiFi = 2,
};

核心实现代码:

- (void)startMonitoring {
    [self stopMonitoring];

    if (!self.networkReachability) {
        return;
    }

    __weak __typeof(self)weakSelf = self;
    AFNetworkReachabilityStatusBlock callback = ^(AFNetworkReachabilityStatus status) {
        __strong __typeof(weakSelf)strongSelf = weakSelf;

        strongSelf.networkReachabilityStatus = status;
        if (strongSelf.networkReachabilityStatusBlock) {
            strongSelf.networkReachabilityStatusBlock(status);
        }

    };

    SCNetworkReachabilityContext context = {0, (__bridge void *)callback, AFNetworkReachabilityRetainCallback, AFNetworkReachabilityReleaseCallback, NULL};
    SCNetworkReachabilitySetCallback(self.networkReachability, AFNetworkReachabilityCallback, &context);
    SCNetworkReachabilityScheduleWithRunLoop(self.networkReachability, CFRunLoopGetMain(), kCFRunLoopCommonModes);

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0),^{
        SCNetworkReachabilityFlags flags;
        if (SCNetworkReachabilityGetFlags(self.networkReachability, &flags)) {
            AFPostReachabilityStatusChange(flags, callback);
        }
    });
}

AFSecurityPolicy

AFSecurityPolicy 主要作用就是验证 HTTPS 请求的证书是否有效，如果应用中有一些敏感信息或者涉及交易信息，一定要使用 HTTPS 来保证交易或者用户信息的安全.验证服务器是否被信任有三种方式:

typedef NS_ENUM(NSUInteger, AFSSLPinningMode) {
    AFSSLPinningModeNone,
    AFSSLPinningModePublicKey,
    AFSSLPinningModeCertificate,
};

AFSSLPinningModeNone:从客户端信任的CA列表中验证证书是否有效.

AFSSLPinningModePublicKey:客户端保存服务端证书的拷贝，验证服务端证书的公钥是否一致;

AFSSLPinningModeCertificate:客户端保存服务端证书的拷贝，首先验证域名/有效期信息，然后验证两个证书是否一致.

实现过程比较简单，主要是验证证书是否受信任需要一些C语言基础:

static NSArray * AFPublicKeyTrustChainForServerTrust(SecTrustRef serverTrust) {
    SecPolicyRef policy = SecPolicyCreateBasicX509();
    CFIndex certificateCount = SecTrustGetCertificateCount(serverTrust);
    NSMutableArray *trustChain = [NSMutableArray arrayWithCapacity:(NSUInteger)certificateCount];
    for (CFIndex i = 0; i < certificateCount; i++) {
        SecCertificateRef certificate = SecTrustGetCertificateAtIndex(serverTrust, i);

        SecCertificateRef someCertificates[] = {certificate};
        CFArrayRef certificates = CFArrayCreate(NULL, (const void **)someCertificates, 1, NULL);

        SecTrustRef trust;
        __Require_noErr_Quiet(SecTrustCreateWithCertificates(certificates, policy, &trust), _out);

        SecTrustResultType result;
        __Require_noErr_Quiet(SecTrustEvaluate(trust, &result), _out);

        [trustChain addObject:(__bridge_transfer id)SecTrustCopyPublicKey(trust)];

    _out:
        if (trust) {
            CFRelease(trust);
        }

        if (certificates) {
            CFRelease(certificates);
        }

        continue;
    }
    CFRelease(policy);

    return [NSArray arrayWithArray:trustChain];
}

AFHTTPRequestSerializer

AFHTTPRequestSerializer是最核心的地方，实现部分开头的代码展现了作者扎实的计算机基础，对URI中的非法字符进行百分比编码.
<pre><code>`NSString * AFPercentEscapedStringFromString(NSString string) {
static NSString * const kAFCharactersGeneralDelimitersToEncode = @":#[]@"; // does not include "?" or "/" due to RFC 3986 - Section 3.4
static NSString * const kAFCharactersSubDelimitersToEncode = @"!$&'()+,;=";

NSMutableCharacterSet * allowedCharacterSet = [[NSCharacterSet URLQueryAllowedCharacterSet] mutableCopy];
[allowedCharacterSet removeCharactersInString:[kAFCharactersGeneralDelimitersToEncode stringByAppendingString:kAFCharactersSubDelimitersToEncode]];

// FIXME: https://github.com/AFNetworking/AFNetworking/pull/3028
// return [string stringByAddingPercentEncodingWithAllowedCharacters:allowedCharacterSet];

static NSUInteger const batchSize = 50;

NSUInteger index = 0;
NSMutableString *escaped = @"".mutableCopy;

while (index < string.length) {

pragma GCC diagnostic push

pragma GCC diagnostic ignored "-Wgnu"

    NSUInteger length = MIN(string.length - index, batchSize);

pragma GCC diagnostic pop

    NSRange range = NSMakeRange(index, length);

    // To avoid breaking up character sequences such as 👴🏻👮🏽
    range = [string rangeOfComposedCharacterSequencesForRange:range];

    NSString *substring = [string substringWithRange:range];
    NSString *encoded = [substring stringByAddingPercentEncodingWithAllowedCharacters:allowedCharacterSet];
    [escaped appendString:encoded];

    index += range.length;
}

return escaped;

}`</code></pre>

参数字典通过&符号拼接:

NSString * AFQueryStringFromParameters(NSDictionary *parameters) {
    NSMutableArray *mutablePairs = [NSMutableArray array];
    for (AFQueryStringPair *pair in AFQueryStringPairsFromDictionary(parameters)) {
        [mutablePairs addObject:[pair URLEncodedStringValue]];
    }

    return [mutablePairs componentsJoinedByString:@"&"];
}

设置Http头部字段:

- (void)setValue:(NSString *)value
forHTTPHeaderField:(NSString *)field
{
    [self.mutableHTTPRequestHeaders setValue:value forKey:field];
}

[self setValue:userAgent forHTTPHeaderField:@"User-Agent"];

KVO观察到的缓存策略，管线化，超时时间字段：

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;
}

加入观察:

    self.mutableObservedChangedKeyPaths = [NSMutableSet set];
    for (NSString *keyPath in AFHTTPRequestSerializerObservedKeyPaths()) {
        if ([self respondsToSelector:NSSelectorFromString(keyPath)]) {
            [self addObserver:self forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:AFHTTPRequestSerializerObserverContext];
        }
    }

AFHTTPResponseSerializer子类包括AFJSONResponseSerializer，AFXMLParserResponseSerializer，AFXMLDocumentResponseSerializer，AFPropertyListResponseSerializer，AFImageResponseSerializer和AFCompoundResponseSerializer.

AFHTTPResponseSerializer及子类都实现了AFURLResponseSerialization协议.

AFJSONResponseSerializer对返回的数据解析如下:

- (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;
}

JSON数据去空:

static id AFJSONObjectByRemovingKeysWithNullValues(id JSONObject, NSJSONReadingOptions readingOptions) {
    if ([JSONObject isKindOfClass:[NSArray class]]) {
        NSMutableArray *mutableArray = [NSMutableArray arrayWithCapacity:[(NSArray *)JSONObject count]];
        for (id value in (NSArray *)JSONObject) {
            [mutableArray addObject:AFJSONObjectByRemovingKeysWithNullValues(value, readingOptions)];
        }

        return (readingOptions & NSJSONReadingMutableContainers) ? mutableArray : [NSArray arrayWithArray:mutableArray];
    } else if ([JSONObject isKindOfClass:[NSDictionary class]]) {
        NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionaryWithDictionary:JSONObject];
        for (id <NSCopying> key in [(NSDictionary *)JSONObject allKeys]) {
            id value = (NSDictionary *)JSONObject[key];
            if (!value || [value isEqual:[NSNull null]]) {
                [mutableDictionary removeObjectForKey:key];
            } else if ([value isKindOfClass:[NSArray class]] || [value isKindOfClass:[NSDictionary class]]) {
                mutableDictionary[key] = AFJSONObjectByRemovingKeysWithNullValues(value, readingOptions);
            }
        }

        return (readingOptions & NSJSONReadingMutableContainers) ? mutableDictionary : [NSDictionary dictionaryWithDictionary:mutableDictionary];
    }

    return JSONObject;
}

参考资料:
url 编码(percentcode 百分号编码)