WP8.1 Study17：网络之后台下载/上传及HttpClient

一、后台下载/上传

1、简介

　　使用BackgroundTransferGroup可以十分方便操作上传及下载文件，BackgroundDownloader和BackgroundUploader类中的方法提供一系列方法交互这一过程，因此我们也可以利用live tile或toast显示传输的状态。

2、下载代码

代码一：

async void DownloadFile(Uri sourceUri, string destFilename)
{
    cts = new CancellationTokenSource();
    StorageFile destinationFile = await KnownFolders.PicturesLibrary.CreateFileAsync(
            destFilename, CreationCollisionOption.GenerateUniqueName);
    BackgroundDownloader downloader = new BackgroundDownloader();
    DownloadOperation download = downloader.CreateDownload(sourceUri, destinationFile);
           
    try
    {
        Progress<DownloadOperation> progressCallback = new Progress<DownloadOperation>(DownloadProgress);
        // Start the download and attach a progress handler.
        await download.StartAsync().AsTask(cts.Token, progressCallback);
    }
    catch (TaskCanceledException) {  /* User cancelled the transfer */ }
    catch (Exception ex)          {  /* ...  */ }
}

代码二：

private void CancelAll_Click(object sender, RoutedEventArgs e)
{
    cts.Cancel();
    cts.Dispose();
    cts = new CancellationTokenSource(); // Re-create the CancellationTokenSource for future downloads.
}

private void DownloadProgress(DownloadOperation download)
{
    double percent = 100;
    if (download.Progress.TotalBytesToReceive > 0)
        percent = download.Progress.BytesReceived * 100 / download.Progress.TotalBytesToReceive; 
    if (download.Progress.HasRestarted) 
    { /* Download restarted  */ };
    if (download.Progress.HasResponseChanged) // We've received new response headers from the server.
        Debug.WriteLine(" - Response updated; Header count: " + download.GetResponseInformation().Headers.Count);
}

二、HttpClient简单应用

　　要使用HttpClicet，需对http协议要有一定的了解才行，可以去找些资料了解Http协议（我是从这篇文章http://www.cnblogs.com/zhili/archive/2012/08/18/HTTP.html初步了解了Http）。过程图可以参考下图：

1、利用Http简单地获取文本例子

try
{
    // Create the HttpClient
    HttpClient httpClient = new HttpClient();

    // Optionally, define HTTP headers
    httpClient.DefaultRequestHeaders.Accept.TryParseAdd("application/json");//表示客户端可接收json类型的文本

    // Make the call
    string responseText = await httpClient.GetStringAsync(
        new Uri("http://services.odata.org/Northwind/Northwind.svc/Suppliers"));
}
catch (Exception ex)
{
   ...
}

ps：这里使用的httpclient来自命名空间Windows.Web.Http，而不是System.Net.Http。（两者有区别）

2、获取http响应

try
{
    var client = new HttpClient();
    var uri = new Uri(" http://example.com/customers/1");
    var response = await client.GetAsync(uri);//GetAsync代替GetstringAsync，可以获得更多内容

    // code and results
    var statusCode = response.StatusCode;
    // EnsureSuccessStatusCode throws exception if not HTTP 200
    response.EnsureSuccessStatusCode();
    var responseText = await response.Content.ReadAsStringAsync();
}
catch (Exception ex)
{
    ...
}

3、写http请求的头部

var client = new HttpClient();
// set some headers
var headers = client.DefaultRequestHeaders;//获取头部
headers.Referer = new Uri("http://contoso.com");//表示的是上一连接的url，如跳转到本页面的上一页面url
var ok = headers.UserAgent.TryParseAdd("testprogram/1.0");//表示客户端软件类型

// make the request
var response = await client.GetAsync(uri);
// get response content length
var length = response.Content.Headers.ContentLength.Value;
byte[] buffer = new byte[length];

我们可以对请求头部进行更多的操作，如下图：

3、对请求行（方法）操作

var client = new HttpClient();
    // we're sending a delete request
    var request = new HttpRequestMessage(HttpMethod.Delete, uri);//http方法Delette
    // we don't expect a response, but we'll snag it anyway
    var response = await client.SendRequestAsync(request);

http请求方法还有Delete，Get，Head，Options，Patch，Post，Put等，具体描述可以参考http://www.cnblogs.com/zhili/archive/2012/08/18/HTTP.html

4、设置及获取cookies

（1）设置cookies

var uri = new Uri("http://example.com/customers/1");

try
{
    var baseFilter = new HttpBaseProtocolFilter();
    var cookieManager = baseFilter.CookieManager;

    var cookie = new HttpCookie("favoriteColor", ".example.com", "/") 
        { Value = "purple"};

    cookieManager.SetCookie(cookie);

    var client = new HttpClient(baseFilter);
    await client.PostAsync(uri, new HttpStringContent("Pete"));
}
catch (Exception ex) { ... }

（2）获取cookies

var baseFilter = new HttpBaseProtocolFilter();
var cookieManager = baseFilter.CookieManager;

var client = new HttpClient(baseFilter);

var cookies = cookieManager.GetCookies(uri);

// display cookies
foreach (var cookie in cookies)
{
    CookieList.Text += cookie.Name + " = " + cookie.Value + "
";
}

5、使用HttpBaseProtocolFilter

HttpBaseProtocolFilter filter = new HttpBaseProtocolFilter();
// When AutomaticDecompression is true (the default), the Accept-Encoding header is added 
// to the headers and set to allow gzip and compress
filter.AutomaticDecompression = true;

PasswordCredential creds = new PasswordCredential("JumpStart", userName, password);
filter.ServerCredential = creds;
filter.ProxyCredential = creds;

// Create the HttpClient
HttpClient httpClient = new HttpClient(filter);

三、使用httpclient完整的demo

　　demo实现从网站一图片下载到手机中并显示在页面上。

1、Page的xaml主要代码如下：

<Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="2*"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Border x:Name="ImageBorder" Grid.Row="1">
            <Image x:Name="MainImage"/>
        </Border>
        <Button x:Name="DownloadButton" Content="Get Image" HorizontalAlignment="Left" Margin="142,47,0,0" Grid.Row="2" VerticalAlignment="Top" Click="DownloadButton_Click"/>
    </Grid>

2、Page的C#主要代码如下：

 private async void DownloadButton_Click(object sender, RoutedEventArgs e)
        {
            Size downloadSize = new Size(
                (int)(ImageBorder.ActualWidth * DisplayInformation.GetForCurrentView().RawPixelsPerViewPixel),
                (int)(ImageBorder.ActualHeight * DisplayInformation.GetForCurrentView().RawPixelsPerViewPixel));
            //Size downloadSize = new Size(744, 360);
            await DownloadAndScale("bg-homepage.jpg", "http://www.qnwfan.com/images/bg-homepage.jpg", downloadSize);

            StorageFolder resultsFolder = await ApplicationData.Current.LocalFolder.GetFolderAsync("Results");//进入图片存放的文件夹
            StorageFile file = await resultsFolder.GetFileAsync("bg-homepage.jpg");//选取对应图片文件

            using (IRandomAccessStream fileStream = await file.OpenAsync(Windows.Storage.FileAccessMode.Read)) //取图片放到MainImage
            { 
                // Set the image source to the selected bitmap 
                 BitmapImage bitmapImage = new BitmapImage(); 
                 await bitmapImage.SetSourceAsync(fileStream); 
                 MainImage.Source = bitmapImage; 
            } 
        }

        private async Task DownloadAndScale(string outfileName, string downloadUriString, Size scaleSize)
        {
            Uri downLoadingUri = new Uri(downloadUriString);
            HttpClient client = new HttpClient();//创建一个http客户端
            using (var response = await client.GetAsync(downLoadingUri))//获取响应
            {
                var buffer = await response.Content.ReadAsBufferAsync();//缓存响应数据内容
                var memoryStream = new InMemoryRandomAccessStream();//创建一个数据流
                await memoryStream.WriteAsync(buffer);//把buffer缓存区里的数据写入数据流中
                await memoryStream.FlushAsync();//数据流刷新
                var decoder = await Windows.Graphics.Imaging.BitmapDecoder.CreateAsync(memoryStream);//数据流解码成图片
                var bt = new Windows.Graphics.Imaging.BitmapTransform();//设置图片大小
                bt.ScaledWidth = (uint)scaleSize.Width;
                bt.ScaledHeight = (uint)scaleSize.Height;
                var pixelProvider = await decoder.GetPixelDataAsync(
                    decoder.BitmapPixelFormat, decoder.BitmapAlphaMode, bt, 
                    ExifOrientationMode.IgnoreExifOrientation, ColorManagementMode.ColorManageToSRgb);//使已初步解码后的图片进一步通过一些设置（设置图片帧格式，图片方向，图片大小等）还原成适合在屏幕显示的图片，成为像素提供源

                // 既然我们有了像素数据，那么可以把数据储存在文件夹中
                var localFolder = Windows.Storage.ApplicationData.Current.LocalFolder;
                var resultsFolder = await localFolder.CreateFolderAsync("Results", CreationCollisionOption.OpenIfExists);
                var scaledFile = await resultsFolder.CreateFileAsync(outfileName, CreationCollisionOption.ReplaceExisting);
                using (var scaledFileStream = await scaledFile.OpenAsync(Windows.Storage.FileAccessMode.ReadWrite))
                {
                    var encoder = await Windows.Graphics.Imaging.BitmapEncoder.CreateAsync(
                        Windows.Graphics.Imaging.BitmapEncoder.JpegEncoderId, scaledFileStream);//
                    var pixels = pixelProvider.DetachPixelData();//获取像素数据
                    encoder.SetPixelData(
                        decoder.BitmapPixelFormat,
                        decoder.BitmapAlphaMode,
                        (uint)scaleSize.Width,
                        (uint)scaleSize.Height,
                        decoder.DpiX,
                        decoder.DpiY,
                        pixels
                        );//设置有关帧的像素数据
                    await encoder.FlushAsync();//刷新
                }

3、实现效果图

点击GetImage前后