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InfiniteScrollingListBox.cs

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Media;

namespace InfiniteScroll
{
    public class InfiniteScrollingListBox : ListBox
    {
        public static readonly DependencyProperty EarlyLoadingFactorProperty =
            DependencyProperty.Register("EarlyLoadingFactor", typeof (float), typeof (InfiniteScrollingListBox),
                                        new PropertyMetadata(0.1f));

        public static readonly DependencyProperty EagernessProperty = DependencyProperty.Register("Eagerness",
                                                                                                  typeof (int),
                                                                                                  typeof (
                                                                                                      InfiniteScrollingListBox
                                                                                                      ),
                                                                                                  new PropertyMetadata(
                                                                                                      10));

        private bool bound = false;

        public float EarlyLoadingFactor
        {
            get { return (float) GetValue(EarlyLoadingFactorProperty); }
            set { SetValue(EarlyLoadingFactorProperty,value);}
        }

        public int Eagerness
        {
            get { return (int) GetValue(EagernessProperty); }
            set { SetValue(EagernessProperty,value);}
        }

        public InfiniteScrollingListBox() : base()
        {
            LayoutUpdated += InfiniteScrollingListBoxLayoutUpdated; // We can't initialize here, there's no layout yet
        }

        void InfiniteScrollingListBoxLayoutUpdated(object source, EventArgs e)
        {
            if (bound) return;
            // find the scrollviewer
            ScrollViewer scrollViewer = GetTemplateChild("ScrollViewer") as ScrollViewer;
            if (scrollViewer == null) throw new Exception("No ScrollViewer found");

            // find the scrollviewer's scrollbar
            ScrollBar scrollBar =
                ((FrameworkElement)VisualTreeHelper.GetChild(scrollViewer, 0)).FindName("VerticalScrollBar") as
                ScrollBar;
            if (scrollBar == null) throw new Exception("Could not find scrollbar");
            //register event handler
            scrollBar.ValueChanged += (sender, args) =>
            {
                Debug.WriteLine(args.NewValue);
                if (scrollBar.Maximum - (scrollBar.Maximum * EarlyLoadingFactor) >= args.NewValue) return;

                IInfiniteScrollingItemsSource src =
                    ItemsSource as IInfiniteScrollingItemsSource;
                if (src == null) return;

                src.GetMoreItems(Eagerness); // if we need to do some loading and we have the right itemssource, do so
            };

            bound = true;
        }

        public void LoadItems()
        {
            IInfiniteScrollingItemsSource src = ItemsSource as IInfiniteScrollingItemsSource;
            if (src == null) return;
            src.GetMoreItems();
        }

    }
}