图像缓存8.1

Question

我写了Windows Phone 8.1（WinRT的）应用程序。它显示从互联网获取的图像。

我想这个图像保存到本地存储，然后显示它。 我的意思是说，每次打开应用程序，它会检查如果图像是在本地存储，显示，并从平行加载互联网这个形象的新副本，并刷新用户界面，新的形象。

意味着，直到从互联网加载图像的新副本，从本地存储器获取的此图像的缓存副本将充当占位符。

但问题是我得到的空白UI直到图像加载从互联网上，而不是局部的图像作为占位符。

C＃

public sealed partial class ProfileView : Page 
{ 
    const string PROFILE_PIC_FILE_NAME = "UmangProfilePic.jpg"; 
    public ProfileView() 
    { 
     this.InitializeComponent(); 

     this.NavigationCacheMode = NavigationCacheMode.Required; 
    } 


    protected override async void OnNavigatedTo(NavigationEventArgs e) 
    { 
     //Testing: 
     //string url = "http://www.nokia.com/sites/default/files/styles/medium/public/media/nokia_white_logo.png"; //small image 
     string url = "http://anderson.chem.ox.ac.uk/images/group2015.jpg"; //large image 

     await UmangImageCacheAsync(url); 
    } 

    /// <summary> 
    /// Check if image exists in local storage, load that image to UI control 
    /// Then, get latest image from internet and storage that image in local storage 
    /// Then, load that new image from local storage again to UI control 
    /// </summary> 
    /// <param name="url"></param> 
    /// <returns></returns> 
    private async Task UmangImageCacheAsync(string url) 
    { 
     loadingProfilePic.IsActive = true; 

     //var dispatcher = CoreWindow.GetForCurrentThread().Dispatcher; 
     //var ignored = dispatcher.RunAsync(CoreDispatcherPriority.Normal, async() => 
     //{ 


     //}); 

     var dispatcher = CoreWindow.GetForCurrentThread().Dispatcher; 
     var ignored = dispatcher.RunAsync(CoreDispatcherPriority.Normal, async() => 
     { 
      if (await FileExists(PROFILE_PIC_FILE_NAME)) 
      { 
       profilePicImage.Source = await GetLocalImageAsync(); 
      } 
     }); 

     var result = await GetRemoteImageAsync(url); 

     if (result is BitmapImage) 
     { 
      profilePicImage.Source = result as BitmapImage; 
      loadingProfilePic.IsActive = false; 
     } 
     else 
     { 
      loadingProfilePic.IsActive = false; 
     } 




    } 

    private async Task<BitmapImage> GetLocalImageAsync() 
    // private async Task<bool> GetLocalImageAsync() 
    { 
     StorageFolder localFolder = ApplicationData.Current.LocalFolder; 
     StorageFile file = await localFolder.GetFileAsync(PROFILE_PIC_FILE_NAME); 
     var image = await FileIO.ReadBufferAsync(file); 
     Uri uri = new Uri(file.Path); 
     return new BitmapImage(new Uri(file.Path)); 

     //return true; 
    } 

    private static async Task<object> GetRemoteImageAsync(string url) 
    //private static async Task GetRemoteImageAsync(string url) 
    { 
     StorageFolder localFolder = ApplicationData.Current.LocalFolder; 
     StorageFile file = await localFolder.CreateFileAsync(PROFILE_PIC_FILE_NAME, CreationCollisionOption.ReplaceExisting); 

     HttpClient client = new HttpClient(); 

     try 
     { 
      byte[] responseBytes = await client.GetByteArrayAsync(url); 

      var stream = await file.OpenAsync(FileAccessMode.ReadWrite); 

      using (var outputStream = stream.GetOutputStreamAt(0)) 
      { 
       DataWriter writer = new DataWriter(outputStream); 
       writer.WriteBytes(responseBytes); 
       await writer.StoreAsync(); 
       await outputStream.FlushAsync(); 
      } 
      var image = await FileIO.ReadBufferAsync(file); 
      Uri uri = new Uri(file.Path); 
      return new BitmapImage(new Uri(file.Path)); 

     } 
     catch (Exception _ex) 
     { 
      return false; 
     } 
    } 


    public static async Task<bool> FileExists(string fileName) 
    { 
     StorageFolder localFolder = ApplicationData.Current.LocalFolder; 
     try 
     { 
      StorageFile file = await localFolder.GetFileAsync(fileName); 
     } 
     catch (Exception _fileEx) 
     { 
      return false; 
     } 
     return true; 
    } 
} 

XAML：

<Grid> 
    <Grid> 
     <Image 
     x:Name="profilePicImage" 
     Width="400" 
     Height="400" /> 

     <ProgressRing x:Name="loadingProfilePic" 
     IsActive="False" 
     IsEnabled="True" > 

     </ProgressRing> 
    </Grid> 
</Grid> 

重现此问题：首先使用 “URL”（小图）执行应用程序， 然后debugg再次使用第二网址评论第一个网址，直到 从互联网下载的新图像，第一个图像应该作为 占位符，但你会看到进步环仅

Answer 1

您的来电GetLocalImageAsync（）得到调用GetRemoteImageAsync（后执行），因为你需要将它发布到调度队列。我想你想用另一种方式来完成你的场景。

感谢， 斯特凡威克 - Windows 8开发平台