AFURLSessionManager

根据作者:
AFURLSessionManager是基于一个指定的NSURLSessionConfiguration对象创建的用于管理NSURLSession对象。 其遵循了NSURLSessionTaskDelegate、NSURLSessionDataDelegate、NSURLSessionDownloadDelegate、NSURLSessionDelegate协议

## NSURLSession & NSURLSessionTask Delegate Methods

 `AFURLSessionManager` implements the following delegate methods:

 ### `NSURLSessionDelegate`

 - `URLSession:didBecomeInvalidWithError:`
 - `URLSession:didReceiveChallenge:completionHandler:`
 - `URLSessionDidFinishEventsForBackgroundURLSession:`

 ### `NSURLSessionTaskDelegate`

 - `URLSession:willPerformHTTPRedirection:newRequest:completionHandler:`
 - `URLSession:task:didReceiveChallenge:completionHandler:`
 - `URLSession:task:didSendBodyData:totalBytesSent:totalBytesExpectedToSend:`
 - `URLSession:task:needNewBodyStream:`
 - `URLSession:task:didCompleteWithError:`

 ### `NSURLSessionDataDelegate`

 - `URLSession:dataTask:didReceiveResponse:completionHandler:`
 - `URLSession:dataTask:didBecomeDownloadTask:`
 - `URLSession:dataTask:didReceiveData:`
 - `URLSession:dataTask:willCacheResponse:completionHandler:`

 ### `NSURLSessionDownloadDelegate`

 - `URLSession:downloadTask:didFinishDownloadingToURL:`
 - `URLSession:downloadTask:didWriteData:totalBytesWritten:totalBytesWritten:totalBytesExpectedToWrite:`
 - `URLSession:downloadTask:didResumeAtOffset:expectedTotalBytes:`

 If any of these methods are overridden in a subclass, they _must_ call the `super` implementation first.

基本功能

  1. 创建和管理NSURLSession
  2. 管理NSURLSessionTask
  3. 实现NSURLSessionDelegate等协议中的代理方法
  4. 使用AFURLSessionManagerTaskDelegate管理进度
  5. 使用_AFURLSessionTaskSwizzling调剂方法
  6. 引入AFSecurityPolicy保证请求的安全
  7. 引入AFNetworkReachabilityManager监控网络状态

创建和管理NSURLSession

初始化方法

- (instancetype)initWithSessionConfiguration:(NSURLSessionConfiguration *)configuration {

    //配置会话设置
    if (!configuration) {
        configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    }
    self.sessionConfiguration = configuration;
    
    self.operationQueue = [[NSOperationQueue alloc] init];
    self.operationQueue.maxConcurrentOperationCount = 1;
    
    //设置NSURLSessionDelegate代理 和 代理queue
    self.session = [NSURLSession sessionWithConfiguration:self.sessionConfiguration delegate:self delegateQueue:self.operationQueue];
    
    //初始化响应序列
    self.responseSerializer = [AFJSONResponseSerializer serializer];
    
    //初始化安全认证
    self.securityPolicy = [AFSecurityPolicy defaultPolicy];
    
    #if !TARGET_OS_WATCH
    //初始化网络监测状态
    self.reachabilityManager = [AFNetworkReachabilityManager sharedManager];
    #endif

    //初始化保存dataTask的字典
    self.mutableTaskDelegatesKeyedByTaskIdentifier = [[NSMutableDictionary alloc] init];
    
    self.lock = [[NSLock alloc] init];
    self.lock.name = AFURLSessionManagerLockName;
    
    //为已有task设置代理  
    [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];
        }
    }];
}

NSURLSessionTask

AFURLSessionManager中提供了一系列的获取dataTask的方法

//upload 
- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request
                                         fromFile:(NSURL *)fileURL
                                         progress:(void (^)(NSProgress *uploadProgress)) uploadProgressBlock
                                completionHandler:(void (^)(NSURLResponse *response, id responseObject, NSError *error))completionHandler

//download                                
- (NSURLSessionDownloadTask *)downloadTaskWithRequest:(NSURLRequest *)request
                                             progress:(void (^)(NSProgress *downloadProgress)) downloadProgressBlock
                                          destination:(NSURL * (^)(NSURL *targetPath, NSURLResponse *response))destination
                                    completionHandler:(void (^)(NSURLResponse *response, NSURL *filePath, NSError *error))completionHandler


//
- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request
                               uploadProgress:(nullable void (^)(NSProgress *uploadProgress)) uploadProgressBlock
                             downloadProgress:(nullable void (^)(NSProgress *downloadProgress)) downloadProgressBlock
                            completionHandler:(nullable void (^)(NSURLResponse *response, id _Nullable responseObject,  NSError * _Nullable error))completionHandler {

    __block NSURLSessionDataTask *dataTask = nil;
    url_session_manager_create_task_safely(^{
        dataTask = [self.session dataTaskWithRequest:request];
    });

    [self addDelegateForDataTask:dataTask uploadProgress:uploadProgressBlock downloadProgress:downloadProgressBlock completionHandler:completionHandler];

    return dataTask;
}

使用url_session_manager_create_task_safely的原因是 苹果的框架的bug，在并行队列中创建task时，因为可能返回的taskIdentifier，先前的complectionHandel被替换为新的，即对第二个任务调用第一个响应.（在iOS8之后问题已修复）

调用了addDelegateForDataTask: uploadProgress: downloadProgress: completionHandler:方法

- (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] initWithTask:dataTask];
    delegate.manager = self;
    delegate.completionHandler = completionHandler;

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

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

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

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

- (void)addNotificationObserverForTask:(NSURLSessionTask *)task {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(taskDidResume:) name:AFNSURLSessionTaskDidResumeNotification object:task];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(taskDidSuspend:) name:AFNSURLSessionTaskDidSuspendNotification object:task];
}

创建了一个AFURLSessionManagerTaskDelegate对象
并在其内部执行了setDelegate来设置代理

AFURLSessionManager就是通过字典mutableTaskDelegatesKeyedByTaskIdentifier来存储和管理每一个NSURLSessionTask，它以taskIdentifier为键存储task

NSURLSessionDelegate

前面说过AFURLSessionManager遵循了一下代理

  • NSURLSessionDelegate
  • NSURLSessionTaskDelegate
  • NSURLSessionDataDelegate
  • NSURLSessionDownloadDelegate

我们在使用AFURLSessionManager的初始化方法时 设置 session的delegate为self

- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(NSError *)error {
    if (self.sessionDidBecomeInvalid) {
        self.sessionDidBecomeInvalid(session, error);
    }

    [[NSNotificationCenter defaultCenter] postNotificationName:AFURLSessionDidInvalidateNotification object:session];
}

- (void)URLSession:(NSURLSession *)session
didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
 completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {
    
 }

遵循这些协议实现其方法，然后将其提供更简洁的block接口
将block存在对应属性中，当代理方法调用时，执行对应的block

- (void)setSessionDidBecomeInvalidBlock:(void (^)(NSURLSession *session, NSError *error))block {
    self.sessionDidBecomeInvalid = block;
}

- (void)setSessionDidReceiveAuthenticationChallengeBlock:(NSURLSessionAuthChallengeDisposition (^)(NSURLSession *session, NSURLAuthenticationChallenge *challenge, NSURLCredential * __autoreleasing *credential))block {
    self.sessionDidReceiveAuthenticationChallenge = block;
}

AFURLSessionManagerTaskDelegate管理进度

主要为task提供进度管理功能，并在task结束时回调，即调用completionHandel

- (instancetype)initWithTask:(NSURLSessionTask *)task {
    _mutableData = [NSMutableData data];
    _uploadProgress = [[NSProgress alloc] initWithParent:nil userInfo:nil];
    _downloadProgress = [[NSProgress alloc] initWithParent:nil userInfo:nil];
    
    __weak __typeof__(task) weakTask = task;
    for (NSProgress *progress in @[ _uploadProgress, _downloadProgress ])
    {
        progress.totalUnitCount = NSURLSessionTransferSizeUnknown;
        progress.cancellable = YES;
        progress.cancellationHandler = ^{
            [weakTask cancel];
        };
        progress.pausable = YES;
        progress.pausingHandler = ^{
            [weakTask suspend];
        };
#if AF_CAN_USE_AT_AVAILABLE
        if (@available(iOS 9, macOS 10.11, *))
#else
        if ([progress respondsToSelector:@selector(setResumingHandler:)])
#endif
        {
            progress.resumingHandler = ^{
                [weakTask resume];
            };
        }
        
        [progress addObserver:self
                   forKeyPath:NSStringFromSelector(@selector(fractionCompleted))
                      options:NSKeyValueObservingOptionNew
                      context:NULL];
    }
    return self;
}

主要设置了uploadProgress和downloadProgress的回调，当NSProgress的状态改变时，调用相应的task方法，如：resume和suspend来改变task状态

代理方法 URLSession:task:didCompleteWithError:

当sessionManager收到代理,将消息转发给task对应的AFURLSessionManagerTaskDelegate

- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error {
    if (delegate) {
        [delegate URLSession:session task:task didCompleteWithError:error];
        [self removeDelegateForTask:task];
    }

    if (self.taskDidComplete) {
        self.taskDidComplete(session, task, error);
    }
}

AFURLSessionManagerTaskDelegate主要调用传入的completionHandle,然后发出AFNetworkingTaskDidCompleteNotification通知

- (void)URLSession:(__unused NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error {
    __strong AFURLSessionManager *manager = self.manager;
    //从mutableData取出数据 设置UserInfo
    __block NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
    userInfo[AFNetworkingTaskDidCompleteResponseSerializerKey] = manager.responseSerializer;
    NSData *data = nil;
    if (self.mutableData) {
        data = [self.mutableData copy];
        self.mutableData = nil;
    }
    if (self.downloadFileURL) {
        userInfo[AFNetworkingTaskDidCompleteAssetPathKey] = self.downloadFileURL;
    } else if (data) {
        userInfo[AFNetworkingTaskDidCompleteResponseDataKey] = data;
    }
    //使用 complectionGroup或者complectionQueue 否则创建一个group和主线程调用completionHandler，并在主线程发出通知
    if (error) {
        dispatch_group_async(manager.completionGroup, manager.completionQueue, ^{
            self.completionHandler(task.response, responseObject, error);
            dispatch_async(dispatch_get_main_queue(), ^{
                [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidCompleteNotification object:task userInfo:userInfo];
            });
        })
    } else {
        dispatch_async(url_session_manager_processing_queue(), ^{
            responseObject = [manager.responseSerializer responseObjectForResponse:task.response data:data error:&serializationError];
            dispatch_group_async(manager.completionGroup, manager.completionQueue, ^{
            self.completionHandler(task.response, responseObject, error);
            dispatch_async(dispatch_get_main_queue(), ^{
                [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidCompleteNotification object:task userInfo:userInfo];
            });
        })
        }
    }

}

代理方法 URLSession:dataTask:didReceiveData: 和 - URLSession:downloadTask:didFinishDownloadingToURL:

//在收到数据时调用
- (void)URLSession:(__unused NSURLSession *)session
          dataTask:(__unused NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data
{
    self.downloadProgress.totalUnitCount = dataTask.countOfBytesExpectedToReceive;
    self.downloadProgress.completedUnitCount = dataTask.countOfBytesReceived;

    [self.mutableData appendData:data];
}

//在下载对应文件时调用
- (void)URLSession:(NSURLSession *)session
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location
{
    self.downloadFileURL = nil;

    if (self.downloadTaskDidFinishDownloading) {
        self.downloadFileURL = self.downloadTaskDidFinishDownloading(session, downloadTask, location);
        if (self.downloadFileURL) {
            NSError *fileManagerError = nil;

            if (![[NSFileManager defaultManager] moveItemAtURL:location toURL:self.downloadFileURL error:&fileManagerError]) {
                [[NSNotificationCenter defaultCenter] postNotificationName:AFURLSessionDownloadTaskDidFailToMoveFileNotification object:downloadTask userInfo:fileManagerError.userInfo];
            }
        }
    }
}

_AFURLSessionTaskSwizzling调剂方法

_AFURLSessionTaskSwizzling功能就是修改NSURLSessionTask的resume和suspend方法，替换原有的实现

- (void)af_resume {
    NSAssert([self respondsToSelector:@selector(state)], @"Does not respond to state");
    NSURLSessionTaskState state = [self state];
    [self af_resume];
    
    if (state != NSURLSessionTaskStateRunning) {
        [[NSNotificationCenter defaultCenter] postNotificationName:AFNSURLSessionTaskDidResumeNotification object:self];
    }
}

- (void)af_suspend {
    NSAssert([self respondsToSelector:@selector(state)], @"Does not respond to state");
    NSURLSessionTaskState state = [self state];
    [self af_suspend];
    
    if (state != NSURLSessionTaskStateSuspended) {
        [[NSNotificationCenter defaultCenter] postNotificationName:AFNSURLSessionTaskDidSuspendNotification object:self];
    }
}

这是为了在调用方法时 发出通知

在load时执行方法替换操作，但是因为NSURLSessionTask在iOS7和8的实现有所不同 导致以下代码有点复杂

在iOS7上，localDataTask是一个__NSCFLocalDataTask，它继承自__NSCFLocalSessionTask，它继承自__NSCFURLSessionTask
在iOS8上，localDataTask是一个__NSCFLocalDataTask，它继承自__NSCFLocalSessionTask，它继承自NSURLSessionTask
在iOS7上，__ NSCFLocalSessionTask和__NSCFURLSessionTask是唯一具有自己的resume和suspend实现的两个类，而__NSCFLocalSessionTask则不会调用超级。这意味着两个类都需要调整。
在iOS 8上，NSURLSessionTask是唯一实现resume和suspend`的类。这意味着这是唯一需要调整的类。

因为NSURLSession是使用类群实现的，因此从API请求的类并不是将获得的类的类型.所以直接简单的使用[NSURLSessionTask class]将不起作用的。需要用NSURLSession来实际创建一个对象，并从该对象处获取该类

因此实现方案如下

  1. 通过获取NSURLSession实例来获取_NSCFLocalDataTask实例
  2. 获取指向af_resume的原始实现指针
  3. 检查当前类是否有resume实现 如果有继续执行步骤4
  4. 获取当前类的父类
  5. 获取父类的resume实现指针
  6. 获取当前类的resume实现指针
  7. 加入当前类的resume实现与父类不同，并且当前类的resume实现和af_resume实现不同，则替换两个方法实现
  8. 设置当前类为父类 重复步骤3-8
+ (void)load {
    if (NSClassFromString(@"NSURLSessionTask")) {
        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];
    }
}

引入AFSecurityPolicy来保证请求的安全

调用了-[AFSecurityPolicy evaluateServerTrust:forDomain:]方法来判断当前服务器是否被信任

引入AFNetworkReachabilityManager监控网络状态

AFURLSessionManager的网络状态由AFNetworkReachabilityManager监控，并持有一个该对象



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