BaconographyWP8.Common.ReorderListBox.dragInterceptor_ManipulationDelta C# (CSharp) Method

        private void dragInterceptor_ManipulationDelta(object sender, ManipulationDeltaEventArgs e)
        {
            if (this.Items.Count <= 1 || this.dragItem == null)
            {
                return;
            }

            if (this.dropTargetIndex == -1)
            {
                if (this.dragItemContainer == null)
                {
                    return;
                }

                // When the drag actually starts, swap out the item for the drag-indicator image of the item.
                // This is necessary because the item itself may be removed from the virtualizing panel
                // if the drag causes a scroll of considerable distance.
                Size dragItemSize = this.dragItemContainer.RenderSize;
                WriteableBitmap writeableBitmap = new WriteableBitmap(
                    (int)dragItemSize.Width, (int)dragItemSize.Height);

                // Swap states to force the transition to complete.
                VisualStateManager.GoToState(this.dragItemContainer, ReorderListBoxItem.NotDraggingState, false);
                VisualStateManager.GoToState(this.dragItemContainer, ReorderListBoxItem.DraggingState, false);
                writeableBitmap.Render(this.dragItemContainer, null);

                writeableBitmap.Invalidate();
                this.dragIndicator.Source = writeableBitmap;

                this.dragIndicator.Visibility = Visibility.Visible;
                this.dragItemContainer.Visibility = Visibility.Collapsed;

                if (this.itemsPanel.Children.IndexOf(this.dragItemContainer) < this.itemsPanel.Children.Count - 1)
                {
                    this.UpdateDropTarget(Canvas.GetTop(this.dragIndicator) + this.dragIndicator.Height + 1, false);
                }
                else
                {
                    this.UpdateDropTarget(Canvas.GetTop(this.dragIndicator) - 1, false);
                }
            }

            double dragItemHeight = this.dragIndicator.Height;

            TranslateTransform translation = (TranslateTransform)this.dragIndicator.RenderTransform;
            double top = Canvas.GetTop(this.dragIndicator);

            // Limit the translation to keep the item within the list area.
            // Use different targeting for the top and bottom edges to allow taller items to
            // move before or after shorter items at the edges.
            double y = top + e.CumulativeManipulation.Translation.Y;
            if (y < 0)
            {
                y = 0;
                this.UpdateDropTarget(0, true);
            }
            else if (y >= this.dragInterceptorRect.Height - dragItemHeight)
            {
                y = this.dragInterceptorRect.Height - dragItemHeight;
                this.UpdateDropTarget(this.dragInterceptorRect.Height - 1, true);
            }
            else
            {
                this.UpdateDropTarget(y + dragItemHeight / 2, true);
            }

            translation.Y = y - top;

            // Check if we're within the margin where auto-scroll needs to happen.
            bool scrolling = (this.dragScrollDelta != 0);
            double autoScrollMargin = this.AutoScrollMargin;
            if (autoScrollMargin > 0 && y < autoScrollMargin)
            {
                this.dragScrollDelta = y - autoScrollMargin;
                if (!scrolling)
                {
                    VisualStateManager.GoToState(this.scrollViewer, ReorderListBox.ScrollViewerScrollingVisualState, true);
                    this.Dispatcher.BeginInvoke(() => this.DragScroll());
                    return;
                }
            }
            else if (autoScrollMargin > 0 && y + dragItemHeight > this.dragInterceptorRect.Height - autoScrollMargin)
            {
                this.dragScrollDelta = (y + dragItemHeight - (this.dragInterceptorRect.Height - autoScrollMargin));
                if (!scrolling)
                {
                    VisualStateManager.GoToState(this.scrollViewer, ReorderListBox.ScrollViewerScrollingVisualState, true);
                    this.Dispatcher.BeginInvoke(() => this.DragScroll());
                    return;
                }
            }
            else
            {
                // We're not within the auto-scroll margin. This ensures any current scrolling is stopped.
                this.dragScrollDelta = 0;
            }
        }