module-navigation-tool/Editor/Toolbar/ToolbarDrawer.cs

using UnityEditor;
using UnityEngine;

#if UNITY_2021_1_OR_NEWER
using System.Collections.Generic;
using UnityEngine.UIElements;
#endif

namespace Module.NavigationTool.Editor.Toolbar
{
    [InitializeOnLoad]
    internal static class ToolbarDrawer
    {
        private static bool IS_INITIALIZED;
        private static AbstractToolbarDrawer[] DRAWERS;
        
        #if UNITY_2021_1_OR_NEWER
        private static readonly Dictionary<AbstractToolbarDrawer, IMGUIContainer> DICT_MAPPING = new Dictionary<AbstractToolbarDrawer, IMGUIContainer>();
        #endif

        static ToolbarDrawer()
        {
            EditorApplication.update -= OnEditorUpdate;
            EditorApplication.update += OnEditorUpdate;
        }
        
        private static void OnEditorUpdate()
        {
            if (!IS_INITIALIZED)
            {
                DRAWERS = ToolbarUtility.GetAllDrawers();
                
                #if !UNITY_2021_1_OR_NEWER
                ToolbarUtility.AddGuiListener(OnGUI);
                #endif
                
                IS_INITIALIZED = true;
            }
            
            #if UNITY_2021_1_OR_NEWER
            ToolbarUtility.OnUpdate(OnUpdateElements);
            #endif

            for (var i = 0; i < DRAWERS.Length; i++)
            {
                if (DRAWERS[i].Visible)
                    DRAWERS[i].Update();
            }
        }

        #if UNITY_2021_1_OR_NEWER
        private static void OnUpdateElements(VisualElement leftAlign, VisualElement rightAlign)
        {
            const float HEIGHT = 22.0f;
            var added = false;

            for (int i = DRAWERS.Length - 1; i >= 0; i--)
            {
                AbstractToolbarDrawer drawer = DRAWERS[i];
                bool valid = CheckValidityOfContainer(drawer, leftAlign, rightAlign);

                if (drawer.Visible && !valid)
                {
                    var rect = new Rect(2.0f, 0.0f, drawer.CalculateWidth(), HEIGHT);
                    drawer.Setup(rect);

                    var container = new ToolbarIMGUIContainer(drawer.OnGUI, drawer.Priority);
                    container.style.width = rect.width + 4.0f;
                    DICT_MAPPING.Add(drawer, container);
                    added = true;

                    if (drawer.Placement == EToolbarPlacement.Left)
                        leftAlign.Add(container);
                    else
                        rightAlign.Add(container);
                }
                else if (!drawer.Visible && valid)
                {
                    IMGUIContainer container = DICT_MAPPING[drawer];
                    DICT_MAPPING.Remove(drawer);
                    container.RemoveFromHierarchy();
                }
            }

            if (added)
            {
                leftAlign.Sort(SortVisualElements);
                rightAlign.Sort(SortVisualElements);
            }
        }

        private static bool CheckValidityOfContainer(AbstractToolbarDrawer drawer, VisualElement leftAlign, VisualElement rightAlign)
        {
            if (!DICT_MAPPING.TryGetValue(drawer, out IMGUIContainer container))
                return false;
            
            bool valid = drawer.Placement == EToolbarPlacement.Left 
                ? leftAlign.Contains(container) 
                : rightAlign.Contains(container);

            if (!valid)
                DICT_MAPPING.Remove(drawer);

            return valid;
        }
        
        private static int SortVisualElements(VisualElement ve0, VisualElement ve1)
        {
            var c0 = ve0 as ToolbarIMGUIContainer;
            var c1 = ve1 as ToolbarIMGUIContainer;

            if (c0 == null && c1 == null)
                return 0;
            if (c0 == null)
                return -1;
            if (c1 == null)
                return 1;

            return c0.Priority.CompareTo(c1.Priority);
        }
        #else
        private static void OnGUI()
        {
            const float Y = 5.0f;
            const float SPACING = 4.0f;
            const float HEIGHT = 22.0f;
            const float PLAY_BUTTON_EXTENT = 75.0f;
            const float PLAY_BUTTON_OFFSET = -23.0f;

            if (DRAWERS == null)
                return;
            
            float xLeft = EditorGUIUtility.currentViewWidth * 0.5f + PLAY_BUTTON_OFFSET - PLAY_BUTTON_EXTENT;
            float xRight = EditorGUIUtility.currentViewWidth * 0.5f + PLAY_BUTTON_OFFSET + PLAY_BUTTON_EXTENT;

            for (var i = 0; i < DRAWERS.Length; i++)
            {
                AbstractToolbarDrawer drawer = DRAWERS[i];

                if (!drawer.Visible)
                    continue;

                GUI.enabled = drawer.Enabled;
                float width = drawer.CalculateWidth();
                
                Rect rect;

                if (drawer.Placement == EToolbarPlacement.Left)
                {
                    rect = new Rect(xLeft - width, Y, width, HEIGHT);
                    xLeft -= width + SPACING;

                    // Note: Magic number for closest right-most item left of play button (2020.3)
                    if (rect.xMin < 414.0f)
                        continue;
                }
                else
                {
                    rect = new Rect(xRight, Y, width, HEIGHT);
                    xRight += width + SPACING;

                    // Note: Magic number for closest left-most item right of play button (2020.3)
                    //       If you don't have collab, which no sane person has
                    if (rect.xMax < 642.0f)
                        continue;
                }

                drawer.Setup(rect);
                drawer.OnGUI();
            }
            
            GUI.enabled = true;
        }
        #endif
    }
}