private void CreateNewItemsSource()
        {
            TimeSpan timeout = TimeSpan.FromSeconds(2);

            if (virtualPDFPages != null)
            {
                virtualPDFPages.CleanUpAllPages();
            }

            virtualPDFPages          = new AsyncVirtualizingCollection <IEnumerable <PDFImage> >(imageProvider, parent.GetPagePer(), timeout);
            itemsControl.ItemsSource = virtualPDFPages;
        }
        public void Unload()
        {
            scrollViewer.Visibility = System.Windows.Visibility.Collapsed;
            scrollViewer.ScrollToHorizontalOffset(0);
            scrollViewer.ScrollToVerticalOffset(0);
            imageProvider = null;

            if (virtualPDFPages != null)
            {
                virtualPDFPages.CleanUpAllPages();
                virtualPDFPages = null;
            }

            itemsControl.ItemsSource = null;
        }