SceneAutoLoader with additive loading

SceneAutoLoader.cs

using System;
using UnityEditor;
using UnityEditor.SceneManagement;
using UnityEngine;
using UnityEngine.SceneManagement;

namespace Editor
{
    /// <summary>
    /// Scene auto loader.  
    /// </summary>
    /// <description>
    /// This class adds a File > Scene Autoload menu containing options to select
    /// a "master scene" enable it to be auto-loaded when the user presses play
    /// in the editor. When enabled, the selected scene will be loaded on play,
    /// then the original scene will be reloaded on stop.
    ///
    /// Based on an idea on this thread:
    /// http://forum.unity3d.com/threads/157502-Executing-first-scene-in-build-settings-when-pressing-play-button-in-editor
    /// </description>
    [InitializeOnLoad]
    internal static class SceneAutoLoader
    {
        // Static constructor binds a playmode-changed callback.
        // [InitializeOnLoad] above makes sure this gets executed.
        static SceneAutoLoader()
        {
            EditorApplication.playModeStateChanged += OnPlayModeChanged;
        }
 
        // Menu items to select the "master" scene and control whether or not to load it.
        [MenuItem("File/Scene Autoload/Select Master Scene...")]
        private static void SelectMasterScene()
        {
            string masterScene = EditorUtility.OpenFilePanel("Select Master Scene", Application.dataPath, "unity");
            if (!string.IsNullOrEmpty(masterScene))
            {
                MasterScene = masterScene;
                LoadMasterOnPlay = true;
            }
        }

        [MenuItem("File/Scene Autoload/----------------------")]
        private static void Separator1()
        {}
 
        [MenuItem("File/Scene Autoload/Load Master On Play", true)]
        private static bool ShowLoadMasterOnPlay()
        {
            return !LoadMasterOnPlay;
        }
        [MenuItem("File/Scene Autoload/Load Master On Play")]
        private static void EnableLoadMasterOnPlay()
        {
            LoadMasterOnPlay = true;
        }
 
        [MenuItem("File/Scene Autoload/Don't Load Master On Play", true)]
        private static bool ShowDontLoadMasterOnPlay()
        {
            return LoadMasterOnPlay;
        }
        [MenuItem("File/Scene Autoload/Don't Load Master On Play")]
        private static void DisableLoadMasterOnPlay()
        {
            LoadMasterOnPlay = false;
        }

        [MenuItem("File/Scene Autoload/-------------------------")]
        private static void Separator2()
        { }

        [MenuItem("File/Scene Autoload/Load current scene additively", true)]
        private static bool ShowLoadAdditiveCurrentScene()
        {
            return !LoadAdditiveCurrentScene;
        }
        [MenuItem("File/Scene Autoload/Load current scene additively")]
        private static void EnableLoadAdditiveCurrentScene()
        {
            LoadAdditiveCurrentScene = true;
        }

        [MenuItem("File/Scene Autoload/Don't Load current scene additively", true)]
        private static bool ShowDontLoadAdditiveCurrentScene()
        {
            return LoadAdditiveCurrentScene;
        }
        [MenuItem("File/Scene Autoload/Don't Load current scene additively")]
        private static void DisableLoadAdditiveCurrentScene()
        {
            LoadAdditiveCurrentScene = false;
        }
 
        // Play mode change callback handles the scene load/reload.
        private static void OnPlayModeChanged(PlayModeStateChange playModeStateChange)
        {
            if (!LoadMasterOnPlay)
            {
                return;
            }
        
            switch (playModeStateChange)
            {
                case PlayModeStateChange.ExitingEditMode:
                    BeginningPlay();
                    break;
                case PlayModeStateChange.EnteredEditMode:
                    StoppingPlay();
                    break;
                case PlayModeStateChange.EnteredPlayMode:
                case PlayModeStateChange.ExitingPlayMode:
                default:
                    return;
            }
        }

        private static void StoppingPlay()
        {
            try
            {
                EditorSceneManager.OpenScene(PreviousScene);
            }
            catch (ArgumentException argEx)
            {
                Debug.LogError($"Failed to open {PreviousScene}.  Error: {argEx.Message}\n{argEx.StackTrace}");
            }
        }

        private static void BeginningPlay()
        {
            PreviousScene = SceneManager.GetActiveScene().path;
            if (EditorSceneManager.SaveCurrentModifiedScenesIfUserWantsTo())
            {
                try
                {
                    EditorSceneManager.OpenScene(MasterScene);
                }
                catch (ArgumentException argEx)
                {
                    Debug.LogError($"Failed to open {MasterScene}.  Error: {argEx.Message}\n{argEx.StackTrace}");
                    EditorApplication.isPlaying = false;
                }
            }
            else
            {
                // User cancelled the save operation -- cancel play as well.
                EditorApplication.isPlaying = false;
            }

            if (LoadAdditiveCurrentScene)
            {
                var sceneName = PreviousScene.Split('/', '.');
                SceneManager.LoadScene(sceneName[^2]);
            }
        }

        // Properties are remembered as editor preferences.
        private const string EditorPrefLoadMasterOnPlay = "SceneAutoLoader.LoadMasterOnPlay";
        private const string EditorPrefMasterScene = "SceneAutoLoader.MasterScene";
        private const string EditorPrefPreviousScene = "SceneAutoLoader.PreviousScene";
        private const string EditorPrefLoadAdditiveCurrentScene = "SceneAutoLoader.LoadAdditiveCurrentScene";
 
        private static bool LoadMasterOnPlay
        {
            get => EditorPrefs.GetBool(EditorPrefLoadMasterOnPlay, false);
            set => EditorPrefs.SetBool(EditorPrefLoadMasterOnPlay, value);
        }
 
        private static string MasterScene
        {
            get => EditorPrefs.GetString(EditorPrefMasterScene, "Master.unity");
            set => EditorPrefs.SetString(EditorPrefMasterScene, value);
        }
 
        private static string PreviousScene
        {
            get => EditorPrefs.GetString(EditorPrefPreviousScene, SceneManager.GetActiveScene().name);
            set => EditorPrefs.SetString(EditorPrefPreviousScene, value);
        }

        private static bool LoadAdditiveCurrentScene
        {
            get => EditorPrefs.GetBool(EditorPrefLoadAdditiveCurrentScene, false);
            set => EditorPrefs.SetBool(EditorPrefLoadAdditiveCurrentScene, value);
        }
    }
}