[Unity3D] A Reliable, user-friendly way to reference SceneAssets by script.

README.md

What is this?

A SceneReference wrapper class that uses ISerializationCallbackReceiver and a custom PropertyDrawerto provide safe, user-friendly scene references in scripts.

Why is this needed?

Sooner or later Unity Developers will want to reference a Scene from script, so they can load it at runtime using the SceneManager.

We can store a string of the scene's path (as the SceneAsset documentation suggests), but that is really not ideal for production: If the scene asset is ever moved or renamed, then our stored path is broken.

Unity has already solved this problem for assets using Object references. Using AssetDatabasse you can find the path for a given asset. The problem there unfortunately is that we don't have access to AssetDatabase at runtime, since it is part of the UnityEditor Assembly.

So to be able to reliably use a scene both in editor and at runtime we need two pieces of serialized information: A reference to the SceneAsset object, and a string path that can be passed into SceneManager at runtime.

We can create our own Wrapper class that uses ISerializationCallbackReceiver to ensure that the stored path is always valid based on the specified SceneAsset Object.

This SceneReference class is an example implementation of this idea.

Key features

• Custom PropertyDrawer that displays the current Build Settings status, including BuildIndex and convenient buttons for managing it with destructive action confirmation dialogues.
• If (and only if) the serialized Object reference is invalid but path is still valid (for example if someone merged incorrectly) will recover object using path.
• Buttons collapse to smaller text if full text cannot be displayed.
  
• Includes detailed tooltips and respects Version Control if build settings is not checked out (tested with Perforce).
  
• It's a single drop-in script. You're welcome to split the Editor-only PropertyDrawer and helpers into their own Editor scripts if you'd like, just for convenience I've put it all in one self-contained file here.

---

For easy runtime verification I've also provided a testing Monobehaviour that lets you view and load scenes via buttons when in playmode:

SceneReference.cs

using System.Collections.Generic;
using System.Linq;
using UnityEngine;

#if UNITY_EDITOR
using UnityEditor;
#endif

// Author: JohannesMP (2018-08-12)
//
// A wrapper that provides the means to safely serialize Scene Asset References.
// 
// Internally we serialize an Object to the SceneAsset which only exists at editor time.
// Any time the object is serialized, we store the path provided by this Asset (assuming it was valid).
// 
// This means that, come build time, the string path of the scene asset is always already stored, which if 
// the scene was added to the build settings means it can be loaded.
// 
// It is up to the user to ensure the scene exists in the build settings so it is loadable at runtime.
// To help with this, a custom PropertyDrawer displays the scene build settings state.
// 
// Known issues:
//     -   When reverting back to a prefab which has the asset stored as null, Unity will show the property 
//         as modified despite having just reverted. This only happens the fist time, and reverting again 
//         fixes it. Under the hood the state is still always valid, and serialized correctly regardless.


/// <summary>
/// A wrapper that provides the means to safely serialize Scene Asset References.
/// </summary>
[System.Serializable]
public class SceneReference : ISerializationCallbackReceiver
{
#if UNITY_EDITOR
    // What we use in editor to select the scene
    [SerializeField] private Object sceneAsset = null;
    bool IsValidSceneAsset
    {
        get
        {
            if (sceneAsset == null)
                return false;
            return sceneAsset.GetType().Equals(typeof(SceneAsset));
        }
    }
#endif

    // This should only ever be set during serialization/deserialization!
    [SerializeField]
    private string scenePath = string.Empty;

    // Use this when you want to actually have the scene path
    public string ScenePath
    {
        get
        {
#if UNITY_EDITOR
            // In editor we always use the asset's path
            return GetScenePathFromAsset();
#else
            // At runtime we rely on the stored path value which we assume was serialized correctly at build time.
            // See OnBeforeSerialize and OnAfterDeserialize
            return scenePath;
#endif
        }
        set
        {
            scenePath = value;
#if UNITY_EDITOR
            sceneAsset = GetSceneAssetFromPath();
#endif
        }
    }

    public static implicit operator string(SceneReference sceneReference)
    {
        return sceneReference.ScenePath;
    }

    // Called to prepare this data for serialization. Stubbed out when not in editor.
    public void OnBeforeSerialize()
    {
#if UNITY_EDITOR
        HandleBeforeSerialize();
#endif
    }

    // Called to set up data for deserialization. Stubbed out when not in editor.
    public void OnAfterDeserialize()
    {
#if UNITY_EDITOR
        // We sadly cannot touch assetdatabase during serialization, so defer by a bit.
        EditorApplication.update += HandleAfterDeserialize;
#endif
    }



#if UNITY_EDITOR
    private SceneAsset GetSceneAssetFromPath()
    {
        if (string.IsNullOrEmpty(scenePath))
            return null;
        return AssetDatabase.LoadAssetAtPath<SceneAsset>(scenePath);
    }

    private string GetScenePathFromAsset()
    {
        if (sceneAsset == null)
            return string.Empty;
        return AssetDatabase.GetAssetPath(sceneAsset);
    }

    private void HandleBeforeSerialize()
    {
        // Asset is invalid but have Path to try and recover from
        if (IsValidSceneAsset == false && string.IsNullOrEmpty(scenePath) == false)
        {
            sceneAsset = GetSceneAssetFromPath();
            if (sceneAsset == null)
                scenePath = string.Empty;

            UnityEditor.SceneManagement.EditorSceneManager.MarkAllScenesDirty();
        }
        // Asset takes precendence and overwrites Path
        else
        {
            scenePath = GetScenePathFromAsset();
        }
    }

    private void HandleAfterDeserialize()
    {
        EditorApplication.update -= HandleAfterDeserialize;
        // Asset is valid, don't do anything - Path will always be set based on it when it matters
        if (IsValidSceneAsset)
            return;

        // Asset is invalid but have path to try and recover from
        if (string.IsNullOrEmpty(scenePath) == false)
        {
            sceneAsset = GetSceneAssetFromPath();
            // No asset found, path was invalid. Make sure we don't carry over the old invalid path
            if (sceneAsset == null)
                scenePath = string.Empty;

            if (Application.isPlaying == false)
                UnityEditor.SceneManagement.EditorSceneManager.MarkAllScenesDirty();
        }
    }
#endif
}


#if UNITY_EDITOR
/// <summary>
/// Display a Scene Reference object in the editor.
/// If scene is valid, provides basic buttons to interact with the scene's role in Build Settings.
/// </summary>
[CustomPropertyDrawer(typeof(SceneReference))]
public class SceneReferencePropertyDrawer : PropertyDrawer
{
    // The exact name of the asset Object variable in the SceneReference object
    const string sceneAssetPropertyString = "sceneAsset";
    // The exact name of  the scene Path variable in the SceneReference object
    const string scenePathPropertyString = "scenePath";

    static readonly RectOffset boxPadding = EditorStyles.helpBox.padding;
    static readonly float padSize = 2f;
    static readonly float lineHeight = EditorGUIUtility.singleLineHeight;
    static readonly float paddedLine = lineHeight + padSize;
    static readonly float footerHeight = 10f;

    /// <summary>
    /// Drawing the 'SceneReference' property
    /// </summary>
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        var sceneAssetProperty = GetSceneAssetProperty(property);

        // Draw the Box Background
        position.height -= footerHeight;
        GUI.Box(EditorGUI.IndentedRect(position), GUIContent.none, EditorStyles.helpBox);
        position = boxPadding.Remove(position);
        position.height = lineHeight;

        // Draw the main Object field
        label.tooltip = "The actual Scene Asset reference.\nOn serialize this is also stored as the asset's path.";

        EditorGUI.BeginProperty(position, GUIContent.none, property);
        EditorGUI.BeginChangeCheck();
        int sceneControlID = GUIUtility.GetControlID(FocusType.Passive);
        var selectedObject = EditorGUI.ObjectField(position, label, sceneAssetProperty.objectReferenceValue, typeof(SceneAsset), false);
        BuildUtils.BuildScene buildScene = BuildUtils.GetBuildScene(selectedObject);

        if (EditorGUI.EndChangeCheck())
        {
            sceneAssetProperty.objectReferenceValue = selectedObject;

            // If no valid scene asset was selected, reset the stored path accordingly
            if (buildScene.scene == null)
                GetScenePathProperty(property).stringValue = string.Empty;
        }
        position.y += paddedLine;

        if (buildScene.assetGUID.Empty() == false)
        {
            // Draw the Build Settings Info of the selected Scene
            DrawSceneInfoGUI(position, buildScene, sceneControlID + 1);
        }

        EditorGUI.EndProperty();
    }

    /// <summary>
    /// Ensure that what we draw in OnGUI always has the room it needs
    /// </summary>
    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        int lines = 2;
        SerializedProperty sceneAssetProperty = GetSceneAssetProperty(property);
        if (sceneAssetProperty.objectReferenceValue == null)
            lines = 1;

        return boxPadding.vertical + lineHeight * lines + padSize * (lines - 1) + footerHeight;
    }

    /// <summary>
    /// Draws info box of the provided scene
    /// </summary>
    private void DrawSceneInfoGUI(Rect position, BuildUtils.BuildScene buildScene, int sceneControlID)
    {
        bool readOnly = BuildUtils.IsReadOnly();
        string readOnlyWarning = readOnly ? "\n\nWARNING: Build Settings is not checked out and so cannot be modified." : "";

        // Label Prefix
        GUIContent iconContent = new GUIContent();
        GUIContent labelContent = new GUIContent();

        // Missing from build scenes
        if (buildScene.buildIndex == -1)
        {
            iconContent = EditorGUIUtility.IconContent("d_winbtn_mac_close");
            labelContent.text = "NOT In Build";
            labelContent.tooltip = "This scene is NOT in build settings.\nIt will be NOT included in builds.";
        }
        // In build scenes and enabled
        else if (buildScene.scene.enabled)
        {
            iconContent = EditorGUIUtility.IconContent("d_winbtn_mac_max");
            labelContent.text = "BuildIndex: " + buildScene.buildIndex;
            labelContent.tooltip = "This scene is in build settings and ENABLED.\nIt will be included in builds." + readOnlyWarning;
        }
        // In build scenes and disabled
        else
        {
            iconContent = EditorGUIUtility.IconContent("d_winbtn_mac_min");
            labelContent.text = "BuildIndex: " + buildScene.buildIndex;
            labelContent.tooltip = "This scene is in build settings and DISABLED.\nIt will be NOT included in builds.";
        }

        // Left status label
        using (new EditorGUI.DisabledScope(readOnly))
        {
            Rect labelRect = DrawUtils.GetLabelRect(position);
            Rect iconRect = labelRect;
            iconRect.width = iconContent.image.width + padSize;
            labelRect.width -= iconRect.width;
            labelRect.x += iconRect.width;
            EditorGUI.PrefixLabel(iconRect, sceneControlID, iconContent);
            EditorGUI.PrefixLabel(labelRect, sceneControlID, labelContent);
        }

        // Right context buttons
        Rect buttonRect = DrawUtils.GetFieldRect(position);
        buttonRect.width = (buttonRect.width) / 3;

        string tooltipMsg = "";
        using (new EditorGUI.DisabledScope(readOnly))
        {
            // NOT in build settings
            if (buildScene.buildIndex == -1)
            {
                buttonRect.width *= 2;
                int addIndex = EditorBuildSettings.scenes.Length;
                tooltipMsg = "Add this scene to build settings. It will be appended to the end of the build scenes as buildIndex: " + addIndex + "." + readOnlyWarning;
                if (DrawUtils.ButtonHelper(buttonRect, "Add...", "Add (buildIndex " + addIndex + ")", EditorStyles.miniButtonLeft, tooltipMsg))
                    BuildUtils.AddBuildScene(buildScene);
                buttonRect.width /= 2;
                buttonRect.x += buttonRect.width;
            }
            // In build settings
            else
            {
                bool isEnabled = buildScene.scene.enabled;
                string stateString = isEnabled ? "Disable" : "Enable";
                tooltipMsg = stateString + " this scene in build settings.\n" + (isEnabled ? "It will no longer be included in builds" : "It will be included in builds") + "." + readOnlyWarning;

                if (DrawUtils.ButtonHelper(buttonRect, stateString, stateString + " In Build", EditorStyles.miniButtonLeft, tooltipMsg))
                    BuildUtils.SetBuildSceneState(buildScene, !isEnabled);
                buttonRect.x += buttonRect.width;

                tooltipMsg = "Completely remove this scene from build settings.\nYou will need to add it again for it to be included in builds!" + readOnlyWarning;
                if (DrawUtils.ButtonHelper(buttonRect, "Remove...", "Remove from Build", EditorStyles.miniButtonMid, tooltipMsg))
                    BuildUtils.RemoveBuildScene(buildScene);
            }
        }

        buttonRect.x += buttonRect.width;

        tooltipMsg = "Open the 'Build Settings' Window for managing scenes." + readOnlyWarning;
        if (DrawUtils.ButtonHelper(buttonRect, "Settings", "Build Settings", EditorStyles.miniButtonRight, tooltipMsg))
        {
            BuildUtils.OpenBuildSettings();
        }

    }

    static SerializedProperty GetSceneAssetProperty(SerializedProperty property)
    {
        return property.FindPropertyRelative(sceneAssetPropertyString);
    }

    static SerializedProperty GetScenePathProperty(SerializedProperty property)
    {
        return property.FindPropertyRelative(scenePathPropertyString);
    }

    private static class DrawUtils
    {
        /// <summary>
        /// Draw a GUI button, choosing between a short and a long button text based on if it fits
        /// </summary>
        static public bool ButtonHelper(Rect position, string msgShort, string msgLong, GUIStyle style, string tooltip = null)
        {
            GUIContent content = new GUIContent(msgLong);
            content.tooltip = tooltip;

            float longWidth = style.CalcSize(content).x;
            if (longWidth > position.width)
                content.text = msgShort;

            return GUI.Button(position, content, style);
        }

        /// <summary>
        /// Given a position rect, get its field portion
        /// </summary>
        static public Rect GetFieldRect(Rect position)
        {
            position.width -= EditorGUIUtility.labelWidth;
            position.x += EditorGUIUtility.labelWidth;
            return position;
        }
        /// <summary>
        /// Given a position rect, get its label portion
        /// </summary>
        static public Rect GetLabelRect(Rect position)
        {
            position.width = EditorGUIUtility.labelWidth - padSize;
            return position;
        }
    }

    /// <summary>
    /// Various BuildSettings interactions
    /// </summary>
    static private class BuildUtils
    {
        // time in seconds that we have to wait before we query again when IsReadOnly() is called.
        public static float minCheckWait = 3;

        static float lastTimeChecked = 0;
        static bool cachedReadonlyVal = true;

        /// <summary>
        /// A small container for tracking scene data BuildSettings
        /// </summary>
        public struct BuildScene
        {
            public int buildIndex;
            public GUID assetGUID;
            public string assetPath;
            public EditorBuildSettingsScene scene;
        }

        /// <summary>
        /// Check if the build settings asset is readonly.
        /// Caches value and only queries state a max of every 'minCheckWait' seconds.
        /// </summary>
        static public bool IsReadOnly()
        {
            float curTime = Time.realtimeSinceStartup;
            float timeSinceLastCheck = curTime - lastTimeChecked;

            if (timeSinceLastCheck > minCheckWait)
            {
                lastTimeChecked = curTime;
                cachedReadonlyVal = QueryBuildSettingsStatus();
            }

            return cachedReadonlyVal;
        }

        /// <summary>
        /// A blocking call to the Version Control system to see if the build settings asset is readonly.
        /// Use BuildSettingsIsReadOnly for version that caches the value for better responsivenes.
        /// </summary>
        static private bool QueryBuildSettingsStatus()
        {
            // If no version control provider, assume not readonly
            if (UnityEditor.VersionControl.Provider.enabled == false)
                return false;

            // If we cannot checkout, then assume we are not readonly
            if (UnityEditor.VersionControl.Provider.hasCheckoutSupport == false)
                return false;

            //// If offline (and are using a version control provider that requires checkout) we cannot edit.
            //if (UnityEditor.VersionControl.Provider.onlineState == UnityEditor.VersionControl.OnlineState.Offline)
            //    return true;

            // Try to get status for file
            var status = UnityEditor.VersionControl.Provider.Status("ProjectSettings/EditorBuildSettings.asset", false);
            status.Wait();

            // If no status listed we can edit
            if (status.assetList == null || status.assetList.Count != 1)
                return true;

            // If is checked out, we can edit
            if (status.assetList[0].IsState(UnityEditor.VersionControl.Asset.States.CheckedOutLocal))
                return false;

            return true;
        }

        /// <summary>
        /// For a given Scene Asset object reference, extract its build settings data, including buildIndex.
        /// </summary>
        static public BuildScene GetBuildScene(Object sceneObject)
        {
            BuildScene entry = new BuildScene()
            {
                buildIndex = -1,
                assetGUID = new GUID(string.Empty)
            };

            if (sceneObject as SceneAsset == null)
                return entry;

            entry.assetPath = AssetDatabase.GetAssetPath(sceneObject);
            entry.assetGUID = new GUID(AssetDatabase.AssetPathToGUID(entry.assetPath));

            for (int index = 0; index < EditorBuildSettings.scenes.Length; ++index)
            {
                if (entry.assetGUID.Equals(EditorBuildSettings.scenes[index].guid))
                {
                    entry.scene = EditorBuildSettings.scenes[index];
                    entry.buildIndex = index;
                    return entry;
                }
            }

            return entry;
        }

        /// <summary>
        /// Enable/Disable a given scene in the buildSettings
        /// </summary>
        static public void SetBuildSceneState(BuildScene buildScene, bool enabled)
        {
            bool modified = false;
            EditorBuildSettingsScene[] scenesToModify = EditorBuildSettings.scenes;
            foreach (var curScene in scenesToModify)
            {
                if (curScene.guid.Equals(buildScene.assetGUID))
                {
                    curScene.enabled = enabled;
                    modified = true;
                    break;
                }
            }
            if (modified)
                EditorBuildSettings.scenes = scenesToModify;
        }

        /// <summary>
        /// Display Dialog to add a scene to build settings
        /// </summary>
        static public void AddBuildScene(BuildScene buildScene, bool force = false, bool enabled = true)
        {
            if (force == false)
            {
                int selection = EditorUtility.DisplayDialogComplex(
                    "Add Scene To Build",
                    "You are about to add scene at " + buildScene.assetPath + " To the Build Settings.",
                    "Add as Enabled",       // option 0
                    "Add as Disabled",      // option 1
                    "Cancel (do nothing)"); // option 2

                switch (selection)
                {
                    case 0: // enabled
                        enabled = true;
                        break;
                    case 1: // disabled
                        enabled = false;
                        break;
                    default:
                    case 2: // cancel
                        return;
                }
            }

            EditorBuildSettingsScene newScene = new EditorBuildSettingsScene(buildScene.assetGUID, enabled);
            List<EditorBuildSettingsScene> tempScenes = EditorBuildSettings.scenes.ToList();
            tempScenes.Add(newScene);
            EditorBuildSettings.scenes = tempScenes.ToArray();
        }

        /// <summary>
        /// Display Dialog to remove a scene from build settings (or just disable it)
        /// </summary>
        static public void RemoveBuildScene(BuildScene buildScene, bool force = false)
        {
            bool onlyDisable = false;
            if (force == false)
            {
                int selection = -1;

                string title = "Remove Scene From Build";
                string details = string.Format("You are about to remove the following scene from build settings:\n    {0}\n    buildIndex: {1}\n\n{2}",
                                buildScene.assetPath, buildScene.buildIndex,
                                "This will modify build settings, but the scene asset will remain untouched.");
                string confirm = "Remove From Build";
                string alt = "Just Disable";
                string cancel = "Cancel (do nothing)";

                if (buildScene.scene.enabled)
                {
                    details += "\n\nIf you want, you can also just disable it instead.";
                    selection = EditorUtility.DisplayDialogComplex(title, details, confirm, alt, cancel);
                }
                else
                {
                    selection = EditorUtility.DisplayDialog(title, details, confirm, cancel) ? 0 : 2;
                }

                switch (selection)
                {
                    case 0: // remove
                        break;
                    case 1: // disable
                        onlyDisable = true;
                        break;
                    default:
                    case 2: // cancel
                        return;
                }
            }

            // User chose to not remove, only disable the scene
            if (onlyDisable)
            {
                SetBuildSceneState(buildScene, false);
            }
            // User chose to fully remove the scene from build settings
            else
            {
                List<EditorBuildSettingsScene> tempScenes = EditorBuildSettings.scenes.ToList();
                tempScenes.RemoveAll(scene => scene.guid.Equals(buildScene.assetGUID));
                EditorBuildSettings.scenes = tempScenes.ToArray();
            }
        }

        /// <summary>
        /// Open the default Unity Build Settings window
        /// </summary>
        static public void OpenBuildSettings()
        {
            EditorWindow.GetWindow(typeof(BuildPlayerWindow));
        }
    }
}

#endif

SceneReferenceTest.cs

using UnityEngine;

public class SceneReferenceTest : MonoBehaviour
{
    private void OnGUI()
    {
        DisplayLevel(exampleNull);
        DisplayLevel(exampleMissing);
        DisplayLevel(exampleDisabled);
        DisplayLevel(exampleEnabled);
    }

    public void DisplayLevel(SceneReference scene)
    {
        GUILayout.Label(new GUIContent("Scene name Path: " + scene));
        if (GUILayout.Button("Load " + scene))
        {
            UnityEngine.SceneManagement.SceneManager.LoadScene(scene);
        }
    }

    public SceneReference exampleNull;
    public SceneReference exampleMissing;
    public SceneReference exampleDisabled;
    public SceneReference exampleEnabled;
    
}