将图像下载到Metro风格应用程序中的本地存储
在WinRT / C#中,如何将映像下载到本地文件夹以支持在线目录的缓存以供脱机使用? 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);
下一步将下载的图像存储为localfile.jpg我该怎么办?
看起来像Windows 8的HttpClient示例中的代码解决了这个问题
HttpRequestMessage request = new HttpRequestMessage( HttpMethod.Get, resourceAddress); HttpResponseMessage response = await rootPage.httpClient.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);
httpClient是一个HttpClient,它的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(如果这是我们正在下载的)
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);
或者将其写入文件
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();
试试这个:
var response = await HttpWebRequest.Create(url).GetResponseAsync(); List allBytes = new List (); 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());
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更多信息,请参阅此页面。
GetResponseAsync将为您提供WebResponse文件而不是图像:
Stream imageStream = downloadedimage.GetResponseStream();
然后你可以写入图像文件:
byte[] imageData = new byte[imageStream.Length]; using(StreamReader reader = new Streamreader(imageStream)) { reader.ReadBytes(imageData, 0, imageData.Length); } await FileIO.WriteBytesAsync(imgfile, imageData);
默认情况下,位图图像由WinRT缓存,开发人员无需干预。