iOS网络开发NSURLSession（二）DataTask＋Demo

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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

获得task的执行情况的属性

countOfBytesExpectedToReceive 
countOfBytesReceived
countOfBytesExpectedToSend
countOfBytesSent

task的综合信息

currentRequest // 当前活跃的request
originalRequest // 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task
taskIdentifier // 用来区分Task的描述符
error //Task失败的错误信息

task状态的枚举

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

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];

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

Session层次收到了授权，证书等问题

- (void)URLSession:(NSURLSession *)session
didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
 completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition,
                             NSURLCredential *credential))completionHandler

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 *)task
didCompleteWithError:(NSError *)error

Task层次收到了授权，证书等问题

- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task
didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
 completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition,
                             NSURLCredential *credential))completionHandler

将会进行HTTP，重定向

- (void)URLSession:(NSURLSession *)session
              task:(NSURLSessionTask *)task
willPerformHTTPRedirection:(NSHTTPURLResponse *)response
        newRequest:(NSURLRequest *)request
 completionHandler:(void (^)(NSURLRequest *))completionHandler

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 *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler

DataTask已经转变成DownloadTask

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
didBecomeDownloadTask:(NSURLSessionDownloadTask *)downloadTask

每收到一次Data时候调用

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data

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;
@end

static 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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