[WP8.1开发]从网络下载图片直接存入手机照片库

            string fileName = "picture"+DateTime.Now.Ticks+".jpg";
            //在SavePictures文件夹下生成一个空文件用于保存图片
            StorageFile destinationFile = await KnownFolders.SavedPictures.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);
           
            using(var response =await HttpWebRequest.Create(savePictureInfo.imageUri).GetResponseAsync())
            {
                //获取到图片流
                using (Stream responseStream = response.GetResponseStream())
                {
                    using (var destinationStream = await destinationFile.OpenAsync(FileAccessMode.ReadWrite))
                    {
                        using (var destinationOutputStream = destinationStream.GetOutputStreamAt(0))
                        {
                            await RandomAccessStream.CopyAndCloseAsync(responseStream.AsInputStream(), destinationStream);
                        }
                    }
                }
                //获取文件更新状态
                FileUpdateStatus status = await CachedFileManager.CompleteUpdatesAsync(destinationFile);
                 if (status == FileUpdateStatus.Complete)
                 {
                     string SAVE_SUCCESS = loader.GetString("uSaveSuccess");
                     await new MessageDialog(SAVE_SUCCESS).ShowAsync();
                 }
                 else
                 {
                     string SAVE_FAILED = loader.GetString("uSaveFailed");
                     await new MessageDialog(SAVE_FAILED).ShowAsync();
                 }