UWP 保存Image的图片到本地文件

　　　　　　  var rtb = new RenderTargetBitmap();
await rtb.RenderAsync(imgCover);

var saveFile = new FileSavePicker();
saveFile.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
saveFile.FileTypeChoices.Add("JPEG files", new List<string>() { ".jpg" });
saveFile.SuggestedFileName = “Test”;
StorageFile sFile = await saveFile.PickSaveFileAsync();
if (sFile == null)
return;

var pixels = await rtb.GetPixelsAsync();
using (IRandomAccessStream stream = await sFile.OpenAsync(FileAccessMode.ReadWrite))
{
var encoder = await
BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, stream);
byte[] bytes = pixels.ToArray();
encoder.SetPixelData(BitmapPixelFormat.Bgra8,
BitmapAlphaMode.Ignore,
(uint)rtb.PixelWidth,
(uint)rtb.PixelHeight,
200,
200,
bytes);

await encoder.FlushAsync();
}

imgCover是Image控件。