Virtualizing list view for Unity IMGUI

GUI2.cs

using System;
using System.Reflection;
using UnityEngine;

public static class GUI2
{
#if UNITY_EDITOR
    private static readonly MethodInfo _internalRepaintEditorWindow = typeof(GUI).GetMethod(
        "InternalRepaintEditorWindow", BindingFlags.Static | BindingFlags.NonPublic);
#endif

    public static float ListView(Rect position, int itemCount, float itemHeight, float scrollPosition,
        Action<int> drawItem)
    {
        float scrollBarWidth = GUI.skin.verticalScrollbar.fixedWidth;

        Rect clientArea = position;
        clientArea.width -= scrollBarWidth;

        GUILayout.BeginArea(clientArea);

        float visibleCount = clientArea.height / itemHeight;
        int start = Mathf.Clamp(Mathf.RoundToInt(scrollPosition), 0, itemCount - 1);
        int end = Mathf.Min(start + Mathf.CeilToInt(visibleCount), itemCount);

        try
        {
            for (int i = start; i < end; i++)
                drawItem(i);
        }
        catch (ArgumentException)
        {
            // If our size changes between layout and repaint, there could be a mismatch between the amount
            // of items laid out and drawn, and an exception could be thrown.
        }

        GUILayout.EndArea();

        var evt = Event.current;
        if (evt.type == EventType.ScrollWheel && position.Contains(evt.mousePosition))
        {
            scrollPosition += evt.delta.y;
#if UNITY_EDITOR
            _internalRepaintEditorWindow.Invoke(null, null);
#endif
        }

        scrollPosition = GUI.VerticalScrollbar(
            new Rect(clientArea.xMax, position.y, scrollBarWidth, position.height),
            scrollPosition, visibleCount, 0f, itemCount + (visibleCount - Mathf.Floor(visibleCount)));

        return scrollPosition;
    }
}

Usage:

_scrollPosition = GUI2.ListView(new Rect(0f, 23f, position.width, position.height - 23f),
    _items.Count, itemHeight, _scrollPosition, index =>
{
    GUILayout.Label(_items[index]);
});