AFNetworking 2.6.3 源碼解析

到了現(xiàn)在大部分項(xiàng)目使用的AFNetworking應(yīng)該是3.0以上版本，源碼分析基本上都是都是3.0的文章介紹，關(guān)于2.0的介紹只有bang神的博客.本文分析以AFNetworking 2.6.3為基礎(chǔ)進(jìn)行分析，如果大家已經(jīng)看過了類似文章，可以忽略我.

AFNetworking主要由NSURLConnction，NSURLSession，Security，Reachability和Serialization五個文件夾組成，AFNetworking對常見的UIKit組件進(jìn)行擴(kuò)展，這一部分就不介紹了，項(xiàng)目中實(shí)際使用的不多.

NSURLConnection

AFURLConnectionOperation是NSOperation的子類，實(shí)現(xiàn)的協(xié)議如下:
<pre><code>@interface AFURLConnectionOperation : NSOperation <NSURLConnectionDelegate, NSURLConnectionDataDelegate, NSSecureCoding, NSCopying></code></pre>

相對于日常開發(fā)中常用的NSCoding協(xié)議NSSecureCoding更加安全，可以防止信息被篡改，如果解檔的類和要求的類類型不一致的時候會拋出異常，詳情見參考鏈接.

單例請求的隊(duì)列queue和隊(duì)列組group，queue的命名堪稱典范:
<pre><code>`static dispatch_group_t url_request_operation_completion_group() {
static dispatch_group_t af_url_request_operation_completion_group;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
af_url_request_operation_completion_group = dispatch_group_create();
});

return af_url_request_operation_completion_group;

}

static dispatch_queue_t url_request_operation_completion_queue() {
static dispatch_queue_t af_url_request_operation_completion_queue;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
af_url_request_operation_completion_queue = dispatch_queue_create("com.alamofire.networking.operation.queue", DISPATCH_QUEUE_CONCURRENT );
});

return af_url_request_operation_completion_queue;

}`</code></pre>

靜態(tài)內(nèi)聯(lián)函數(shù)，static inline可以理解為static函數(shù)加入了inline屬性，編譯的時候會將其展開編譯而不是為函數(shù)生成獨(dú)立的匯編碼.
<pre><code>`static inline NSString * AFKeyPathFromOperationState(AFOperationState state) {
switch (state) {
case AFOperationReadyState:
return @"isReady";
case AFOperationExecutingState:
return @"isExecuting";
case AFOperationFinishedState:
return @"isFinished";
case AFOperationPausedState:
return @"isPaused";
default: {

pragma clang diagnostic push

pragma clang diagnostic ignored "-Wunreachable-code"

        return @"state";

pragma clang diagnostic pop

}
}

}`</code></pre>

創(chuàng)建線程默認(rèn)是主線的Runloop下，Runloop模式默認(rèn)為NSDefaultRunLoopMode模式，如果用戶操作UI滑動屏幕，會切換到UITrackingRunLoopMode模式下，導(dǎo)致請求失效.AFNetworking創(chuàng)建一條常駐線程:
<pre><code>`+ (void)networkRequestThreadEntryPoint:(id)__unused object {
@autoreleasepool {
[[NSThread currentThread] setName:@"AFNetworking"];

    NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
    [runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
    [runLoop run];
}

}

(NSThread *)networkRequestThread {
static NSThread *_networkRequestThread = nil;
static dispatch_once_t oncePredicate;
dispatch_once(&oncePredicate, ^{
_networkRequestThread = [[NSThread alloc] initWithTarget:self selector:@selector(networkRequestThreadEntryPoint:) object:nil];
[_networkRequestThread start];
});

return _networkRequestThread;
}`</code></pre>

批量請求的代碼，線程的異步轉(zhuǎn)同步，線程之間的依賴寫的很??:
<pre><code>`+ (NSArray *)batchOfRequestOperations:(NSArray *)operations
progressBlock:(void (^)(NSUInteger numberOfFinishedOperations, NSUInteger totalNumberOfOperations))progressBlock
completionBlock:(void (^)(NSArray *operations))completionBlock
{
if (!operations || [operations count] == 0) {
return @[[NSBlockOperation blockOperationWithBlock:^{
dispatch_async(dispatch_get_main_queue(), ^{
if (completionBlock) {
completionBlock(@[]);
}
});
}]];
}

__block dispatch_group_t group = dispatch_group_create();
NSBlockOperation *batchedOperation = [NSBlockOperation blockOperationWithBlock:^{
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        if (completionBlock) {
            completionBlock(operations);
        }
    });
}];

for (AFURLConnectionOperation *operation in operations) {
    operation.completionGroup = group;
    void (^originalCompletionBlock)(void) = [operation.completionBlock copy];
    __weak __typeof(operation)weakOperation = operation;
    operation.completionBlock = ^{
        __strong __typeof(weakOperation)strongOperation = weakOperation;

pragma clang diagnostic push

pragma clang diagnostic ignored "-Wgnu"

        dispatch_queue_t queue = strongOperation.completionQueue ?: dispatch_get_main_queue();

pragma clang diagnostic pop

        dispatch_group_async(group, queue, ^{
            if (originalCompletionBlock) {
                originalCompletionBlock();
            }

            NSUInteger numberOfFinishedOperations = [[operations indexesOfObjectsPassingTest:^BOOL(id op, NSUInteger __unused idx,  BOOL __unused *stop) {
                return [op isFinished];
            }] count];

            if (progressBlock) {
                progressBlock(numberOfFinishedOperations, [operations count]);
            }

            dispatch_group_leave(group);
        });
    };

    dispatch_group_enter(group);
    [batchedOperation addDependency:operation];
}

return [operations arrayByAddingObject:batchedOperation];

}`</code></pre>

AFHTTPRequestOperation繼承自AFURLConnectionOperation，其中的暫停的操作對文件流的操作代碼如下:
<pre><code>`- (void)pause {
[super pause];

u_int64_t offset = 0;
if ([self.outputStream propertyForKey:NSStreamFileCurrentOffsetKey]) {
    offset = [(NSNumber *)[self.outputStream propertyForKey:NSStreamFileCurrentOffsetKey] unsignedLongLongValue];
} else {
    offset = [(NSData *)[self.outputStream propertyForKey:NSStreamDataWrittenToMemoryStreamKey] length];
}

NSMutableURLRequest *mutableURLRequest = [self.request mutableCopy];
if ([self.response respondsToSelector:@selector(allHeaderFields)] && [[self.response allHeaderFields] valueForKey:@"ETag"]) {
    [mutableURLRequest setValue:[[self.response allHeaderFields] valueForKey:@"ETag"] forHTTPHeaderField:@"If-Range"];
}
[mutableURLRequest setValue:[NSString stringWithFormat:@"bytes=%llu-", offset] forHTTPHeaderField:@"Range"];
self.request = mutableURLRequest;

}
`</code></pre>

AFHTTPRequestOperationManager對所有的Operation進(jìn)行管理，通過初始化就覆蓋了所有的核心類文件，序列化，安全協(xié)議和網(wǎng)絡(luò)請求管理文件.
<pre><code>`- (instancetype)initWithBaseURL:(NSURL *)url {
self = [super init];
if (!self) {
return nil;
}

// Ensure terminal slash for baseURL path, so that NSURL +URLWithString:relativeToURL: works as expected
if ([[url path] length] > 0 && ![[url absoluteString] hasSuffix:@"/"]) {
    url = [url URLByAppendingPathComponent:@""];
}

self.baseURL = url;

self.requestSerializer = [AFHTTPRequestSerializer serializer];
self.responseSerializer = [AFJSONResponseSerializer serializer];

self.securityPolicy = [AFSecurityPolicy defaultPolicy];

self.reachabilityManager = [AFNetworkReachabilityManager sharedManager];

self.operationQueue = [[NSOperationQueue alloc] init];

self.shouldUseCredentialStorage = YES;

return self;

}
`</code></pre>

提供http請求的六種方式GET，HEAD，POST，PUT，PATCH，DELETE.
<pre><code>`- (nullable AFHTTPRequestOperation *)GET:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a HEAD request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes a single arguments: the request operation.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)HEAD:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a POST request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)POST:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a multipart POST request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param block A block that takes a single argument and appends data to the HTTP body. The block argument is an object adopting the AFMultipartFormData protocol.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)POST:(NSString *)URLString
parameters:(nullable id)parameters
constructingBodyWithBlock:(nullable void (^)(id <AFMultipartFormData> formData))block
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a PUT request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)PUT:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a PATCH request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)PATCH:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;

/**
Creates and runs an AFHTTPRequestOperation with a DELETE request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the request operation finishes successfully. This block has no return value and takes two arguments: the request operation, and the response object created by the client response serializer.
@param failure A block object to be executed when the request operation finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the request operation and the error describing the network or parsing error that occurred.

@see -HTTPRequestOperationWithRequest:success:failure:
*/

(nullable AFHTTPRequestOperation *)DELETE:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(AFHTTPRequestOperation *operation, id responseObject))success
failure:(nullable void (^)(AFHTTPRequestOperation * __nullable operation, NSError *error))failure;`</code></pre>

整個網(wǎng)絡(luò)請求回調(diào)順序:

AFNetworking.png

NSURLSession

NSURLSession文件夾下有兩個文件AFURLSessionManager和AFHTTPSessionManager.
<pre><code>@interface AFURLSessionManager : NSObject <NSURLSessionDelegate, NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate, NSSecureCoding, NSCopying></code></pre>

AFURLSessionManager有一個私有類_AFURLSessionTaskSwizzling，交換NSURLSessionTask中的暫停和繼續(xù)方法.

交換函數(shù):

<pre><code>`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));
}
`</code></pre>

交換過程:

<pre><code>`

(void)load {
/**
WARNING: Trouble Ahead
https://github.com/AFNetworking/AFNetworking/pull/2702
*/

if (NSClassFromString(@"NSURLSessionTask")) {
/**
iOS 7 and iOS 8 differ in NSURLSessionTask implementation, which makes the next bit of code a bit tricky.
Many Unit Tests have been built to validate as much of this behavior has possible.
Here is what we know:
- NSURLSessionTasks are implemented with class clusters, meaning the class you request from the API isn't actually the type of class you will get back.
- Simply referencing [NSURLSessionTask class] will not work. You need to ask an NSURLSession to actually create an object, and grab the class from there.
- On iOS 7, localDataTask is a __NSCFLocalDataTask, which inherits from __NSCFLocalSessionTask, which inherits from __NSCFURLSessionTask.
- On iOS 8, localDataTask is a __NSCFLocalDataTask, which inherits from __NSCFLocalSessionTask, which inherits from NSURLSessionTask.
- On iOS 7, __NSCFLocalSessionTask and __NSCFURLSessionTask are the only two classes that have their own implementations of resume and suspend, and __NSCFLocalSessionTask DOES NOT CALL SUPER. This means both classes need to be swizzled.
- On iOS 8, NSURLSessionTask is the only class that implements resume and suspend. This means this is the only class that needs to be swizzled.
- Because NSURLSessionTask is not involved in the class hierarchy for every version of iOS, its easier to add the swizzled methods to a dummy class and manage them there.
```
   Some Assumptions:
      - No implementations of `resume` or `suspend` call super. If this were to change in a future version of iOS, we'd need to handle it.
      - No background task classes override `resume` or `suspend`
   
   The current solution:
      1) Grab an instance of `__NSCFLocalDataTask` by asking an instance of `NSURLSession` for a data task.
      2) Grab a pointer to the original implementation of `af_resume`
      3) Check to see if the current class has an implementation of resume. If so, continue to step 4.
      4) Grab the super class of the current class.
      5) Grab a pointer for the current class to the current implementation of `resume`.
      6) Grab a pointer for the super class to the current implementation of `resume`.
      7) If the current class implementation of `resume` is not equal to the super class implementation of `resume` AND the current implementation of `resume` is not equal to the original implementation of `af_resume`, THEN swizzle the methods
      8) Set the current class to the super class, and repeat steps 3-8
   */
  NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration ephemeralSessionConfiguration];
  NSURLSession * session = [NSURLSession sessionWithConfiguration:configuration];
```

pragma GCC diagnostic push

pragma GCC diagnostic ignored "-Wnonnull"

    NSURLSessionDataTask *localDataTask = [session dataTaskWithURL:nil];

pragma clang diagnostic pop

    IMP originalAFResumeIMP = method_getImplementation(class_getInstanceMethod([self class], @selector(af_resume)));
    Class currentClass = [localDataTask class];
    
    while (class_getInstanceMethod(currentClass, @selector(resume))) {
        Class superClass = [currentClass superclass];
        IMP classResumeIMP = method_getImplementation(class_getInstanceMethod(currentClass, @selector(resume)));
        IMP superclassResumeIMP = method_getImplementation(class_getInstanceMethod(superClass, @selector(resume)));
        if (classResumeIMP != superclassResumeIMP &&
            originalAFResumeIMP != classResumeIMP) {
            [self swizzleResumeAndSuspendMethodForClass:currentClass];
        }
        currentClass = [currentClass superclass];
    }
    
    [localDataTask cancel];
    [session finishTasksAndInvalidate];
}

}

(void)swizzleResumeAndSuspendMethodForClass:(Class)theClass {
Method afResumeMethod = class_getInstanceMethod(self, @selector(af_resume));
Method afSuspendMethod = class_getInstanceMethod(self, @selector(af_suspend));

if (af_addMethod(theClass, @selector(af_resume), afResumeMethod)) {
af_swizzleSelector(theClass, @selector(resume), @selector(af_resume));
}

if (af_addMethod(theClass, @selector(af_suspend), afSuspendMethod)) {
af_swizzleSelector(theClass, @selector(suspend), @selector(af_suspend));
}
}`</code></pre>

初始化過程與AFURLConnectionOperation的常駐線程處理方式不一樣，設(shè)置最大并發(fā)數(shù)量為1，想當(dāng)于是串行隊(duì)列.
<pre><code>`- (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 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];
    }
}];

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(taskDidResume:) name:AFNSURLSessionTaskDidResumeNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(taskDidSuspend:) name:AFNSURLSessionTaskDidSuspendNotification object:nil];

return self;

}`</code></pre>

任務(wù)的暫停和繼續(xù)通過通知來執(zhí)行:
<pre><code>`- (void)taskDidResume:(NSNotification *)notification {
NSURLSessionTask *task = notification.object;
if ([task respondsToSelector:@selector(taskDescription)]) {
if ([task.taskDescription isEqualToString:self.taskDescriptionForSessionTasks]) {
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidResumeNotification object:task];
});
}
}
}

(void)taskDidSuspend:(NSNotification *)notification {
NSURLSessionTask *task = notification.object;
if ([task respondsToSelector:@selector(taskDescription)]) {
if ([task.taskDescription isEqualToString:self.taskDescriptionForSessionTasks]) {
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidSuspendNotification object:task];
});
}
}
}`</code></pre>

進(jìn)度管理通過AFURLSessionManagerTaskDelegate來實(shí)現(xiàn)：
<pre><code>@interface AFURLSessionManagerTaskDelegate : NSObject <NSURLSessionTaskDelegate, NSURLSessionDataDelegate, NSURLSessionDownloadDelegate> @property (nonatomic, weak) AFURLSessionManager *manager; @property (nonatomic, strong) NSMutableData *mutableData; @property (nonatomic, strong) NSProgress *progress; @property (nonatomic, copy) NSURL *downloadFileURL; @property (nonatomic, copy) AFURLSessionDownloadTaskDidFinishDownloadingBlock downloadTaskDidFinishDownloading; @property (nonatomic, copy) AFURLSessionTaskCompletionHandler completionHandler; @end</code></pre>

異步轉(zhuǎn)同步的另外一種實(shí)現(xiàn)方式:
<pre><code>`- (NSArray *)tasksForKeyPath:(NSString *)keyPath {
__block NSArray *tasks = nil;
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
[self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
if ([keyPath isEqualToString:NSStringFromSelector(@selector(dataTasks))]) {
tasks = dataTasks;
} else if ([keyPath isEqualToString:NSStringFromSelector(@selector(uploadTasks))]) {
tasks = uploadTasks;
} else if ([keyPath isEqualToString:NSStringFromSelector(@selector(downloadTasks))]) {
tasks = downloadTasks;
} else if ([keyPath isEqualToString:NSStringFromSelector(@selector(tasks))]) {
tasks = [@[dataTasks, uploadTasks, downloadTasks] valueForKeyPath:@"@unionOfArrays.self"];
}

    dispatch_semaphore_signal(semaphore);
}];

dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

return tasks;

}

(NSArray *)tasks {
return [self tasksForKeyPath:NSStringFromSelector(_cmd)];
}
`</code></pre>

AFHTTPSessionManger是AFURLSessionManager的子類，提供了GET，HEAD，POST，PUT，PATCH和DELETE六種訪問方式.
<pre><code>`- (nullable NSURLSessionDataTask *)GET:(NSString *)URLString
parameters:(nullable id)parameters
success:(nullable void (^)(NSURLSessionDataTask *task, id responseObject))success
failure:(nullable void (^)(NSURLSessionDataTask * __nullable task, NSError *error))failure;

/**
Creates and runs an NSURLSessionDataTask with a HEAD request.

@param URLString The URL string used to create the request URL.
@param parameters The parameters to be encoded according to the client request serializer.
@param success A block object to be executed when the task finishes successfully. This block has no return value and takes a single arguments: the data task.
@param failure A block object to be executed when the task finishes unsuccessfully, or that finishes successfully, but encountered an error while parsing the response data. This block has no return value and takes a two arguments: the data task and the error describing the network or parsing error that occurred.