NSURLSession - NSURLSessionTask 和 delegate

转：http://blog.csdn.net/hello_hwc/article/details/44565115

前言： 
这是IOS 网络开发系列的第三篇文章，这篇文章主要介绍了NSURLSession以及NSURLSessionTask这个抽象类，和NSURLSessionDataTask的使用和代理方法。 
本篇的顺序， 

1. demo效果 
2. NSURLSessionTask属性方法介绍  
3. NSURLSessionDataTask的使用和代理方法  
4. Demo的源码讲解

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一 Demo效果

暂停－继续 
 
取消 

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三 NSURLSessionTask

Task是由Session创建的，Session会保持对Task的一个强引用，直到Task完成或者出错才会释放。通过NSURLSessionTask可以获得Task的各种状态，以及对Task进行取消，挂起，继续等操作。一共包括三种Task，三种Task的结构如图。本文主要讲解的是DataTask。 

管理task的状态的方法

- (void)cancel //取消一个task- (void)resume //如果task在挂起状态，则继续执行task- (void)suspend //挂起task
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获得task的执行情况的属性

countOfBytesExpectedToReceive countOfBytesReceivedcountOfBytesExpectedToSendcountOfBytesSent
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task的综合信息

currentRequest // 当前活跃的requestoriginalRequest // 在task创建的时候传入的request（有可能会重定向）response // 服务器对当前活跃请求的响应taskDescription // 描述当前tasktaskIdentifier // 用来区分Task的描述符error //Task失败的错误信息
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task状态的枚举

typedef NS_ENUM (NSInteger,   NSURLSessionTaskState ) {   NSURLSessionTaskStateRunning = 0,   NSURLSessionTaskStateSuspended = 1,   NSURLSessionTaskStateCanceling = 2,   NSURLSessionTaskStateCompleted = 3,};
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四 NSURLSessionDataTask代理

DataTask是用来干嘛的呢？ 
用来下载数据到内存里，数据的格式是NSData 
dataTask使用的过程中，有两种方式来处理结果

第一种，通过block的方式（不关注过程，只关注结果，使用简单）

举个例子 
下载一幅图片，完成后显示到ImageView。下面代码看起来很简单吧。

NSURLSessionDataTask * task = [self.session dataTaskWithURL:[NSURL URLWithString:imageURL] completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {        if (!error) {            UIImage * image = [UIImage imageWithData:data];            self.imageview.image = image;        }    }];    [task resume];
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第二种通过代理方法来管理session和task。( 可以获得全部过程，但是较为复杂)

主要使用到三种代理中的事件 
NSURLSessionDelegate用来处理Session层次的事件

Session被 invalide得到的事件- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(NSError *)error
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Session层次收到了授权，证书等问题

- (void)URLSession:(NSURLSession *)sessiondidReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition,                             NSURLCredential *credential))completionHandler
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NSURLSessionTaskDelegate是使用代理的时候，任何种类task都要实现的代理 
Task完成的事件

- (void)URLSession:(NSURLSession *)session              task:(NSURLSessionTask *)taskdidCompleteWithError:(NSError *)error
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Task层次收到了授权，证书等问题

- (void)URLSession:(NSURLSession *)session              task:(NSURLSessionTask *)taskdidReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition,                             NSURLCredential *credential))completionHandler
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将会进行HTTP，重定向

- (void)URLSession:(NSURLSession *)session              task:(NSURLSessionTask *)taskwillPerformHTTPRedirection:(NSHTTPURLResponse *)response        newRequest:(NSURLRequest *)request completionHandler:(void (^)(NSURLRequest *))completionHandler
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NSURLSessionDataDelegate特别用来处理dataTask的事件 
收到了Response，这个Response包括了HTTP的header（数据长度，类型等信息），这里可以决定DataTask以何种方式继续（继续，取消，转变为Download）

- (void)URLSession:(NSURLSession *)session          dataTask:(NSURLSessionDataTask *)dataTaskdidReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler
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DataTask已经转变成DownloadTask

- (void)URLSession:(NSURLSession *)session          dataTask:(NSURLSessionDataTask *)dataTaskdidBecomeDownloadTask:(NSURLSessionDownloadTask *)downloadTask
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每收到一次Data时候调用

- (void)URLSession:(NSURLSession *)session          dataTask:(NSURLSessionDataTask *)dataTask    didReceiveData:(NSData *)data
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是否把Response存储到Cache中

- (void)URLSession:(NSURLSession *)session          dataTask:(NSURLSessionDataTask *)dataTask willCacheResponse:(NSCachedURLResponse *)proposedResponse completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler
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五 Demo源码讲解

#import "ViewController.h"@interface ViewController ()<NSURLSessionDelegate,NSURLSessionTaskDelegate,NSURLSessionDataDelegate>@property (strong,nonatomic)UIImageView * imageview;@property (strong,nonatomic)NSURLSession * session;@property (strong,nonatomic)NSURLSessionDataTask * dataTask;@property (weak, nonatomic) IBOutlet UIProgressView *progressview;@property (nonatomic)NSUInteger expectlength;@property (strong,nonatomic) NSMutableData * buffer;@endstatic NSString * imageURL = @"http://f12.topit.me/o129/10129120625790e866.jpg";@implementation ViewController//属性全部采用惰性初始化#pragma mark - lazy property-(UIImageView *)imageview{    if (!_imageview) {        _imageview = [[UIImageView alloc] initWithFrame:CGRectMake(40,40,300,200)];        _imageview.backgroundColor = [UIColor lightGrayColor];        _imageview.contentMode = UIViewContentModeScaleToFill;    }    return _imageview;}-(NSMutableData *)buffer{    if (!_buffer) {        _buffer = [[NSMutableData alloc] init];    }    return _buffer;}-(NSURLSession*)session{    if (!_session) {        NSURLSessionConfiguration * configuration = [NSURLSessionConfiguration ephemeralSessionConfiguration];        _session = [NSURLSession sessionWithConfiguration:configuration                                                 delegate:self                                            delegateQueue:[NSOperationQueue mainQueue]];    }    return _session;}-(NSURLSessionDataTask *)dataTask{    if (!_dataTask) {        _dataTask = [self.session dataTaskWithURL:[NSURL URLWithString:imageURL]];    }    return _dataTask;}#pragma mark - life circle of viewcontroller- (void)viewDidLoad {    [super viewDidLoad];    [self.view addSubview:self.imageview];    [self.dataTask resume];//Task要resume彩绘进行实际的数据传输    [self.session finishTasksAndInvalidate];//完成task就invalidate}#pragma mark - target-action//注意判断当前Task的状态- (IBAction)pause:(UIButton *)sender {    if (self.dataTask.state == NSURLSessionTaskStateRunning) {        [self.dataTask suspend];    }}- (IBAction)cancel:(id)sender {    switch (self.dataTask.state) {        case NSURLSessionTaskStateRunning:        case NSURLSessionTaskStateSuspended:            [self.dataTask cancel];            break;        default:            break;    }}- (IBAction)resume:(id)sender {    if (self.dataTask.state == NSURLSessionTaskStateSuspended) {        [self.dataTask resume];    }}#pragma mark -  URLSession delegate method-(void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition))completionHandler{    NSUInteger length = [response expectedContentLength];    if (length != -1) {        self.expectlength = [response expectedContentLength];//存储一共要传输的数据长度        completionHandler(NSURLSessionResponseAllow);//继续数据传输    }else{        completionHandler(NSURLSessionResponseCancel);//如果Response里不包括数据长度的信息，就取消数据传输        UIAlertView * alert = [[UIAlertView alloc] initWithTitle:@"error"                                                         message:@"Do not contain property of expectedlength"                                                        delegate:nil                                               cancelButtonTitle:@"OK"                                               otherButtonTitles:nil];        [alert show];    }}-(void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data{    [self.buffer appendData:data];//数据放到缓冲区里    self.progressview.progress = [self.buffer length]/((float) self.expectlength);//更改progressview的progress}-(void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error{    if (!error) {        dispatch_async(dispatch_get_main_queue(), ^{//用GCD的方式，保证在主线程上更新UI            UIImage * image = [UIImage imageWithData:self.buffer];            self.imageview.image = image;            self.progressview.hidden = YES;            self.session = nil;            self.dataTask = nil;        });    }else{        NSDictionary * userinfo = [error userInfo];        NSString * failurl = [userinfo objectForKey:NSURLErrorFailingURLStringErrorKey];        NSString * localDescription = [userinfo objectForKey:NSLocalizedDescriptionKey];        if ([failurl isEqualToString:imageURL] && [localDescription isEqualToString:@"cancelled"]) {//如果是task被取消了，就弹出提示框            UIAlertView * alert = [[UIAlertView alloc] initWithTitle:@"Message"                                                             message:@"The task is canceled"                                                            delegate:nil                                                   cancelButtonTitle:@"OK"                                                   otherButtonTitles:nil];            [alert show];        }else{            UIAlertView * alert = [[UIAlertView alloc] initWithTitle:@"Unknown type error"//其他错误，则弹出错误描述                                                             message:error.localizedDescription                                                            delegate:nil                                                   cancelButtonTitle:@"OK"                                                   otherButtonTitles:nil];            [alert show];        }        self.progressview.hidden = YES;        self.session = nil;        self.dataTask = nil;    }}@end
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