Auto opens a scene on Unity Editor play

SceneAutoLoader.cs

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

[InitializeOnLoad]
internal static class SceneAutoLoader
{
    private const string MenuFolder = "Edit/Scene Auto Load/";
    private const string ToggleMenuItem = MenuFolder + "Enabled";
    private const string SelectSceneMenuItem = MenuFolder + "Select Scene...";

    private const string PrefLoadMasterOnPlay = "SceneAutoLoader.Enabled";
    private const string PrefMasterScene = "SceneAutoLoader.MasterScene";
    private const string PrefPreviousScene = "SceneAutoLoader.PreviousScene";

    static SceneAutoLoader()
    {
        // Subscribe to event if enabled
        if (Enabled) EditorApplication.playModeStateChanged += OnPlayModeChanged;

        // TODO this can sometimes throw an error on editor opening (harmless but ugly)
        Menu.SetChecked(ToggleMenuItem, Enabled);
    }

    /// <summary>
    ///     Returns true if the tool is currently enabled
    /// </summary>
    private static bool Enabled
    {
        get { return PlayerPrefs.GetInt(PrefLoadMasterOnPlay, 0) == 1; }
        set { PlayerPrefs.SetInt(PrefLoadMasterOnPlay, value ? 1 : 0); }
    }

    /// <summary>
    ///     The currently set master scene
    /// </summary>
    private static string MasterScene
    {
        get { return PlayerPrefs.GetString(PrefMasterScene, DefaultMasterScene); }
        set { PlayerPrefs.SetString(PrefMasterScene, value); }
    }

    /// <summary>
    ///     Last scene the user was on before they pressed play
    /// </summary>
    private static string PreviousScene
    {
        get { return PlayerPrefs.GetString(PrefPreviousScene, SceneManager.GetActiveScene().path); }
        set { PlayerPrefs.SetString(PrefPreviousScene, value); }
    }

    /// <summary>
    ///     Get path to default master scene. Used when no master scene is set
    /// </summary>
    private static string DefaultMasterScene
    {
        get { return EditorBuildSettings.scenes.Length > 0 ? EditorBuildSettings.scenes[0].path : null; }
    }

    [MenuItem(SelectSceneMenuItem)]
    public static void SelectMasterScene()
    {
        var masterScene = EditorUtility.OpenFilePanel("Select Master Scene", Application.dataPath, "unity");
        masterScene = masterScene.Replace(Application.dataPath, "Assets");

        if (string.IsNullOrEmpty(masterScene)) return;
        MasterScene = masterScene;

        // Enable if disabled
        if (!Enabled) Toggle();
    }

    [MenuItem(ToggleMenuItem)]
    public static void Toggle()
    {
        Enabled = !Enabled;
        Menu.SetChecked(ToggleMenuItem, Enabled);

        if (Enabled) EditorApplication.playModeStateChanged += OnPlayModeChanged;
        else EditorApplication.playModeStateChanged -= OnPlayModeChanged;
    }

    [MenuItem(ToggleMenuItem, true)]
    private static bool ToggleCheck()
    {
        Menu.SetChecked(ToggleMenuItem, Enabled);
        return true;
    }

    private static void OnPlayModeChanged(PlayModeStateChange state)
    {
        switch (state)
        {
            case PlayModeStateChange.ExitingEditMode:
            {
                OnPlay();
                break;
            }
            case PlayModeStateChange.EnteredEditMode:
            {
                OnStop();
                break;
            }
        }
    }

    private static void OnPlay()
    {
        // If no master scene is set, don't do anything
        if (string.IsNullOrEmpty(MasterScene)) return;

        // Cache previous scene path, ignore if already on the master scene
        PreviousScene = SceneManager.GetActiveScene().path;
        if (PreviousScene == MasterScene)
        {
            PreviousScene = null;
            return;
        }

        // Show save dialog
        if (EditorSceneManager.SaveCurrentModifiedScenesIfUserWantsTo())
            // Open master scene
            try
            {
                EditorSceneManager.OpenScene(MasterScene);
            }
            catch (Exception)
            {
                EditorApplication.isPlaying = false;

                // Disable if currently enabled as current master scene selection does not exist
                if (Enabled) Toggle();
            }
        else
            // Cancel playing if user chooses to cancel the save dialog
            EditorApplication.isPlaying = false;
    }

    private static void OnStop()
    {
        // Don't attempt to load previous scene if it's not set
        if (string.IsNullOrEmpty(PreviousScene)) return;

        try
        {
            // Open previous scene
            EditorSceneManager.OpenScene(PreviousScene);
        }
        catch (Exception)
        {
            // Previous scene doesn't exist anymore
            Debug.LogErrorFormat("Scene Auto Loader: Couldn't find previous scene to load.\n{0}", PreviousScene);
        }
    }
}