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();
            }