ExtendedListBox

ExtendedListBox

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace HamGuyToolkit.Controls
{
    public class ExtendedListBox : ListBox
    {
        public event RoutedEventHandler OnScrollToBottom;

        private ScrollViewer _scrollViewer = null;
        private bool _isHookedScrollEvent = false;

        public ExtendedListBox()
        {
            this.Loaded += ExtendedListBox_Loaded;
            this.LayoutUpdated += ExtendedListBox_LayoutUpdated;
        }

        void ExtendedListBox_LayoutUpdated(object sender, EventArgs e)
        {
            if (_scrollViewer != null)
                return; 
            
            _scrollViewer = FindFirstVisualElement<ScrollViewer>(this);

            if (_isHookedScrollEvent)
            {
                return;
            }

            if (_scrollViewer != null)
            {
                _isHookedScrollEvent = true;

                FrameworkElement element = VisualTreeHelper.GetChild(_scrollViewer, 0) as FrameworkElement;
                if (element != null)
                {
                    VisualStateGroup visualStateGroup = FindVisualState(element, "ScrollStates");
                    visualStateGroup.CurrentStateChanged += new EventHandler<VisualStateChangedEventArgs>(visualStateGroup_CurrentStateChanged);

                }
            }
        }

        



        void ExtendedListBox_Loaded(object sender, System.Windows.RoutedEventArgs e)
        {
            if (_scrollViewer != null)
                return;

            _scrollViewer = FindFirstVisualElement<ScrollViewer>(this);

            if (_isHookedScrollEvent)
            {
                return;
            }

            if (_scrollViewer != null)
            {
                _isHookedScrollEvent = true;

                FrameworkElement element = VisualTreeHelper.GetChild(_scrollViewer, 0) as FrameworkElement;
                if (element != null)
                {
                    VisualStateGroup visualStateGroup = FindVisualState(element, "ScrollStates");
                    visualStateGroup.CurrentStateChanged += new EventHandler<VisualStateChangedEventArgs>(visualStateGroup_CurrentStateChanged);

                }
            }

        }

        void visualStateGroup_CurrentStateChanged(object sender, VisualStateChangedEventArgs e)
        {
            var visualState = e.NewState.Name;

            if (visualState == "NotScrolling")
            {
                var v1 = Math.Round(_scrollViewer.ExtentHeight - _scrollViewer.VerticalOffset);
                var v2 = Math.Round(_scrollViewer.ViewportHeight);


                if (v1 <= v2)
                {
                    // TODO: Load more data action
                    if (OnScrollToBottom != null)
                        OnScrollToBottom(this, new RoutedEventArgs());
                }

            }
        }


        private T FindFirstVisualElement<T>(DependencyObject container) where T : DependencyObject
        {
            var childQueue = new Queue<DependencyObject>();

            childQueue.Enqueue(container);

            while (childQueue.Count > 0)
            {
                var current = childQueue.Dequeue();
                T result = current as T;
                if (result != null && result != container)
                {
                    return result;
                }

                int childCount = VisualTreeHelper.GetChildrenCount(current);

                for (int childIndex = 0; childIndex < childCount; childIndex++)
                {
                    childQueue.Enqueue(VisualTreeHelper.GetChild(current, childIndex));
                }
            }

            return null;
        }


        private VisualStateGroup FindVisualState(FrameworkElement element, string name)
        {
            if (element == null)
            {
                return null;
            }

            var groups = VisualStateManager.GetVisualStateGroups(element);
            foreach (VisualStateGroup group in groups)
            {
                if (group.Name == name)
                {
                    return group;
                }
            }

            return null;
        }
    }
}