Silverlight ListBox Part II (Drag and drop in the same listbox and scroll)

using System;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using System.Collections.Generic;
using System.Diagnostics;

namespace MultipleDataTemplateInListBox
{
    public class WrapPanel : Panel
    {
        private static readonly DependencyProperty WrapPanelAttachedDataProperty =
           DependencyProperty.RegisterAttached("WrapPanelAttachedDataProperty", typeof(WrapPanelAttachedData), typeof(WrapPanel), null);

        private readonly List<double> rowHeights = new List<double>();

        // To maintain the row no. for each item.
        private readonly Dictionary<int, int> indexRowMapping = new Dictionary<int, int>();

        public double GetOffsetFromTop(int indexOfItem)
        {
            double offset = 0;
            if (!this.indexRowMapping.ContainsKey(indexOfItem))
            {
                return 0;
            }

            // Get the row no. for the given item.
            int rowNo = this.indexRowMapping[indexOfItem];

            // If row no. is 0 then offset would be 0.
            if (rowNo == 0)
            {
                return 0;
            }
            else
            {
                // Add the height of the rows which are before the row of the given element.
                for (int index = rowNo - 1; index >= 0; index--)
                {
                    offset += this.rowHeights[index];
                }
            }

            return offset;
        }

        protected override Size MeasureOverride(Size availableSize)
        {
            foreach (FrameworkElement child in this.Children)
            {
                child.Measure(availableSize);
            }

            double rowHeight = 0;
            int row = 0;
            int elementNo = 0;
            this.rowHeights.Clear();
            this.indexRowMapping.Clear();

            Size desiredSize = Size.Empty;

            Point nextChildPosition = new Point(0, 0);

            foreach (FrameworkElement child in this.Children)
            {
                if (nextChildPosition.X + child.DesiredSize.Width > availableSize.Width)
                {
                    this.rowHeights.Add(rowHeight);
                    ++row;

                    nextChildPosition.X = 0;
                    nextChildPosition.Y += rowHeight;
                    rowHeight = 0;
                }

                // Insert the index of the item and the row no.
                this.indexRowMapping.Add(elementNo, row);
                elementNo++;

                WrapPanelAttachedData data = GetWrapPanelAttachedData(child);

                if (data.TargetPosition != nextChildPosition)
                {
                    data.TargetPosition = nextChildPosition;
                    data.Row = row;
                }

                desiredSize.Width = Math.Max(desiredSize.Width, nextChildPosition.X + child.DesiredSize.Width);
                desiredSize.Height = Math.Max(desiredSize.Height, nextChildPosition.Y + child.DesiredSize.Height);

                rowHeight = Math.Max(rowHeight, child.DesiredSize.Height);

                nextChildPosition.X += child.DesiredSize.Width;
            }

            this.rowHeights.Add(rowHeight);

            return desiredSize;
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            foreach (UIElement child in this.Children)
            {
                WrapPanelAttachedData data = GetWrapPanelAttachedData(child);
                child.Arrange(new Rect(data.TargetPosition.X, data.TargetPosition.Y, child.DesiredSize.Width, this.rowHeights[data.Row]));
            }

            return finalSize;
        }

        private static WrapPanelAttachedData GetWrapPanelAttachedData(DependencyObject obj)
        {
            object value = obj.GetValue(WrapPanelAttachedDataProperty);

            if (value == null)
            {
                WrapPanelAttachedData data = new WrapPanelAttachedData();
                SetAnimatedWrapPanelAttachedData(obj, data);
                return data;
            }

            return (WrapPanelAttachedData)value;
        }

        private static void SetAnimatedWrapPanelAttachedData(DependencyObject obj, WrapPanelAttachedData value)
        {
            obj.SetValue(WrapPanelAttachedDataProperty, value);
        }

        private class WrapPanelAttachedData
        {
            public static readonly Point Unset = new Point(-1, -1);

            public WrapPanelAttachedData()
            {
                this.TargetPosition = Unset;
            }

            public Point TargetPosition { get; set; }

            public int Row { get; set; }
        }
    }
}