using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;

namespace WxOrder.Framework.Commons
{
	/// <summary>
	/// 让页面级 ScrollViewer 在鼠标悬停在子控件（如 DataGrid、图表等）上时也能随鼠标滚轮滚动。
	/// 用法：在 ScrollViewer 上设置 common:ScrollViewerBehavior.FixMouseWheel="True"。
	/// </summary>
	public static class ScrollViewerBehavior
	{
		public static readonly DependencyProperty FixMouseWheelProperty =
			DependencyProperty.RegisterAttached(
				"FixMouseWheel",
				typeof(bool),
				typeof(ScrollViewerBehavior),
				new PropertyMetadata(false, OnFixMouseWheelChanged));

		public static bool GetFixMouseWheel(DependencyObject obj)
			=> (bool)obj.GetValue(FixMouseWheelProperty);

		public static void SetFixMouseWheel(DependencyObject obj, bool value)
			=> obj.SetValue(FixMouseWheelProperty, value);

		private static void OnFixMouseWheelChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
		{
			if (!(d is ScrollViewer scrollViewer))
			{
				return;
			}

			if ((bool)e.NewValue)
			{
				scrollViewer.PreviewMouseWheel += OnPreviewMouseWheel;
			}
			else
			{
				scrollViewer.PreviewMouseWheel -= OnPreviewMouseWheel;
			}
		}

		private static void OnPreviewMouseWheel(object sender, MouseWheelEventArgs e)
		{
			if (e.Handled || !(sender is ScrollViewer scrollViewer) || scrollViewer.ScrollableHeight <= 0)
			{
				return;
			}

			if (!(e.OriginalSource is DependencyObject original))
			{
				return;
			}

			// 从鼠标所在控件向上查找，确认它属于当前 ScrollViewer
			var current = original;
			var inside = false;
			while (current != null)
			{
				if (ReferenceEquals(current, scrollViewer))
				{
					inside = true;
					break;
				}

				// 若鼠标位于一个能自行纵向滚动的内部 ScrollViewer 上（例如下拉列表、
				// 启用了滚动的表格），则交给内部处理，避免抢走它的滚动。
				if (current is ScrollViewer inner &&
					inner.ComputedVerticalScrollBarVisibility == Visibility.Visible &&
					inner.ScrollableHeight > 0)
				{
					return;
				}

				current = VisualTreeHelper.GetParent(current);
			}

			if (!inside)
			{
				return;
			}

			// 按系统默认的滚轮行数滚动当前页面
			var lines = (int)(e.Delta / Mouse.MouseWheelDeltaForOneLine * SystemParameters.WheelScrollLines);
			if (lines == 0)
			{
				lines = e.Delta > 0 ? 1 : -1;
			}

			if (lines < 0)
			{
				for (var i = 0; i < -lines; i++)
				{
					scrollViewer.LineDown();
				}
			}
			else
			{
				for (var i = 0; i < lines; i++)
				{
					scrollViewer.LineUp();
				}
			}

			e.Handled = true;
		}
	}
}