【WPF】ページングできるListBox

using System;
using System.Collections;
using System.Collections.ObjectModel;
using System.Linq;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

namespace PagingListBoxSample
{
    [TemplatePart(Name = PartScrollViewer)]
    [TemplatePart(Name = PartMoreButton)]
    public class PagingListBox : ListBox
    {
        private const string PartScrollViewer = "PART_ScrollViewer";
        private const string PartMoreButton = "PART_MoreButton";

        static PagingListBox()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(PagingListBox), new FrameworkPropertyMetadata(typeof(PagingListBox)));
        }



        public new IEnumerable ItemsSource
        {
            get { return (IEnumerable)GetValue(ItemsSourceProperty); }
            set { SetValue(ItemsSourceProperty, value); }
        }

        // Using a DependencyProperty as the backing store for ItemsSource.  This enables animation, styling, binding, etc...
        public new static readonly DependencyProperty ItemsSourceProperty =
            DependencyProperty.Register(nameof(ItemsSource), typeof(IEnumerable), typeof(PagingListBox), new PropertyMetadata(null, (d, e) =>
            {
                if (!(d is PagingListBox plv)) { return; }
                if (!(e.NewValue is IEnumerable enu)) { return; }

                plv.InitializeInnerCollection(enu);

                // INotifyCollectionChangedだったら増減を監視する必要あり
            }));


        /// <summary>
        /// 最初から表示されている要素数
        /// </summary>
        public int DefaultDisplayNum
        {
            get { return (int)GetValue(DefaultDisplayNumProperty); }
            set { SetValue(DefaultDisplayNumProperty, value); }
        }

        // Using a DependencyProperty as the backing store for DefaultDisplayNum.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty DefaultDisplayNumProperty =
            DependencyProperty.Register("DefaultDisplayNum", typeof(int), typeof(PagingListBox), new PropertyMetadata(20, (d, e) =>
            {
                if (!(d is PagingListBox plv)) { return; }
                if (!(e.NewValue is int defaultDisplayNum)) { return; }

                plv.currentDisplayNum = defaultDisplayNum;

                plv.InitializeInnerCollection(plv.ItemsSource);
            }));


        /// <summary>
        /// 新しいページを表示したときに追加される要素数
        /// </summary>
        public int DisplayNumOnPage
        {
            get { return (int)GetValue(PageCountProperty); }
            set { SetValue(PageCountProperty, value); }
        }

        // Using a DependencyProperty as the backing store for AddCount.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty PageCountProperty =
            DependencyProperty.Register(nameof(DisplayNumOnPage), typeof(int), typeof(PagingListBox), new PropertyMetadata(10));

        /// <summary>
        /// Bindingしたコレクションの要素が全て表示された状態かどうか
        /// </summary>
        public bool IsDisplayAll
        {
            get { return (bool)GetValue(IsDisplayAllProperty); }
            private set { SetValue(IsDisplayAllPropertyKey, value); }
        }

        private static readonly DependencyPropertyKey IsDisplayAllPropertyKey =
            DependencyProperty.RegisterReadOnly(nameof(IsDisplayAll), typeof(bool), typeof(PagingListBox), new PropertyMetadata(false));
        public static readonly DependencyProperty IsDisplayAllProperty = IsDisplayAllPropertyKey.DependencyProperty;

        private readonly ObservableCollection<object> innerCollection
            = new ObservableCollection<object>();
        private int currentDisplayNum;
        private int totalCount;

        private ScrollViewer partScrollViewer;
        private Button partMoreButton;

        public PagingListBox()
        {
            currentDisplayNum = DefaultDisplayNum;
            base.ItemsSource = innerCollection;
        }

        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            partScrollViewer = GetTemplateChild(PartScrollViewer) as ScrollViewer;

            if (partMoreButton != null)
            {
                partMoreButton.Click -= OnPartMoreButtonClick;
            }

            partMoreButton = GetTemplateChild(PartMoreButton) as Button;
            partMoreButton.Click += OnPartMoreButtonClick;
            UpdateState();
        }

        private void OnPartMoreButtonClick(object sender, RoutedEventArgs e)
        {
            TryAddNewPage();
        }

        protected override void OnPreviewMouseWheel(MouseWheelEventArgs e)
        {
            base.OnPreviewMouseWheel(e);

            if (e.Delta <= 0
                && partScrollViewer.VerticalOffset >= partScrollViewer.ScrollableHeight)
            {
                if (TryAddNewPage())
                {
                    e.Handled = true;
                }
            }
        }

        private bool TryAddNewPage()
        {
            // 現在の表示数がオリジナルのコレクションの合計数に到達している場合は何もしない
            if (IsDisplayAll)
            {
                return false;
            }

            var enumerable = ItemsSource
                .OfType<object>()
                .Skip(currentDisplayNum);

            var count = 1;
            foreach (var item in enumerable)
            {
                innerCollection.Add(item);
                if (count++ >= DisplayNumOnPage)
                {
                    break;
                }
            }

            currentDisplayNum = Math.Min(currentDisplayNum + DisplayNumOnPage, totalCount);
            UpdateState();
            return true;
        }

        private void InitializeInnerCollection(IEnumerable enu)
        {
            var enumerable = enu.OfType<object>();
            totalCount = enumerable.Count();
            innerCollection.Clear();
            var count = 1;
            foreach (var item in enumerable)
            {
                innerCollection.Add(item);
                if (count++ >= currentDisplayNum)
                {
                    break;
                }
            }
            UpdateState();
        }

        private void UpdateState()
        {
            IsDisplayAll = currentDisplayNum >= totalCount;

            if (partMoreButton == null)
            {
                return;
            }
            partMoreButton.Visibility = IsDisplayAll
                ? Visibility.Collapsed
                : Visibility.Visible;
        }
    }
}