using System;
using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Input;
using System.Windows.Media;

namespace Juick.Classes
{
    public class ScrollViewerMonitor
    {
        public static DependencyProperty AtEndCommandProperty =
            DependencyProperty.RegisterAttached("AtEndCommand", typeof(ICommand), typeof(ScrollViewerMonitor), new PropertyMetadata(OnAtEndCommandChanged));

        public static ICommand GetAtEndCommand(DependencyObject obj)
        {
            return (ICommand)obj.GetValue(AtEndCommandProperty);
        }

        public static void SetAtEndCommand(DependencyObject obj, ICommand value)
        {
            obj.SetValue(AtEndCommandProperty, value);
        }

        public static void OnAtEndCommandChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var element = (FrameworkElement)d;
            if (element != null)
            {
                element.Loaded -= element_Loaded;
                element.Loaded += element_Loaded;
            }
        }

        static void element_Loaded(object sender, RoutedEventArgs e)
        {
            var element = (FrameworkElement)sender;
            element.Loaded -= element_Loaded;
            var scrollViewer = FindChildOfType<ScrollViewer>(element);
            if (scrollViewer == null)
            {
                throw new InvalidOperationException("ScrollViewer not found.");
            }

            var listener = new DependencyPropertyListener();
            listener.Changed += (s, eArgs) =>
                {
                    var atBottom = scrollViewer.VerticalOffset >= scrollViewer.ScrollableHeight;
                    if (atBottom)
                    {
                        var atEnd = GetAtEndCommand(element);
                        if (atEnd != null && atEnd.CanExecute(null))
                        {
                            atEnd.Execute(null);
                        }
                    }
                };
            var binding = new Binding("VerticalOffset") { Source = scrollViewer };
            listener.Attach(scrollViewer, binding);
        }

        static T FindChildOfType<T>(DependencyObject root)
            where T : class
        {
            var queue = new Queue<DependencyObject>();
            queue.Enqueue(root);

            while (queue.Count > 0)
            {
                var current = queue.Dequeue();
                for (int i = VisualTreeHelper.GetChildrenCount(current) - 1; 0 <= i; i--)
                {
                    var child = VisualTreeHelper.GetChild(current, i);
                    var typedChild = child as T;
                    if (typedChild != null)
                    {
                        return typedChild;
                    }
                    queue.Enqueue(child);
                }
            }
            return null;
        }
    }
}