Question

在WinRT / C＃中，如何将图像下载到本地文件夹以支持在线目录的缓存以供离线使用？有没有办法直接下载图像并链接控件以从缓存中获取它们作为后备？

    var downloadedimage = await HttpWebRequest.Create(url).GetResponseAsync();
    StorageFile imgfile = await ApplicationData.Current.LocalFolder.CreateFileAsync(
                        "localfile.png", CreationCollisionOption.FailIfExists);

我将下载的图像存储为localfile.jpg？

Answer 1

看起来以下来自Windows 8的HttpClient示例中的代码解决了问题

    HttpRequestMessage request = new HttpRequestMessage(
        HttpMethod.Get, resourceAddress);
    HttpResponseMessage response = await rootPage.httpClient.SendAsync(request,
        HttpCompletionOption.ResponseHeadersRead);

httpClient是一个HttpClient，其BaseAddress需要设置为资源的服务器文件夹。然后，我们可以将其转换为图像源（如果这就是我们正在下载的内容）

    InMemoryRandomAccessStream randomAccessStream = 
        new InMemoryRandomAccessStream();
    DataWriter writer = new DataWriter(randomAccessStream.GetOutputStreamAt(0));
    writer.WriteBytes(await response.Content.ReadAsByteArrayAsync());
    await writer.StoreAsync();
    BitmapImage image = new BitmapImage();
    imagecontrol.SetSource(randomAccessStream);

或将其写入文件

    var imageFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(
         filename, CreationCollisionOption.ReplaceExisting);
    var fs = await imageFile.OpenAsync(FileAccessMode.ReadWrite);
    DataWriter writer = new DataWriter(fs.GetOutputStreamAt(0));
    writer.WriteBytes(await response.Content.ReadAsByteArrayAsync());
    await writer.StoreAsync();
    writer.DetachStream();
    await fs.FlushAsync();

Answer 2

试试这个：

        var response = await HttpWebRequest.Create(url).GetResponseAsync();
        List<Byte> allBytes = new List<byte>();
        using (Stream imageStream = response.GetResponseStream())
        {
            byte[] buffer = new byte[4000];
            int bytesRead = 0;
            while ((bytesRead = await imageStream.ReadAsync(buffer, 0, 4000)) > 0)
            {
                allBytes.AddRange(buffer.Take(bytesRead));
            }
        }
        var file = await ApplicationData.Current.LocalFolder.CreateFileAsync(
                    "localfile.png", CreationCollisionOption.FailIfExists);
        await FileIO.WriteBytesAsync(file, allBytes.ToArray());

Answer 3

如果要下载由URL表示的文件，最简单的方法是使用BackgroundDownloader类。有关如何使用后台下载程序的详细信息，请参阅this页面。

Answer 4

GetResponseAsync将为您提供WebResponse文件，而不是图像：

Stream imageStream = downloadedimage.GetResponseStream();

然后你可以写到图像文件：

byte[] imageData = new byte[imageStream.Length];
using(StreamReader reader = new Streamreader(imageStream))
{
    reader.ReadBytes(imageData, 0, imageData.Length);
}
await FileIO.WriteBytesAsync(imgfile, imageData);

Answer 5

默认情况下，位图图像由WinRT缓存，开发人员无需干预。

将图像下载到Metro风格应用程序中的本地存储

5 个答案: