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AFNetworking 的核心 AFURLSessionManager(二).md

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AFNetworking 的核心 AFURLSessionManager(二)

Blog: Draveness

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AFURLSessionManager 绝对可以称得上是 AFNetworking 的核心。

  1. 负责创建和管理 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

我们会在这里着重介绍上面七个功能中的前五个,分析它是如何包装 NSURLSession 以及众多代理方法的。

创建和管理 NSURLSession

在使用 AFURLSessionManager 时,第一件要做的事情一定是初始化:

- (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];

    self.reachabilityManager = [AFNetworkReachabilityManager sharedManager];

    self.mutableTaskDelegatesKeyedByTaskIdentifier = [[NSMutableDictionary alloc] init];

    self.lock = [[NSLock alloc] init];
    self.lock.name = AFURLSessionManagerLockName;
    
    #1: 为已有的 task 设置代理, 略

    return self;
}

在初始化方法中,需要完成初始化一些自己持有的实例:

  1. 初始化会话配置(NSURLSessionConfiguration),默认为 defaultSessionConfiguration
  2. 初始化会话(session),并设置会话的代理以及代理队列
  3. 初始化管理响应序列化(AFJSONResponseSerializer),安全认证(AFSecurityPolicy)以及监控网络状态(AFNetworkReachabilityManager)的实例
  4. 初始化保存 data task 的字典(mutableTaskDelegatesKeyedByTaskIdentifier)

管理 NSURLSessionTask

接下来,在获得了 AFURLSessionManager 的实例之后,我们可以通过以下方法创建 NSURLSessionDataTask 的实例:

- (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;

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

...

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

...

这里省略了一些返回 NSURLSessionTask 的方法,因为这些接口的形式都是差不多的。

我们将以 - [AFURLSessionManager dataTaskWithRequest:uploadProgress:downloadProgress:completionHandler:] 方法的实现为例,分析它是如何实例化并返回一个 NSURLSessionTask 的实例的:

- (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;
}

url_session_manager_create_task_safely 的调用是因为苹果框架中的一个 bug #2093,如果有兴趣可以看一下,在这里就不说明了

  1. 调用 - [NSURLSession dataTaskWithRequest:] 方法传入 NSURLRequest
  2. 调用 - [AFURLSessionManager addDelegateForDataTask:uploadProgress:downloadProgress:completionHandler:] 方法创建一个 AFURLSessionManagerTaskDelegate 对象
  3. completionHandler uploadProgressBlockdownloadProgressBlock 传入该对象并在相应事件发生时进行回调
- (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;
}

在这个方法中同时调用了另一个方法 - [AFURLSessionManager setDelegate:forTask:] 来设置代理:

- (void)setDelegate:(AFURLSessionManagerTaskDelegate *)delegate
            forTask:(NSURLSessionTask *)task
{
		
	#1: 检查参数, 略

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

正如上面所提到的,AFURLSessionManager 就是通过字典 mutableTaskDelegatesKeyedByTaskIdentifier 来存储并管理每一个 NSURLSessionTask,它以 taskIdentifier 为键存储 task。

该方法使用 NSLock 来保证不同线程使用 mutableTaskDelegatesKeyedByTaskIdentifier 时,不会出现线程竞争的问题。

同时调用 - setupProgressForTask:,我们会在下面具体介绍这个方法。

实现 NSURLSessionDelegate 等协议中的代理方法

AFURLSessionManager 的头文件中可以看到,它遵循了多个协议,其中包括:

  • NSURLSessionDelegate
  • NSURLSessionTaskDelegate
  • NSURLSessionDataDelegate
  • NSURLSessionDownloadDelegate

它在初始化方法 - [AFURLSessionManager initWithSessionConfiguration:]NSURLSession 的代理指向 self,然后实现这些方法,提供更简洁的 block 的接口:

- (void)setSessionDidBecomeInvalidBlock:(nullable void (^)(NSURLSession *session, NSError *error))block;
- (void)setSessionDidReceiveAuthenticationChallengeBlock:(nullable NSURLSessionAuthChallengeDisposition (^)(NSURLSession *session, NSURLAuthenticationChallenge *challenge, NSURLCredential * _Nullable __autoreleasing * _Nullable credential))block;
...

它为所有的代理协议都提供了对应的 block 接口,方法实现的思路都是相似的,我们以 - [AFNRLSessionManager setSessionDidBecomeInvalidBlock:] 为例。

首先调用 setter 方法,将 block 存入 sessionDidBecomeInvalid 属性中:

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

当代理方法调用时,如果存在对应的 block,会执行对应的 block:

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

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

其他相似的接口实现也都差不多,这里直接跳过了。

使用 AFURLSessionManagerTaskDelegate 管理进度

在上面我们提到过 AFURLSessionManagerTaskDelegate 类,它主要为 task 提供进度管理功能,并在 task 结束时回调, 也就是调用在 - [AFURLSessionManager dataTaskWithRequest:uploadProgress:downloadProgress:completionHandler:] 等方法中传入的 completionHandler

我们首先分析一下 AFURLSessionManagerTaskDelegate 是如何对进度进行跟踪的:

- (void)setupProgressForTask:(NSURLSessionTask *)task {

	#1:设置在上传进度或者下载进度状态改变时的回调
	
	#2:KVO

}

该方法的实现有两个部分,一部分是对代理持有的两个属性 uploadProgressdownloadProgress 设置回调

__weak __typeof__(task) weakTask = task;

self.uploadProgress.totalUnitCount = task.countOfBytesExpectedToSend;
[self.uploadProgress setCancellable:YES];
[self.uploadProgress setCancellationHandler:^{
   __typeof__(weakTask) strongTask = weakTask;
   [strongTask cancel];
}];
[self.uploadProgress setPausable:YES];
[self.uploadProgress setPausingHandler:^{
   __typeof__(weakTask) strongTask = weakTask;
   [strongTask suspend];
}];
if ([self.uploadProgress respondsToSelector:@selector(setResumingHandler:)]) {
   [self.uploadProgress setResumingHandler:^{
       __typeof__(weakTask) strongTask = weakTask;
       [strongTask resume];
   }];
}

这里只有对 uploadProgress 设置回调的代码,设置 downloadProgress 与这里完全相同

主要目的是在对应 NSProgress 的状态改变时,调用 resume suspend 等方法改变 task 的状态。

第二部分是对 task 和 NSProgress 属性进行键值观测:

[task addObserver:self
      forKeyPath:NSStringFromSelector(@selector(countOfBytesReceived))
         options:NSKeyValueObservingOptionNew
         context:NULL];
[task addObserver:self
      forKeyPath:NSStringFromSelector(@selector(countOfBytesExpectedToReceive))
         options:NSKeyValueObservingOptionNew
         context:NULL];

[task addObserver:self
      forKeyPath:NSStringFromSelector(@selector(countOfBytesSent))
         options:NSKeyValueObservingOptionNew
         context:NULL];
[task addObserver:self
      forKeyPath:NSStringFromSelector(@selector(countOfBytesExpectedToSend))
         options:NSKeyValueObservingOptionNew
         context:NULL];

[self.downloadProgress addObserver:self
                       forKeyPath:NSStringFromSelector(@selector(fractionCompleted))
                          options:NSKeyValueObservingOptionNew
                          context:NULL];
[self.uploadProgress addObserver:self
                     forKeyPath:NSStringFromSelector(@selector(fractionCompleted))
                        options:NSKeyValueObservingOptionNew
                        context:NULL];

observeValueForKeypath:ofObject:change:context: 方法中改变进度,并调用 block

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
    if ([object isKindOfClass:[NSURLSessionTask class]]) {
        if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesReceived))]) {
            self.downloadProgress.completedUnitCount = [change[@"new"] longLongValue];
        } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesExpectedToReceive))]) {
            self.downloadProgress.totalUnitCount = [change[@"new"] longLongValue];
        } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesSent))]) {
            self.uploadProgress.completedUnitCount = [change[@"new"] longLongValue];
        } else if ([keyPath isEqualToString:NSStringFromSelector(@selector(countOfBytesExpectedToSend))]) {
            self.uploadProgress.totalUnitCount = [change[@"new"] longLongValue];
        }
    }
    else if ([object isEqual:self.downloadProgress]) {
        if (self.downloadProgressBlock) {
            self.downloadProgressBlock(object);
        }
    }
    else if ([object isEqual:self.uploadProgress]) {
        if (self.uploadProgressBlock) {
            self.uploadProgressBlock(object);
        }
    }
}

对象的某些属性改变时更新 NSProgress 对象或使用 block 传递 NSProgress 对象 self.uploadProgressBlock(object)

代理方法 URLSession:task:didCompleteWithError:

在每一个 NSURLSessionTask 结束时,都会在代理方法 URLSession:task:didCompleteWithError: 中:

  1. 调用传入的 completionHander block
  2. 发出 AFNetworkingTaskDidCompleteNotification 通知
- (void)URLSession:(__unused NSURLSession *)session
              task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
    #1:获取数据, 存储 `responseSerializer` 和 `downloadFileURL`

    if (error) {
    	#2:在存在错误时调用 `completionHandler`
    } else {
		#3:调用 `completionHandler`
    }
}

这是整个代理方法的骨架,先看一下最简单的第一部分代码:

__block NSMutableDictionary *userInfo = [NSMutableDictionary dictionary];
userInfo[AFNetworkingTaskDidCompleteResponseSerializerKey] = manager.responseSerializer;

//Performance Improvement from #2672
NSData *data = nil;
if (self.mutableData) {
   data = [self.mutableData copy];
   //We no longer need the reference, so nil it out to gain back some memory.
   self.mutableData = nil;
}

if (self.downloadFileURL) {
   userInfo[AFNetworkingTaskDidCompleteAssetPathKey] = self.downloadFileURL;
} else if (data) {
   userInfo[AFNetworkingTaskDidCompleteResponseDataKey] = data;
}

这部分代码从 mutableData 中取出了数据,设置了 userInfo

userInfo[AFNetworkingTaskDidCompleteErrorKey] = error;

dispatch_group_async(manager.completionGroup ?: url_session_manager_completion_group(), manager.completionQueue ?: dispatch_get_main_queue(), ^{
    if (self.completionHandler) {
        self.completionHandler(task.response, responseObject, error);
    }

    dispatch_async(dispatch_get_main_queue(), ^{
        [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidCompleteNotification object:task userInfo:userInfo];
    });
});

如果当前 manager 持有 completionGroup 或者 completionQueue 就使用它们。否则会创建一个 dispatch_group_t 并在主线程中调用 completionHandler 并发送通知(在主线程中)。

如果在执行当前 task 时没有遇到错误,那么先对数据进行序列化,然后同样调用 block 并发送通知。

dispatch_async(url_session_manager_processing_queue(), ^{
    NSError *serializationError = nil;
    responseObject = [manager.responseSerializer responseObjectForResponse:task.response data:data error:&serializationError];

    if (self.downloadFileURL) {
        responseObject = self.downloadFileURL;
    }

    if (responseObject) {
        userInfo[AFNetworkingTaskDidCompleteSerializedResponseKey] = responseObject;
    }

    if (serializationError) {
        userInfo[AFNetworkingTaskDidCompleteErrorKey] = serializationError;
    }

    dispatch_group_async(manager.completionGroup ?: url_session_manager_completion_group(), manager.completionQueue ?: dispatch_get_main_queue(), ^{
        if (self.completionHandler) {
            self.completionHandler(task.response, responseObject, serializationError);
        }

        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:AFNetworkingTaskDidCompleteNotification object:task userInfo:userInfo];
        });
    });
});

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

这两个代理方法分别会在收到数据或者完成下载对应文件时调用,作用分别是为 mutableData 追加数据和处理下载的文件:

- (void)URLSession:(__unused NSURLSession *)session
          dataTask:(__unused NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data
{
    [self.mutableData appendData:data];
}

- (void)URLSession:(NSURLSession *)session
      downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location
{
    NSError *fileManagerError = nil;
    self.downloadFileURL = nil;

    if (self.downloadTaskDidFinishDownloading) {
        self.downloadFileURL = self.downloadTaskDidFinishDownloading(session, downloadTask, location);
        if (self.downloadFileURL) {
            [[NSFileManager defaultManager] moveItemAtURL:location toURL:self.downloadFileURL error:&fileManagerError];

            if (fileManagerError) {
                [[NSNotificationCenter defaultCenter] postNotificationName:AFURLSessionDownloadTaskDidFailToMoveFileNotification object:downloadTask userInfo:fileManagerError.userInfo];
            }
        }
    }
}

使用 _AFURLSessionTaskSwizzling 调剂方法

_AFURLSessionTaskSwizzling 的唯一功能就是修改 NSURLSessionTaskresumesuspend 方法,使用下面的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];
    }
}

这样做的目的是为了在方法 resume 或者 suspend 被调用时发出通知。

具体方法调剂的过程是在 + load 方法中进行的

load 方法只会在整个文件被引入时调用一次

+ (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];
    }
}
  1. 首先用 NSClassFromString(@"NSURLSessionTask") 判断当前部署的 iOS 版本是否含有类 NSURLSessionTask
  2. 因为 iOS7 和 iOS8 上对于 NSURLSessionTask 的实现不同,所以会通过 - [NSURLSession dataTaskWithURL:] 方法返回一个 NSURLSessionTask 实例
  3. 取得当前类 _AFURLSessionTaskSwizzling 中的实现 af_resume
  4. 如果当前类 currentClassresume 方法
    • 真:5
    • 假:6
  5. 使用 swizzleResumeAndSuspendMethodForClass: 调剂该类的 resumesuspend 方法
  6. currentClass = [currentClass superclass]

这里复杂的实现是为了解决 bug #2702

引入 AFSecurityPolicy 保证请求的安全

AFSecurityPolicyAFNetworking 用来保证 HTTP 请求安全的类,它被 AFURLSessionManager 持有,如果你在 AFURLSessionManager 的实现文件中搜索 self.securityPolicy,你只会得到三条结果:

  1. 初始化 self.securityPolicy = [AFSecurityPolicy defaultPolicy]
  2. 收到连接层的验证请求时
  3. 任务接收到验证请求时

在 API 调用上,后两者都调用了 - [AFSecurityPolicy evaluateServerTrust:forDomain:] 方法来判断当前服务器是否被信任,我们会在接下来的文章中具体介绍这个方法的实现的作用。

- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task
didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
 completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler
{
    NSURLSessionAuthChallengeDisposition disposition = NSURLSessionAuthChallengePerformDefaultHandling;
    __block NSURLCredential *credential = nil;

    if (self.taskDidReceiveAuthenticationChallenge) {
        disposition = self.taskDidReceiveAuthenticationChallenge(session, task, challenge, &credential);
    } else {
        if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {
            if ([self.securityPolicy evaluateServerTrust:challenge.protectionSpace.serverTrust forDomain:challenge.protectionSpace.host]) {
                disposition = NSURLSessionAuthChallengeUseCredential;
                credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
            } else {
                disposition = NSURLSessionAuthChallengeRejectProtectionSpace;
            }
        } else {
            disposition = NSURLSessionAuthChallengePerformDefaultHandling;
        }
    }

    if (completionHandler) {
        completionHandler(disposition, credential);
    }
}

如果没有传入 taskDidReceiveAuthenticationChallenge block,只有在上述方法返回 YES 时,才会获得认证凭证 credential

引入 AFNetworkReachabilityManager 监控网络状态

AFSecurityPolicy 相同,AFURLSessionManager 对网络状态的监控是由 AFNetworkReachabilityManager 来负责的,它仅仅是持有一个 AFNetworkReachabilityManager 的对象。

真正需要判断网络状态时,仍然需要开发者调用对应的 API 获取网络状态

小结

  1. AFURLSessionManager 是对 NSURLSession 的封装
  2. 它通过 - [AFURLSessionManager dataTaskWithRequest:completionHandler:] 等接口创建 NSURLSessionDataTask 的实例
  3. 持有一个字典 mutableTaskDelegatesKeyedByTaskIdentifier 管理这些 data task 实例
  4. 引入 AFURLSessionManagerTaskDelegate 来对传入的 uploadProgressBlock downloadProgressBlock completionHandler 在合适的时间进行调用
  5. 实现了全部的代理方法来提供 block 接口
  6. 通过方法调剂在 data task 状态改变时,发出通知

相关文章

关于其他 AFNetworking 源代码分析的其他文章:

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