Question

如何以异步方式将文件下载并保存到IsolatedStorage？我首先使用WebClient作为目的，但我无法等到完成，所以我找到了一些有用的代码here。

但是，这也不是完整的格式。我发现了这个功能：

public static Task<Stream> DownloadFile(Uri url)
{
    var tcs = new TaskCompletionSource<Stream>();
    var wc = new WebClient();
    wc.OpenReadCompleted += (s, e) =>
    {
        if (e.Error != null) tcs.TrySetException(e.Error);
        else if (e.Cancelled) tcs.TrySetCanceled();
        else tcs.TrySetResult(e.Result);
    };
    wc.OpenReadAsync(url);
    return tcs.Task;
}

但是如何将此文件保存到IsolatedStorage？有人帮我完成这个功能！

Answer 1

尝试这样的事情（未经测试）：

try
{
    using (IsolatedStorageFile myIsolatedStorage = IsolatedStorageFile.GetUserStoreForApplication())
    {
        IsolatedStorageFileStream fileStream = myIsolatedStorage.CreateFile("fileNameHere");

        BitmapImage bitmap = new BitmapImage();
        var stream = await DownloadFile(new Uri("http://someuri.com", UriKind.Absolute));
        bitmap.SetSource(stream);
        WriteableBitmap wb = new WriteableBitmap(bitmap);

        // Encode WriteableBitmap object to a JPEG stream.
        Extensions.SaveJpeg(wb, fileStream, wb.PixelWidth, wb.PixelHeight, 0, 85);
        fileStream.Close();
    }
}
catch (Exception ex)
{
    //Exception handle appropriately for your app  
}

[Reference]

Answer 2

这个har07发布的内容应该有效。如果你需要稍微扩展你的方法，我会尝试做这样的事情（尽管它没有经过测试，但应该有效）：

（评论后重建 - 取消）

// first define Cancellation Token Source - I've made it global so that CancelButton has acces to it
CancellationTokenSource cts = new CancellationTokenSource();
enum Problem { Ok, Cancelled, Other }; // results of my Task

// cancelling button event
private void CancellButton_Click(object sender, RoutedEventArgs e)
{
   if (this.cts != null)
        this.cts.Cancel();
}

// the main method - I've described it a little below in the text
public async Task<Problem> DownloadFileFromWeb(Uri uriToDownload, string fileName, CancellationToken cToken)
{
   try
   {
       using (Stream mystr = await DownloadFile(uriToDownload))
           using (IsolatedStorageFile ISF = IsolatedStorageFile.GetUserStoreForApplication())
           {
           if (ISF.FileExists(fileName)) return Problem.Other;
           using (IsolatedStorageFileStream file = ISF.CreateFile(fileName))
           {
               const int BUFFER_SIZE = 1024;
               byte[] buf = new byte[BUFFER_SIZE];

               int bytesread = 0;
               while ((bytesread = await mystr.ReadAsync(buf, 0, BUFFER_SIZE)) > 0)
               {
                  cToken.ThrowIfCancellationRequested();
                  file.Write(buf, 0, bytesread);
               }
           }
       }
       return Problem.Ok;
    }
    catch (Exception exc)
    {
       if (exc is OperationCanceledException)
           return Problem.Cancelled;
       else return Problem.Other; 
    }
}

// and download
private async void Downlaod_Click(object sender, RoutedEventArgs e)
{
   cts = new CancellationTokenSource();
   Problem fileDownloaded = await DownloadFileFromWeb(new Uri(@"http://filedress/myfile.txt", UriKind.Absolute), "myfile.txt", cts.Token);
   switch(fileDownloaded)
   {
      case Problem.Ok:
           MessageBox.Show("File downloaded");
           break;
      case Problem.Cancelled:
           MessageBox.Show("Download cancelled");
           break;
      case Problem.Other:
      default:
           MessageBox.Show("Other problem with download");
           break;
    }
}

我添加了取消令牌 - 这意味着您可以在Button.Click之后取消下载操作。另一方面，如果等待DownloadFile（uriToDownload）被取消，它会自动抛出OperationCancelled - 然后你捕获该异常并返回足够的结果。

我没有运行该代码，但它可能显示主要想法。

Answer 3

在我看来，使用这样的本机实现的最佳方式（包装开始/结束异步API）：

var url = new Uri(UriString, UriKind.Absolute);
var fileName = Path.GetFileName(url.LocalPath);

var w = WebRequest.CreateHttp(url);
var response = await Task.Factory.FromAsync<WebResponse>(w.BeginGetResponse, w.EndGetResponse, null);
await response.GetResponseStream().CopyToAsync(new FileStream(ApplicationData.Current.LocalFolder.Path + @"\" + fileName, FileMode.CreateNew));

在Windows Phone 8中下载并保存文件Async

3 个答案: