/// <summary>
        /// 添加
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btn_VideoItemadd_Click(object sender, RoutedEventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();

            ofd.Multiselect = false;
            ofd.Filter      = "媒体文件|*.jpg;*.bmp;*.jpeg;*.png;*.wmv";
            ofd.ShowDialog();
            if (!string.IsNullOrEmpty(ofd.FileName))
            {
                SchoolNoteEditer.ViewModel.ViewModel_VideoItem newVideo = viewModel.GetNewItme(ofd.FileName);
                if (newVideo != null)
                {
                    viewModel.VideoItems.Add(newVideo);
                }
            }
            LB_VideoItem.SelectedIndex = LB_VideoItem.Items.Count - 1;
        }
        /// <summary>
        /// 更换图片
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btn_VideoItemSelectImage_Click(object sender, RoutedEventArgs e)
        {
            if (LB_VideoItem.SelectedIndex < 0)
            {
                viewModel.ErrorMessage = "请先选择一个修改的项";
                return;
            }
            OpenFileDialog ofd = new OpenFileDialog();

            ofd.Multiselect = false;
            ofd.Filter      = "媒体文件|*.jpg;*.bmp;*.jpeg;*.png;*.wmv";
            ofd.ShowDialog();
            if (!string.IsNullOrEmpty(ofd.FileName))
            {
                SchoolNoteEditer.ViewModel.ViewModel_VideoItem newVideo = viewModel.GetNewItme(ofd.FileName);
                if (newVideo != null)
                {
                    viewModel.VideoItems[LB_VideoItem.SelectedIndex] = newVideo;
                }
            }
        }