版本记录
| 版本号 | 时间 |
|---|---|
| V1.0 | 2018.02.24 |
前言
我们做APP，文字和图片是绝对不可缺少的元素，特别是图片一般存储在图床里面，一般公司可以委托第三方保存，NB的公司也可以自己存储图片，ios有很多图片加载的第三方框架，其中最优秀的莫过于SDWebImage，它几乎可以满足你所有的需求，用了好几年这个框架，今天想总结一下。感兴趣的可以看其他几篇。
1. SDWebImage探究（一）
2. SDWebImage探究（二）
3. SDWebImage探究（三）
4. SDWebImage探究（四）
5. SDWebImage探究（五）
6. SDWebImage探究（六） —— 图片类型判断深入研究
7. SDWebImage探究（七） —— 深入研究图片下载流程（一）之有关option的位移枚举的说明
8. SDWebImage探究（八） —— 深入研究图片下载流程（二）之开始下载并返回下载结果的总的方法
9. SDWebImage探究（九） —— 深入研究图片下载流程（三）之下载之前的缓存查询操作
10. SDWebImage探究（十） —— 深入研究图片下载流程（四）之查询缓存后的block回调处理
11. SDWebImage探究（十一） —— 深入研究图片下载流程（五）之SDWebImageDownloadToken和操作对象的生成和返回
12. SDWebImage探究（十二） —— 深入研究图片下载流程（六）之下载器到具体下载操作的代理分发实现
NSURLSession中的几种代理
这些代理方法都在NSURLSession中，我们先简单的看一下该类的功能。
- 通过URL将数据下载到内存
- 通过URL将数据下载到文件系统
- 将数据上传到指定URL
- 在后台完成上述功能
NSURLSession是下载的上下文，其中包含了很多下载相关的代理分发，包括NSURLSessionDelegate、NSURLSessionTaskDelegate 、NSURLSessionDataDelegate 、NSURLSessionDownloadDelegate 、NSURLSessionStreamDelegate ，它们之间的关系看继承就已经很清楚了。
它们之间的关系可以参考下图：
NSURLSessionDelegate
先看一下API文档
/*
* Messages related to the URL session as a whole
与整个URL session相关的消息
*/
@protocol NSURLSessionDelegate
@optional
/* The last message a session receives. A session will only become
* invalid because of a systemic error or when it has been
* explicitly invalidated, in which case the error parameter will be nil.
*/
会话收到的最后一条消息。 一个会话只会因为系统错误或者当它明确地失效而失效，
在这种情况下，错误参数将为nil。
- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(nullable NSError *)error;
/* If implemented, when a connection level authentication challenge
* has occurred, this delegate will be given the opportunity to
* provide authentication credentials to the underlying
* connection. Some types of authentication will apply to more than
* one request on a given connection to a server (SSL Server Trust
* challenges). If this delegate message is not implemented, the
* behavior will be to use the default handling, which may involve user
* interaction.
*/
如果已实现，则在发生连接级别身份验证质询时，此代理将有机会向底层连接提供身份验证凭据。
某些类型的身份验证将适用于给定服务器连接上的多个请求（SSL服务器信任SSL Server Trust挑战）。
如果此代理消息未实现，则行为将使用默认处理，这可能涉及用户交互。
- (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential * _Nullable credential))completionHandler;
/* 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.
*/
如果应用程序收到了-application：handleEventsForBackgroundURLSession：completionHandler：
消息，则会话代理将收到此消息，指示之前已为此会话入队的所有消息已传递。 此时，调用先前存储的
完成处理程序或开始任何内部更新将导致调用完成处理程序，这一过程是安全的。
- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session API_AVAILABLE(ios(7.0), watchos(2.0), tvos(9.0)) API_UNAVAILABLE(macos);
@end
NSURLSessionTaskDelegate
下面看一下API
/*
* Messages related to the operation of a specific task.
与特定任务的操作有关的消息
*/
@protocol NSURLSessionTaskDelegate
@optional
/*
* Sent when the system is ready to begin work for a task with a delayed start
* time set (using the earliestBeginDate property). The completionHandler must
* be invoked in order for loading to proceed. The disposition provided to the
* completion handler continues the load with the original request provided to
* the task, replaces the request with the specified task, or cancels the task.
* If this delegate is not implemented, loading will proceed with the original
* request.
*
* Recommendation: only implement this delegate if tasks that have the
* earliestBeginDate property set may become stale and require alteration prior
* to starting the network load.
*
* If a new request is specified, the allowsCellularAccess property from the
* new request will not be used; the allowsCellularAccess property from the
* original request will continue to be used.
*
* Canceling the task is equivalent to calling the task's cancel method; the
* URLSession:task:didCompleteWithError: task delegate will be called with error
* NSURLErrorCancelled.
*/
当系统准备好开始工作时设置延迟的开始时间（使用earliestBeginDate属性）时发送。必须调用
completionHandler才能继续加载。提供给完成处理程序的处置将继续使用提供给该任务的原始请求的
负载，用指定的任务替换该请求或取消该任务。如果这个委托没有实现，加载将继续处理原始请求。
建议：如果具有设置earliestBeginDate属性的任务可能会过时并且需要在开始网络加载
之前进行更改，那么仅实现此代理。
如果指定了新请求，则不会使用来自新请求的allowsCellularAccess
属性；来自原始请求的allowsCellularAccess属性将继续使用。
取消任务等同于调用任务的cancel方法； URLSession：task：didCompleteWithError：任务代理将被调用，error为NSURLErrorCancelled。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
willBeginDelayedRequest:(NSURLRequest *)request
completionHandler:(void (^)(NSURLSessionDelayedRequestDisposition disposition, NSURLRequest * _Nullable newRequest))completionHandler
API_AVAILABLE(macos(10.13), ios(11.0), watchos(4.0), tvos(11.0));
/*
* Sent when a task cannot start the network loading process because the current
* network connectivity is not available or sufficient for the task's request.
*
* This delegate will be called at most one time per task, and is only called if
* the waitsForConnectivity property in the NSURLSessionConfiguration has been
* set to YES.
*
* This delegate callback will never be called for background sessions, because
* the waitForConnectivity property is ignored by those sessions.
*/
当任务无法启动网络加载过程时发送，因为当前网络连接不可用或不足以满足任务请求。
每个任务最多只能调用一次该委托，并且只有在NSURLSessionConfiguration中
的waitsForConnectivity属性设置为YES时才会调用该代理。
此代理回调将永远不会被后台会话调用，因为这些会话会忽略waitForConnectivity属性。
- (void)URLSession:(NSURLSession *)session taskIsWaitingForConnectivity:(NSURLSessionTask *)task
API_AVAILABLE(macos(10.13), ios(11.0), watchos(4.0), tvos(11.0));
/* An HTTP request is attempting to perform a redirection to a different
* URL. You must invoke the completion routine to allow the
* redirection, allow the redirection with a modified request, or
* pass nil to the completionHandler to cause the body of the redirection
* response to be delivered as the payload of this request. The default
* is to follow redirections.
*
* For tasks in background sessions, redirections will always be followed and this method will not be called.
*/
一个HTTP请求正试图执行重定向到一个不同的URL。 您必须调用完成例程以允许重定向，允许使用修改后
的请求进行重定向，或将nil传递给completionHandler以使重定向响应的主体作为此请求的有效内容传递。
默认是遵循重定向。
对于后台会话中的任务，将始终遵循重定向，并且不会调用此方法。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
willPerformHTTPRedirection:(NSHTTPURLResponse *)response
newRequest:(NSURLRequest *)request
completionHandler:(void (^)(NSURLRequest * _Nullable))completionHandler;
/* The task has received a request specific authentication challenge.
* If this delegate is not implemented, the session specific authentication challenge
* will *NOT* be called and the behavior will be the same as using the default handling
* disposition.
*/
该任务已收到请求特定的身份验证质询。 如果这个代理没有被实现，会话特定的认证挑战不会被调用，
并且行为将与使用默认处理处置相同。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential * _Nullable credential))completionHandler;
/* Sent if a task requires a new, unopened body stream. This may be
* necessary when authentication has failed for any request that
* involves a body stream.
*/
如果任务需要新的未打开的正文流，则发送。
当涉及正文流的任何请求的身份验证失败时，这可能是必需的。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
needNewBodyStream:(void (^)(NSInputStream * _Nullable bodyStream))completionHandler;
/* Sent periodically to notify the delegate of upload progress. This
* information is also available as properties of the task.
*/
定期发送以通知代理上传进度。 该信息也可作为任务的属性。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didSendBodyData:(int64_t)bytesSent
totalBytesSent:(int64_t)totalBytesSent
totalBytesExpectedToSend:(int64_t)totalBytesExpectedToSend;
/*
* Sent when complete statistics information has been collected for the task.
*/
当完成任务的完整统计信息时发送。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didFinishCollectingMetrics:(NSURLSessionTaskMetrics *)metrics API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));
/* Sent as the last message related to a specific task. Error may be
* nil, which implies that no error occurred and this task is complete.
*/
作为与特定任务相关的最后一条消息发送。 错误可能为nil，
这意味着没有发生错误，并且此任务已完成。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didCompleteWithError:(nullable NSError *)error;
@end
NSURLSessionDataDelegate
下面看一下API文档
/*
* Messages related to the operation of a task that delivers data
* directly to the delegate.
与将数据直接传递给代理的任务的操作有关的消息
*/
@protocol NSURLSessionDataDelegate
@optional
/* The task has received a response and no further messages will be
* received until the completion block is called. The disposition
* allows you to cancel a request or to turn a data task into a
* download task. This delegate message is optional - if you do not
* implement it, you can get the response as a property of the task.
*
* This method will not be called for background upload tasks (which cannot be converted to download tasks).
*/
该任务已收到响应，直到完成块被调用不会收到更多消息。 该配置允许您取消请求或将数据任务转换
为下载任务。 此代理消息是可选的 - 如果您没有实现它，则可以将响应作为任务的属性获取。
此方法不会被后台上传任务（不能转换为下载任务）调用。
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler;
/* Notification that a data task has become a download task. No
* future messages will be sent to the data task.
*/
通知数据任务已成为下载任务。 没有更多的消息将被发送到数据任务。
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
didBecomeDownloadTask:(NSURLSessionDownloadTask *)downloadTask;
/*
* Notification that a data task has become a bidirectional stream
* task. No future messages will be sent to the data task. The newly
* created streamTask will carry the original request and response as
* properties.
*
* For requests that were pipelined, the stream object will only allow
* reading, and the object will immediately issue a
* -URLSession:writeClosedForStream:. Pipelining can be disabled for
* all requests in a session, or by the NSURLRequest
* HTTPShouldUsePipelining property.
*
* The underlying connection is no longer considered part of the HTTP
* connection cache and won't count against the total number of
* connections per host.
*/
通知数据任务已成为双向流任务。 没有更多的消息将被发送到数据任务。 新创建的streamTask将作为属性携带原始请求和响应。
对于流水线的请求，流对象只允许读取，而对象将立即发出-URLSession：writeClosedForStream :。
对于会话中的所有请求，或者NSURLRequest 的HTTPShouldUsePipelining属性，都可以禁用管道传输。
底层连接不再被认为是HTTP连接缓存的一部分，并且不会计入每个主机的连接总数。
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
didBecomeStreamTask:(NSURLSessionStreamTask *)streamTask;
/* Sent when data is available for the delegate to consume. It is
* assumed that the delegate will retain and not copy the data. As
* the data may be discontiguous, you should use
* [NSData enumerateByteRangesUsingBlock:] to access it.
*/
数据可供代理使用时发送。 假定代理将保留并不复制数据。 由于数据可能不连续，因此应该
使用[NSData enumerateByteRangesUsingBlock：]来访问它。
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
didReceiveData:(NSData *)data;
/* Invoke the completion routine with a valid NSCachedURLResponse to
* allow the resulting data to be cached, or pass nil to prevent
* caching. Note that there is no guarantee that caching will be
* attempted for a given resource, and you should not rely on this
* message to receive the resource data.
*/
用有效的NSCachedURLResponse调用完成例程以允许缓存结果数据，或通过nil来防止缓存。
请注意，不能保证将为给定资源尝试缓存，并且不应该依赖此消息来接收资源数据。
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
willCacheResponse:(NSCachedURLResponse *)proposedResponse
completionHandler:(void (^)(NSCachedURLResponse * _Nullable cachedResponse))completionHandler;
@end
NSURLSessionDownloadDelegate
下面看一下API
/*
* Messages related to the operation of a task that writes data to a
* file and notifies the delegate upon completion.
与将数据写入文件并在完成时通知代理的任务的操作有关的消息
*/
@protocol NSURLSessionDownloadDelegate
/* Sent when a download task that has completed a download. The delegate should
* copy or move the file at the given location to a new location as it will be
* removed when the delegate message returns. URLSession:task:didCompleteWithError: will
* still be called.
*/
当完成下载的下载任务时发送。 代理应该将指定位置的文件复制或移动到新位置，因为代理消息返回
时将被删除。 URLSession：task：didCompleteWithError：仍然会被调用。
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location;
@optional
/* Sent periodically to notify the delegate of download progress. */
定期发送以通知代理下载进度
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didWriteData:(int64_t)bytesWritten
totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite;
/* Sent when a download has been resumed. If a download failed with an
* error, the -userInfo dictionary of the error will contain an
* NSURLSessionDownloadTaskResumeData key, whose value is the resume
* data.
*/
下载已恢复时发送。 如果下载失败并显示错误，则error的-userInfo字典将包含NSURLSessionDownloadTaskResumeData键，其值为恢复的数据。
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didResumeAtOffset:(int64_t)fileOffset
expectedTotalBytes:(int64_t)expectedTotalBytes;
@end
NSURLSessionStreamDelegate
下面看一下API文档
@protocol NSURLSessionStreamDelegate
@optional
/* Indiciates that the read side of a connection has been closed. Any
* outstanding reads complete, but future reads will immediately fail.
* This may be sent even when no reads are in progress. However, when
* this delegate message is received, there may still be bytes
* available. You only know that no more bytes are available when you
* are able to read until EOF. */
指示连接的读取端已关闭。 任何未完成的读取都会结束，但将来的读取将立即失败。即使没有
读取正在进行，也可能会发送此信息。 但是，收到此代理消息时，可能仍有可用字节。
只有在能够读取EOF时，您才知道没有更多的字节可用。
- (void)URLSession:(NSURLSession *)session readClosedForStreamTask:(NSURLSessionStreamTask *)streamTask;
/* Indiciates that the write side of a connection has been closed.
* Any outstanding writes complete, but future writes will immediately
* fail.
*/
表示连接的写入端已关闭。 任何未完成的写入都会完成，但将来写入将立即失败。
- (void)URLSession:(NSURLSession *)session writeClosedForStreamTask:(NSURLSessionStreamTask *)streamTask;
/* A notification that the system has determined that a better route
* to the host has been detected (eg, a wi-fi interface becoming
* available.) This is a hint to the delegate that it may be
* desirable to create a new task for subsequent work. Note that
* there is no guarantee that the future task will be able to connect
* to the host, so callers should should be prepared for failure of
* reads and writes over any new interface. */
系统已经确定检测到到主机的更好路由的通知（例如，Wi-Fi界面变得可用）。这是对代理的提示，
可能需要为后续工作创建新任务。 请注意，不能保证将来的任务将能够连接到主机，因此调用者
应该准备好通过任何新接口进行读写失败
- (void)URLSession:(NSURLSession *)session betterRouteDiscoveredForStreamTask:(NSURLSessionStreamTask *)streamTask;
/* The given task has been completed, and unopened NSInputStream and
* NSOutputStream objects are created from the underlying network
* connection. This will only be invoked after all enqueued IO has
* completed (including any necessary handshakes.) The streamTask
* will not receive any further delegate messages.
*/
给定的任务已完成，未打开的NSInputStream和NSOutputStream对象是从底层网络连接创建的。
这只会在所有入队的IO完成（包括任何必要的握手）后才会被调用。streamTask不会接收到
任何进一步的代理消息。
- (void)URLSession:(NSURLSession *)session streamTask:(NSURLSessionStreamTask *)streamTask
didBecomeInputStream:(NSInputStream *)inputStream
outputStream:(NSOutputStream *)outputStream;
@end
参考文章
1. NSURLSession的一些代理方法
2. ios NSURLSession使用说明及后台工作流程分析
后记
本篇已结束，主要是介绍NSURLSession这几个代理的关系以及它们的简单用法，资料主要来自苹果的API，后续会进一步结合SDWebImage等框架和工程实践进行深化。喜欢的关注我，待续~~~
