AFNetworking 的核心 AFURLSessionManager(二)

阅读数:5 2019 年 12 月 10 日 17:51

AFNetworking 的核心 AFURLSessionManager(二)

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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:

Objective-C

复制代码
- (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:

Objective-C

复制代码
- (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:

Objective-C

复制代码
- (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

Objective-C

复制代码
- (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:

Objective-C

复制代码
- (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:

Objective-C

复制代码
- (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:

Objective-C

复制代码
- (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:

Objective-C

复制代码
- (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:

Objective-C

复制代码
- (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

Objective-C

复制代码
__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:

Objective-C

复制代码
[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

Objective-C

复制代码
- (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

Objective-C

复制代码
- (void)URLSession:(__unused NSURLSession *)session
task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
#1:获取数据, 存储 `responseSerializer``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:

Objective-C

复制代码
__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

Objective-C

复制代码
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。

Objective-C

复制代码
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:

Objective-C

复制代码
- (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

Objective-C

复制代码
- (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

Objective-C

复制代码
+ (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。

Objective-C

复制代码
- (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:

本文转载自 Draveness 技术博客。

原文链接: https://draveness.me/afnetworking2

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