Explorer-Workbench/src/Explorer.App/FolderViewport.cs

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using Explorer.Presentation;
using Explorer.Presentation.ViewModels;

namespace Explorer.App;

public static class FolderViewport
{
    public static readonly DependencyProperty IsTrackedProperty =
        DependencyProperty.RegisterAttached(
            "IsTracked",
            typeof(bool),
            typeof(FolderViewport),
            new PropertyMetadata(false, OnTrackedChanged));

    public static void SetIsTracked(ListView element, bool value) => element.SetValue(IsTrackedProperty, value);

    public static bool GetIsTracked(ListView element) => (bool)element.GetValue(IsTrackedProperty);

    private static void OnTrackedChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not ListView list)
        {
            return;
        }

        if (e.NewValue is true)
        {
            list.AddHandler(ScrollViewer.ScrollChangedEvent, new ScrollChangedEventHandler(OnScrollChanged), true);
            list.Loaded += OnLoaded;
        }
        else
        {
            list.RemoveHandler(ScrollViewer.ScrollChangedEvent, new ScrollChangedEventHandler(OnScrollChanged));
            list.Loaded -= OnLoaded;
        }
    }

    private static void OnLoaded(object sender, RoutedEventArgs e) => Report(sender as ListView);

    private static void OnScrollChanged(object sender, ScrollChangedEventArgs e)
    {
        var list = sender as ListView ?? FindAncestor<ListView>(e.OriginalSource as DependencyObject);
        Report(list);
    }

    private static void Report(ListView? list)
    {
        if (list?.DataContext is not ExplorerPaneViewModel pane || list.Visibility != Visibility.Visible)
        {
            return;
        }

        var count = list.Items.Count;
        if (count == 0)
        {
            pane.NotifyViewport([]);
            return;
        }

        int first;
        int lastExclusive;
        if (FindWrapPanel(list) is { } wrap)
        {
            first = wrap.FirstVisibleIndex;
            lastExclusive = Math.Max(first, wrap.LastVisibleIndex);
        }
        else
        {
            var viewer = FindScrollViewer(list);
            int visible;
            if (viewer is null || viewer.ExtentHeight <= 0)
            {
                first = 0;
                visible = Math.Min(80, count);
            }
            else
            {
                first = (int)Math.Clamp(Math.Floor(viewer.VerticalOffset / viewer.ExtentHeight * count), 0, count - 1);
                visible = Math.Max(8, (int)Math.Ceiling(viewer.ViewportHeight / viewer.ExtentHeight * count) + 8);
            }

            lastExclusive = Math.Min(count, first + visible);
        }

        lastExclusive = Math.Clamp(lastExclusive, 0, count);
        first = Math.Clamp(first, 0, count);
        var paths = new string[Math.Max(0, lastExclusive - first)];
        for (var i = first; i < lastExclusive; i++)
        {
            if (list.Items[i] is FolderItemViewModel item)
            {
                paths[i - first] = item.FullPath;
            }
        }

        pane.NotifyViewport(paths);
    }

    private static ScrollViewer? FindScrollViewer(DependencyObject root)
    {
        if (root is ScrollViewer viewer)
        {
            return viewer;
        }

        for (var i = 0; i < VisualTreeHelper.GetChildrenCount(root); i++)
        {
            var found = FindScrollViewer(VisualTreeHelper.GetChild(root, i));
            if (found is not null)
            {
                return found;
            }
        }

        return null;
    }

    private static VirtualizingWrapPanel? FindWrapPanel(DependencyObject root)
    {
        if (root is VirtualizingWrapPanel wrap)
        {
            return wrap;
        }

        for (var i = 0; i < VisualTreeHelper.GetChildrenCount(root); i++)
        {
            var found = FindWrapPanel(VisualTreeHelper.GetChild(root, i));
            if (found is not null)
            {
                return found;
            }
        }

        return null;
    }

    private static T? FindAncestor<T>(DependencyObject? current)
        where T : DependencyObject
    {
        while (current is not null)
        {
            if (current is T match)
            {
                return match;
            }

            current = VisualTreeHelper.GetParent(current);
        }

        return null;
    }
}