下载并保存Windows Phone 8中的文件异步

Question

如何异步下载并保存文件到IsolatedStorage？ 我首先使用WebClient为目的，但我不能等待，直到完成，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。我发现的功能：

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

这个，贴什么har07，应该工作。如果你需要扩展你的方法一点点，我会尝试做这样的事情（尽管它没有进行测试，但应工作）：

（评论重修后 - 与取消）

// 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 - 这意味着您的下载操作可以在Button.Click后取消。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。

我还没有运行该代码，但它可能会显示主要想法。

Answer 2

尝试这样的事情（未测试）：

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 3

在我使用本机实现这样的观点最好的方式（包装纸开始/结束异步API）：基于任务

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));