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:] 方法返回一个 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,AFNRUSessionManager 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:7
  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

关于其他 AFNetworking 源代码分析的其他文章:

关注仓库,及时获得更新:iOS-Source-Code-Analyze

Blog: Draveness

Draveness

iOS Developer / Rails / Elixir

Maine, USA draveness.me
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