实战iOS7之NSURLSession

NSURLSession VS NSURLConnection 
NSURLSession可以看做是NSURLConnection的进化版，其对NSURLConnection的改进点有： 

• * 根据每个Session做配置(http header，Cache,Cookie,protocal,Credential)，不再在整个App层面共享配置.
• * 支持网络操作的取消和断点续传
• * 改进了授权机制的处理
• * 丰富的Delegate模型
• * 分离了真实数据和网络配置数据。
• * 后台处理上传和下载，即使你点击了“Home”按钮，后台仍然可以继续下载，并且提供了根据网络状况，电力情况进行处理的配置。

知识点 



用法 
使用NSURLSession的一般套路如下： 

• 1. 定义一个NSURLRequest
• 2. 定义一个NSURLSessionConfiguration，配置各种网络参数
• 3. 使用NSURLSession的工厂方法获取一个所需类型的NSURLSession
• 4. 使用定义好的NSURLRequest和NSURLSession构建一个NSURLSessionTask
• 5. 使用Delegate或者CompletionHandler处理任务执行过程的所有事件。

实战 
这儿我简单的实现了一个下载任务的断点续传功能，具体效果如下： 

 

实现代码如下： 

#import "UrlSessionDemoViewController.h"

@interface UrlSessionDemoViewController ()

@end

@implementation UrlSessionDemoViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.progressBar.progress = 0;
}


- (NSURLSession *)session
{
    //创建NSURLSession
    NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession  *session = [NSURLSession sessionWithConfiguration:sessionConfig delegate:self delegateQueue:nil];
    return session;
}

- (NSURLRequest *)request
{
    //创建请求
    NSURL *url = [NSURL URLWithString:@"http://p1.pichost.me/i/40/1639665.png"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    return request;
}

-(IBAction)start:(id)sender
{
    //用NSURLSession和NSURLRequest创建网络任务
    self.task = [[self session] downloadTaskWithRequest:[self request]];
    [self.task resume];
}

-(IBAction)pause:(id)sender
{
    NSLog(@"Pause download task");
    if (self.task) {
        //取消下载任务，把已下载数据存起来
        [self.task cancelByProducingResumeData:^(NSData *resumeData) {
            self.partialData = resumeData;
            self.task = nil;
        }];
    }
}

-(IBAction)resume:(id)sender
{
    NSLog(@"resume download task");
    if (!self.task) {
        //判断是否又已下载数据，有的话就断点续传，没有就完全重新下载
        if (self.partialData) {
            self.task = [[self session] downloadTaskWithResumeData:self.partialData];
        }else{
            self.task = [[self session] downloadTaskWithRequest:[self request]];
        }
    }
    [self.task resume];
}

//创建文件本地保存目录
-(NSURL *)createDirectoryForDownloadItemFromURL:(NSURL *)location
{
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSArray *urls = [fileManager URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask];
    NSURL *documentsDirectory = urls[0];
    return [documentsDirectory URLByAppendingPathComponent:[location lastPathComponent]];
}
//把文件拷贝到指定路径
-(BOOL) copyTempFileAtURL:(NSURL *)location toDestination:(NSURL *)destination
{

    NSError *error;
    NSFileManager *fileManager = [NSFileManager defaultManager];
    [fileManager removeItemAtURL:destination error:NULL];
    [fileManager copyItemAtURL:location toURL:destination error:&error];
    if (error == nil) {
        return true;
    }else{
        NSLog(@"%@",error);
        return false;
    }
}

#pragma mark NSURLSessionDownloadDelegate
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location
{
    //下载成功后，文件是保存在一个临时目录的，需要开发者自己考到放置该文件的目录
    NSLog(@"Download success for URL: %@",location.description);
    NSURL *destination = [self createDirectoryForDownloadItemFromURL:location];
    BOOL success = [self copyTempFileAtURL:location toDestination:destination];

    if(success){
//        文件保存成功后，使用GCD调用主线程把图片文件显示在UIImageView中
        dispatch_async(dispatch_get_main_queue(), ^{
            UIImage *image = [UIImage imageWithContentsOfFile:[destination path]];
            self.imageView.image = image;
            self.imageView.contentMode = UIViewContentModeScaleAspectFit;
            self.imageView.hidden = NO;
        });
    }else{
        NSLog(@"Meet error when copy file");
    }
    self.task = nil;
}

/* Sent periodically to notify the delegate of download progress. */
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    //刷新进度条的delegate方法，同样的，获取数据，调用主线程刷新UI
    double currentProgress = totalBytesWritten/(double)totalBytesExpectedToWrite;
    dispatch_async(dispatch_get_main_queue(), ^{
        self.progressBar.progress = currentProgress;
        self.progressBar.hidden = NO;
    });
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
 didResumeAtOffset:(int64_t)fileOffset
expectedTotalBytes:(int64_t)expectedTotalBytes
{
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
}

@end

所有代码在这儿：https://github.com/xianlinbox/iOS7_New/tree/master/iOS7_New/NSURLSession/ViewController 

参考文章：http://www.objc.io/issue-5/from-nsurlconnection-to-nsurlsession.html 
http://www.shinobicontrols.com/blog/posts/2013/09/20/ios7-day-by-day-day-1-nsurlsession