EditorBase.cs

using System;
using UnityEditor;
using UnityEngine;

namespace Pixeland.Tools.Router.Editor
{
    public abstract class EditorBase : UnityEditor.Editor
    {
        protected void ChangeCheck<T>(T val, Action<T> onChanged) where T : class
        {
            EditorGUI.BeginChangeCheck();
            T newVal;
            if (val is string)
                newVal = EditorGUILayout.TextField(val as string) as T;
            else if (val is SceneAsset)
                newVal = EditorGUILayout.ObjectField(val as SceneAsset, typeof(SceneAsset), false) as T;
            else
                throw new Exception("Unsupported ChangeCheck type: " + val.GetType().Name);

            if (EditorGUI.EndChangeCheck())
            {
                onChanged(newVal);
            }
        }

        protected void Vertical(GUIStyle style, Action<Rect> addContent)
        {
            Rect layout = style != null ? EditorGUILayout.BeginVertical(style) : EditorGUILayout.BeginVertical();
            addContent(layout);
            EditorGUILayout.EndVertical();
        }

        protected void Vertical(Action<Rect> addContent)
        {
            Vertical(null, addContent);
        }

        protected void Horizontal(Action<Rect> addContent)
        {
            Rect layout = EditorGUILayout.BeginHorizontal();
            addContent(layout);
            EditorGUILayout.EndHorizontal();
        }
    }
}

GameRouterEditor.cs

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

namespace Pixeland.Tools.Router.Editor
{
    [CustomEditor(typeof(GameRouter))]
    public class GameRouterEditor : EditorBase
    {
        private GameRouter m_Router;

        private void OnEnable()
        {
            m_Router = target as GameRouter;
        }

        private void DrawRouteItem(int index)
        {
            RouteDefinition route = m_Router.routes[index];

            // Find all routes that have a parent id of this route's id 
            List<RouteDefinition> children = m_Router.routes
            .Where(r => r.parentId == route.id)
            .ToList();

            Horizontal(_ =>
            {
                EditorGUILayout.PrefixLabel("Route:",
                EditorStyles.label,
                EditorStyles.miniBoldLabel);

                if (GUILayout.Button("Delete", EditorStyles.miniButton))
                {
                    Undo.RecordObject(m_Router, "Delete subroute");

                    ArrayUtility.RemoveAt(ref m_Router.routes, index);

                    // Clear out all the children 
                    for (int i = m_Router.routes.Length - 1; i >= 0; i--)
                        if (m_Router.routes[i].parentId == route.id)
                            ArrayUtility.RemoveAt(ref m_Router.routes, i);

                    EditorUtility.SetDirty(m_Router);
                }
            });

            // Get current scene and update it in the inspector... 
            var currentScene = AssetDatabase.LoadAssetAtPath<SceneAsset>(route.scenePath);
            serializedObject.Update();

            // Start drawing our list item  
            Horizontal(_ =>
            {
                // Name field 
                ChangeCheck(route.name, newName =>
                {
                    Undo.RecordObject(m_Router, "Update route name");
                    m_Router.routes[index].name = newName;
                    EditorUtility.SetDirty(m_Router);
                });

                // Scene field
                if (!children.Any())
                {
                    ChangeCheck(currentScene, newScene =>
                    {
                        Undo.RecordObject(m_Router, "Update route scene");
                        string newPath = AssetDatabase.GetAssetPath(newScene);
                        m_Router.routes[index].scenePath = newPath;
                        EditorUtility.SetDirty(m_Router);
                    });
                }

                serializedObject.ApplyModifiedProperties();
            });

            // Child routes... 
            if (children.Any())
            {
                Vertical(new GUIStyle { padding = new RectOffset(16, 0, 16, 0) }, rect =>
                {
                    foreach (RouteDefinition subroute in children)
                    {
                        Horizontal(_ =>
                        {
                            EditorGUILayout.PrefixLabel("Subroute:", EditorStyles.label, EditorStyles.miniBoldLabel);

                            if (GUILayout.Button("Delete", EditorStyles.miniButton))
                            {
                                Undo.RecordObject(m_Router, "Delete subroute");
                                ArrayUtility.RemoveAt(ref m_Router.routes, m_Router.routes.ToList().IndexOf(subroute));
                                EditorUtility.SetDirty(m_Router);
                            }
                        });

                        // Get current scene and update it in the inspector... 
                        var currentChildScene = AssetDatabase.LoadAssetAtPath<SceneAsset>(subroute.scenePath);
                        serializedObject.Update();

                        // Start drawing our list item  
                        Horizontal(_ =>
                        {
                            // Name field 
                            ChangeCheck(subroute.name, subNewName =>
                            {
                                Undo.RecordObject(m_Router, "Update subroute name");
                                subroute.name = subNewName;
                                EditorUtility.SetDirty(m_Router);
                            });

                            // Scene field 
                            ChangeCheck(currentChildScene, newSubScene =>
                            {
                                Undo.RecordObject(m_Router, "Update route scene");
                                string newPath = AssetDatabase.GetAssetPath(newSubScene);
                                subroute.scenePath = newPath;
                                EditorUtility.SetDirty(m_Router);
                            });

                            serializedObject.ApplyModifiedProperties();
                        });

                        EditorGUILayout.Space();
                    }
                });
            }

            Horizontal(_r =>
            {
                if (GUILayout.Button("+ subroute", EditorStyles.miniButton))
                {
                    Undo.RecordObject(m_Router, "Add subroute");
                    ArrayUtility.Add(ref m_Router.routes, new RouteDefinition
                    {
                        id = m_Router.routes.Max(r => r.id) + 1,
                        name = "subroute",
                        parentId = route.id
                    });
                    EditorUtility.SetDirty(m_Router);
                }
            });
        }

        public override void OnInspectorGUI()
        {
            serializedObject.Update();

            Vertical(_ =>
            {
                for (var i = 0; i < m_Router.routes.Length; i++)
                {
                    // If this is a subroute, don't render it (yet) 
                    if (m_Router.routes[i].parentId >= 0)
                        continue;

                    DrawRouteItem(i);

                    // Draw separator 
                    Vertical(sep =>
                    {
                        EditorGUI.DrawRect(new Rect(sep.x, sep.y += EditorGUIUtility.standardVerticalSpacing, sep.width, 1.0f), Color.grey);
                        EditorGUILayout.Space();
                    });
                }
            });

            Horizontal(_ =>
            {
                if (GUILayout.Button("+ route", EditorStyles.miniButton))
                {
                    Undo.RecordObject(m_Router, "Add route");
                    ArrayUtility.Add(ref m_Router.routes, new RouteDefinition
                    {
                        id = m_Router.routes.Any() ? m_Router.routes.Max(r => r.id) + 1 : 0,
                        name = "my-route"
                    });
                    EditorUtility.SetDirty(m_Router);
                }
            });

            serializedObject.ApplyModifiedProperties();
        }
    }
}