NSURLSession下载和断点续传

　　NSURLSession是iOS7以后新的网络接口，和常常用到NSURLConnection是类似的。在程序在前台时，NSURLSession与NSURLConnection可以相互的替换。但是当用户在对程序进行强迫关闭的时候此时NSURLSession会默许的自动断开。相比而言NSURLSession的优势主要体现在后台操作时候，而且在最流行的框架AFNetworking中也对NSURLSession提供了更好的支持。

　　主要提供的功能以下：

1 下载文件到内存中

2 下载文件到路径

3 上传制定的文件等

案例演示：图片下载断点续传

主要代码：

1、定义几个全局变量

@interface ViewController ()
{
    NSURLSessionDownloadTask * _task;
    NSData * _data;
    NSURLSession * _session;
    NSURLRequest * _request;
    UIProgressView * _pro;
    UIImageView * _imageView;
    
}

2、向视图中添加图片进度条

_imageView=[[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 300, 300)];

    _imageView.center=self.view.center;
    [self.view addSubview:_imageView];
    
_pro=[[UIProgressView alloc] initWithFrame:CGRectMake(_imageView.frame.origin.x, _imageView.frame.origin.y+400, 300, 40)];

3、向视图中添加按钮（一样的方式添加3个）

UIButton * button=[[UIButton alloc] initWithFrame:CGRectMake(50, _imageView.frame.origin.y+400+20, 50, 40)];
    button.backgroundColor=[UIColor blueColor];
    [button setTitle:@"开始" forState:UIControlStateNormal];
    [button addTarget:self action:@selector(ddLoad) forControlEvents:UIControlEventTouchUpInside];
    button.layer.borderWidth=1;
    button.layer.borderColor=[UIColor blueColor].CGColor;
    button.layer.cornerRadius=5;
    [self.view addSubview:button];

4、通过AFNetworkReachabilityManager网络状态监测

- (void) _checkNet{
    //开启网络状态监控
    [[AFNetworkReachabilityManager sharedManager] startMonitoring];
 
    [[AFNetworkReachabilityManager sharedManager] setReachabilityStatusChangeBlock:^(AFNetworkReachabilityStatus status) {
        
        if(status==AFNetworkReachabilityStatusReachableViaWiFi){
            NSLog(@"当前是wifi");
        }
        if(status==AFNetworkReachabilityStatusReachableViaWWAN){
             NSLog(@"当前是3G");
        }
if(status==AFNetworkReachabilityStatusNotReachable){
             NSLog(@"当前是没有网络");
        }
if(status==AFNetworkReachabilityStatusUnknown){
             NSLog(@"当前是未知网络");
        }
    }];
}

5、开始下载

- (void) ddLoad{
    NSURLSessionConfiguration * config=[NSURLSessionConfiguration defaultSessionConfiguration];
    _session=[NSURLSession sessionWithConfiguration:config delegate:self delegateQueue:nil];
    //
    NSURL *url=[NSURL URLWithString:@src];
    _request=[NSURLRequest requestWithURL:url];
    _task= [_session downloadTaskWithRequest:_request];
    
    NSLog(@"开始加载");
    [_task resume];
}

6、设置暂停和回复

- (void) pause{
    //暂停
    NSLog(@"暂停下载");
    [_task cancelByProducingResumeData:^(NSData *resumeData) {
        _data=resumeData;
    }];
    _task=nil;

}
- (void) resume{
    //恢复
     NSLog(@"恢复下载");
    if(!_data){
        NSURL *url=[NSURL URLWithString:@src];
        _request=[NSURLRequest requestWithURL:url]； 
        _task=[_session downloadTaskWithRequest:_request];
    
    }else{
        _task=[_session downloadTaskWithResumeData:_data];
    }
    [_task resume]; 
}

7、代理方法保存下载文件监控下载进度

#pragma mark - delegate
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location{
  
    NSURL * url=[NSURL fileURLWithPath:@"/Users/jredu/Desktop/tt.png"];
    
    NSFileManager * manager=[NSFileManager defaultManager];
    
    [manager moveItemAtURL:location toURL:url error:nil];
    dispatch_async(dispatch_get_main_queue(), ^{
        
        NSData * data=[manager contentsAtPath:@"/Users/jredu/Desktop/tt.png"];
        UIImage * image=[[UIImage alloc ]initWithData:data];
        _imageView.image=image;
        UIAlertView * alert=[[UIAlertView alloc] initWithTitle:nil message:@"下载完成" delegate:self cancelButtonTitle:@"OK" otherButtonTitles: nil];
        [alert show];

    }) ;
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite{
    CGFloat progress=(totalBytesWritten*1.0)/totalBytesExpectedToWrite;
    dispatch_async(dispatch_get_main_queue(), ^{
        _pro.progress=progress;
    }) ;

}