NSURLSession用法初探(一)

一、URL Session的基本概念

1.三种工作模式:

默认会话模式(default):工作模式类似于原来的NSURLConnection,使用的是基于磁盘缓存的持久化策略,使用用户keychain中保存的证书进行认证授权。

瞬时会话模式(ephemeral):该模式不使用磁盘保存任何数据。所有和会话相关的caches,证书,cookies等都被保存在RAM中,因此当程序使会话无效,这些缓存的数据就会被自动清空。

后台会话模式(background):该模式在后台完成上传和下载,在创建Configuration对象的时候需要提供一个NSString类型的ID用于标识完成工作的后台会话。

2.NSURLSession支持的三种任务

NSURLSession类支持三种类型的任务:加载数据,下载和上传。

二、相关的类

NSURLConnection这个名字,实际上指的是一组构成Foundation框架中URL加载系统的相互关联的组件:NSURLRequest,NSURLResponse,NSURLProtocol,NSURLCache,NSHTTPCookieStorage,NSURLCredentialStorage,以及和它同名的NSURLConnection。

在WWDC 2013中,Apple的团队对NSURLConnection进行了重构,并推出了NSURLSession作为替代。

NSURLSession也是一组相互依赖的类,它的大部分组件和NSURLConnection中的组件相同如NSURLRequest,NSURLCache等。而NSURLSession的不同之处在于,它将NSURLConnection替换为NSURLSession和NSURLSessionConfiguration,以及3个NSURLSessionTask的子类:NSURLSessionDataTask, NSURLSessionUploadTask, 和NSURLSessionDownloadTask。

下面来说下NSURLSession新推出的类:

1.NSURLSessionConfiguration类

其中NSURLSessionConfiguration用于配置会话的属性,可以通过该类配置会话的工作模式:

+ (NSURLSessionConfiguration *)defaultSessionConfiguration;  

+ (NSURLSessionConfiguration *)ephemeralSessionConfiguration;  

+ (NSURLSessionConfiguration *)backgroundSessionConfiguration:(NSString *)identifier;  

在backgroundSessionConfiguration:方法中的identifier参数指定了会话的ID,用于标记后台的session。

该类的其中两个属性:

/* allow request to route over cellular. */ 

 @property BOOL allowsCellularAccess;   

 /* allows background tasks to be scheduled at the discretion of the system for optimal performance. */ 

 @property (getter=isDiscretionary) BOOL discretionary NS_AVAILABLE(NA, 7_0);  

allowsCellularAccess 属性指定是否允许使用蜂窝连接, discretionary属性为YES时表示当程序在后台运作时由系统自己选择最佳的网络连接配置,该属性可以节省通过蜂窝连接的带宽。在使用后台传输数据的时候,建议使用discretionary属性,而不是allowsCellularAccess属性,因为它会把WiFi和电源可用性考虑在内。补充:这个标志允许系统为分配任务进行性能优化。这意味着只有当设备有足够电量时,设备才通过Wifi进行数据传输。如果电量低,或者只仅有一个蜂窝连接,传输任务是不会运行的。后台传输总是在discretionary模式下运行。

2.NSURLSession类

获取NSURLSession类对象有几种方式:

/*  * The shared session uses the currently set global NSURLCache,  * NSHTTPCookieStorage and NSURLCredentialStorage objects.  */ 

 + (NSURLSession *)sharedSession;   

 /*  * Customization of NSURLSession occurs during creation of a new session.  

* If you only need to use the convenience routines with custom  

* configuration options it is not necessary to specify a delegate.  

* If you do specify a delegate, the delegate will be retained until after 

 * the delegate has been sent the URLSession:didBecomeInvalidWithError: message. 

 */

  + (NSURLSession *)sessionWithConfiguration:(NSURLSessionConfiguration *)configuration;  

+ (NSURLSession *)sessionWithConfiguration:(NSURLSessionConfiguration *)configuration delegate:(id)delegate delegateQueue:(NSOperationQueue *)queue; 

第一种方式是使用静态的sharedSession方法,该类使用共享的会话,该会话使用全局的Cache,Cookie和证书。

第二种方式是通过sessionWithConfiguration:方法创建对象,也就是创建对应配置的会话,与NSURLSessionConfiguration合作使用。

第三种方式是通过sessionWithConfiguration:delegate:delegateQueue方法创建对象,二三两种方式可以创建一个新会话并定制其会话类型。该方式中指定了session的委托和委托所处的队列。当不再需要连接时,可以调用Session的invalidateAndCancel直接关闭,或者调用finishTasksAndInvalidate等待当前Task结束后关闭。这时Delegate会收到URLSession:didBecomeInvalidWithError:这个事件。Delegate收到这个事件之后会被解引用。

3.NSURLSessionTask类

NSURLSessionTask是一个抽象子类,它有三个子类:NSURLSessionDataTask,NSURLSessionUploadTask和NSURLSessionDownloadTask。这三个类封装了现代应用程序的三个基本网络任务:获取数据,比如JSON或XML,以及上传和下载文件。

下面是其继承关系:

NSURLSession用法初探(一)_第1张图片
继承关系

有多种方法创建对应的任务对象:

(1)NSURLSessionDataTask通过request对象或url创建:

/* Creates a data task with the given request.  The request may have a body stream. */  

- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request;  

  /* Creates a data task to retrieve the contents of the given URL. */ 

 - (NSURLSessionDataTask *)dataTaskWithURL:(NSURL *)url; 

通过request对象或url创建,同时指定任务完成后通过completionHandler指定回调的代码块:

/*  * data task convenience methods.  These methods create tasks that 

 * bypass the normal delegate calls for response and data delivery, 

 * and provide a simple cancelable asynchronous interface to receiving  * data.  Errors will be returned in the NSURLErrorDomain,  

 * see.  The delegate, if any, will still be

* called for authentication challenges.

*/

- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request completionHandler:(void (^)(NSData *data, NSURLResponse *response, NSError *error))completionHandler;

- (NSURLSessionDataTask *)dataTaskWithURL:(NSURL *)url completionHandler:(void (^)(NSData *data, NSURLResponse *response, NSError *error))completionHandler;

(2)NSURLSessionUploadTask

通过request创建,在上传时指定文件源或数据源。

/* Creates an upload task with the given request.  The body of the request will be created from the file referenced by fileURL */

- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromFile:(NSURL *)fileURL;

/* Creates an upload task with the given request.  The body of the request is provided from the bodyData. */

- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromData:(NSData *)bodyData;

/* Creates an upload task with the given request.  The previously set body stream of the request (if any) is ignored and the URLSession:task:needNewBodyStream: delegate will be called when the body payload is required. */

- (NSURLSessionUploadTask *)uploadTaskWithStreamedRequest:(NSURLRequest *)request;

在创建upload task对象时,通过completionHandler指定任务完成后的回调代码块:

/*

* upload convenience method.

*/

- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromFile:(NSURL *)fileURL completionHandler:(void (^)(NSData *data, NSURLResponse *response, NSError *error))completionHandler;

- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromData:(NSData *)bodyData completionHandler:(void (^)(NSData *data, NSURLResponse *response, NSError *error))completionHandler;

(3)NSURLSessionDownloadTask

/* Creates a download task with the given request. */

- (NSURLSessionDownloadTask *)downloadTaskWithRequest:(NSURLRequest *)request;

/* Creates a download task to download the contents of the given URL. */

- (NSURLSessionDownloadTask *)downloadTaskWithURL:(NSURL *)url;

/* Creates a download task with the resume data.  If the download cannot be successfully resumed, URLSession:task:didCompleteWithError: will be called. */

- (NSURLSessionDownloadTask *)downloadTaskWithResumeData:(NSData *)resumeData;

下载任务支持断点续传,第三种方式是通过之前已经下载的数据来创建下载任务。

同样地可以通过completionHandler指定任务完成后的回调代码块:

/*

* download task convenience methods.  When a download successfully

* completes, the NSURL will point to a file that must be read or

* copied during the invocation of the completion routine.  The file

* will be removed automatically.

*/

- (NSURLSessionDownloadTask *)downloadTaskWithRequest:(NSURLRequest *)request completionHandler:(void (^)(NSURL *location, NSURLResponse *response, NSError *error))completionHandler;

- (NSURLSessionDownloadTask *)downloadTaskWithURL:(NSURL *)url completionHandler:(void (^)(NSURL *location, NSURLResponse *response, NSError *error))completionHandler;

- (NSURLSessionDownloadTask *)downloadTaskWithResumeData:(NSData *)resumeData completionHandler:(void (^)(NSURL *location, NSURLResponse *response, NSError *error))completionHandler;

4.NSURLSessionDelegate和NSURLSessionTaskDelegate协议

在协议的方法中可以完成各种各样的回调动作,如身份验证、完成任务后的动作、错误处理和后台任务完成的动作等。委托方法指定在NSURLSession中一定数量的字节传输使用int64_t类型的参数。

这里只说下后台任务的一个委托方法:

/* If an application has received an

* -application:handleEventsForBackgroundURLSession:completionHandler:

* message, the session delegate will receive this message to indicate

* that all messages previously enqueued for this session have been

* delivered.  At this time it is safe to invoke the previously stored

* completion handler, or to begin any internal updates that will

* result in invoking the completion handler.

*/

- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session NS_AVAILABLE_IOS(7_0);

配合使用的ApplicationDelegate方法:

// Applications using an NSURLSession with a background configuration may be launched or resumed in the background in order to handle the

// completion of tasks in that session, or to handle authentication. This method will be called with the identifier of the session needing

// attention. Once a session has been created from a configuration object with that identifier, the session's delegate will begin receiving

// callbacks. If such a session has already been created (if the app is being resumed, for instance), then the delegate will start receiving

// callbacks without any action by the application. You should call the completionHandler as soon as you're finished handling the callbacks.

- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler NS_AVAILABLE_IOS(7_0);

将任务切换到后台之后,Session的Delegate不会再收到和Task相关的消息。当所有Task全都完成后,程序将被唤醒,并调用ApplicationDelegate的application:handleEventsForBackgroundURLSession:completionHandler:回调,在这里要为后台session(由background session的identifier标识)指定对应的回调代码块。

随后,对于每一个完成的后台Task调用该Session的Delegate中的URLSession:downloadTask:didFinishDownloadingToURL:(成功的话)和URLSession:task:didCompleteWithError:(成功或者失败都会调用)方法做处理,以上的回调代码块可以在这里调用。

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