trail-into-darkness/Assets/Code/Util/Editor/SceneReferenceEditor.cs

using UnityEditor;
using UnityEngine;

namespace Nox.Core
{
    [CustomEditor(typeof(SceneReference))]
    public class SceneReferenceEditor : Editor {
        private SceneReference sceneReference;

        private SerializedProperty gameState;
        private SerializedProperty playMode;

        private void OnEnable() {
            sceneReference = (SceneReference)target;
            playMode = serializedObject.FindProperty("playMode");
            gameState = serializedObject.FindProperty("gameState");
        }

        public override void OnInspectorGUI() {
            EditorGUILayout.PropertyField(gameState, new GUIContent("Game Active State"));
            if(sceneReference.gameState == GameState.GameMode) {
                EditorGUILayout.PropertyField(playMode, new GUIContent("Play Mode"));
            }

            serializedObject.ApplyModifiedProperties();
        }
    }
}