A Construction Script is a system to help generate procedural Gameobjects at Edit-Time in a Unity scene.

ConstructionScript.cs

using System.Linq;
using UnityEngine;
#if UNITY_EDITOR
using System;
using UnityEditor;

#endif

namespace Global.Scripts.ConstructionScript
{
    /// <summary>
    ///     A Construction Script is a system to help generate procedural Gameobjects at Edit-Time in a Unity scene. It does
    ///     three main things:
    ///     <para>1. Generates a child gameobject wrapped in a <see cref="HideFlags.DontSave" /> gameobject.</para>
    ///     <para>
    ///         2. Runs a set of <see cref="IOnConstruct" /> scripts on this generated object, whenever this component's
    ///         gameobject updates in Edit mode.
    ///     </para>
    ///     <para>3. On Standalone build, bakes this game object (after running construct scripts) into the scene file.</para>
    /// </summary>
    /// <remarks>
    ///     There are two main types: <see cref="ConstructionScript" /> and <see cref="IOnConstruct" />. You can either
    ///     extend this class, implementing IOnConstruct, or add a IOnConstruct script on the child (within the prefab). Often
    ///     you may want to extend this script to store properties in the scene, but have a separate IOnConstruct script
    ///     running on the prefab to actually modify the gameobject. Having a separate IOnConstruct script allows that script
    ///     to directly reference objects within the prefab.
    /// </remarks>
    [ExecuteInEditMode]
    public class ConstructionScript : MonoBehaviour
    {
        public GameObject Child
        {
            get
            {
                // In standalone, we need to hook up the baked child object, as it's not generated at runtime.
#if !UNITY_EDITOR
                if (child == null)
                {
                    child = transform.GetChild(0).GetChild(0).gameObject;
                }
#endif
                return child;
            }
        }

        /// <summary>The Prefab to use as the base for generation, before running construction scripts.</summary>
        [Header("Construction Script")]
        [Tooltip("This prefab will be spawned when the child is regenerated.")]
        public GameObject ConstructedPrefab;

        /// <summary>
        ///     Stores a reference to the generated object. This object is not marked "DontSave", because marking it as such
        ///     keeps you from applying prefab changes in the editor. Instead we create an additional "DontSave" gameobject as the
        ///     parent of this object, which correctly lets you apply prefab changes.
        /// </summary>
        private GameObject child;

        // The vast majority of code in this class only acts in the editor. At build time, we bake the generated
        // objects directly into the scene files.
#if UNITY_EDITOR
        /// <summary>
        ///     The immediate child object of this gameobject. In the editor, it is marked with
        ///     <see cref="HideFlags.DontSave" /> so that it is not serialized into the scene.
        /// </summary>
        private GameObject dontSave;

        /// <summary>The previous value of ToSpawn in the editor.</summary>
        private GameObject prevToSpawn;

        private void Awake()
        {
            // When a script is ExecuteInEditMode Awake is called in Editor as soon the component is created
            // or when the scene is loaded. However, in edit mode Awake, we aren't able to do normal scene
            // changes like creating gameobjects. Because of this, we have to recreate the prefab async, 
            // doing the actual operation on the next Editor GUI update frame.

            // However, when Unity is building the player, it reloads all scenes (calling Awake here), 
            // and oddly, normal scene changes *are* allowed, so we're able to recreate the child immediately.
            // This is lucky, because it allows us to save the generated GameObject into the scene at build time.
            RecreateChild(!BuildPipeline.isBuildingPlayer);
        }

        private void RecreateChild(bool async)
        {
            if (async)
            {
                RunOnceDelayed(RecreateChildImmediate);
            }
            else
            {
                RecreateChildImmediate();
            }
        }

        /// <summary>Re-creates the generated child object. This can only be done on the Unity main thread.</summary>
        /// <remarks>
        ///     There are certain phases of the Unity editor (OnValidate, [ExecuteInEditMode]Awake) that do not allow scene
        ///     modifications. We can't run this function during those phases.
        /// </remarks>
        private void RecreateChildImmediate()
        {
            if (dontSave != null)
            {
                DestroyImmediate(dontSave);
            }

            dontSave = new GameObject("(Generated)");

            dontSave.transform.parent = transform;
            dontSave.transform.localPosition = Vector3.zero;
            dontSave.transform.localRotation = Quaternion.identity;
            dontSave.transform.localScale = Vector3.one;

            // Don't save the object that's created, in the editor.
            // The only time we save the object is during the standalone build
            // process, such that the constructed object is packaged into the scene file of the build.
            if (!BuildPipeline.isBuildingPlayer)
            {
                dontSave.hideFlags = HideFlags.DontSave;
            }

            if (ConstructedPrefab != null)
            {
                // This should only happen in the editor, when creating a new component of this type.
                child = (GameObject) PrefabUtility.InstantiatePrefab(ConstructedPrefab, dontSave.transform);
            }
            else
            {
                if (BuildPipeline.isBuildingPlayer)
                {
                    child = new GameObject(gameObject.name);
                    child.transform.parent = dontSave.transform;
                }
                else
                {
                    child = new GameObject(gameObject.name);
                    child.transform.parent = dontSave.transform;
                }
            }

            RunConstruction();
        }

        /// <summary>Called when the inspector is modified.</summary>
        private void OnValidate()
        {
            if (ConstructedPrefab != prevToSpawn)
            {
                // Can only modify the scene on the main thread.
                // OnValidate isn't allowed to run any scene modifications.
                RecreateChild(true);
            }

            prevToSpawn = ConstructedPrefab;
        }

        /// <summary>Runs a given action on the next editor gui update.</summary>
        private void RunOnceDelayed(Action action)
        {
            void OnDelayCall()
            {
                action();
                EditorApplication.delayCall -= OnDelayCall;
            }

            EditorApplication.delayCall += OnDelayCall;
        }

        /// <summary>Runs in the editor whenever this gameobject is modified, such as dragging it or changing its properties.</summary>
        private void Update()
        {
            if (Application.isPlaying)
            {
                return;
            }

            if (child == null)
            {
                return;
            }

            // Skip updating the construction if we have the child selected.
            if (Selection.activeGameObject == null
                || !Selection.activeGameObject.transform.IsChildOf(Child.transform)
                || Selection.activeGameObject == Child)
            {
                RunConstruction();
            }
        }

        /// <summary>
        ///     Runs all construction scripts on this object and on the child. While this script controls the lifecycle of the
        ///     generated object, the Construction scripts actually setup the generated object and do the modification.
        /// </summary>
        private void RunConstruction()
        {
            // Any scripts on this object or on the child.
            var constructionScripts = child.GetComponents<IOnConstruct>().Concat(GetComponents<IOnConstruct>());

            foreach (IOnConstruct onConstruct in constructionScripts)
            {
                onConstruct.OnConstruct();
            }
        }

        /// <summary>When this component is destroyed, remove the generated object.</summary>
        private void OnDestroy()
        {
            if (child != null)
            {
                DestroyImmediate(dontSave);
            }
        }
#endif
    }
}

IOnConstruct.cs

namespace Global.Scripts.ConstructionScript
{
    /// <summary>
    ///     Implement this interface, and place the component on a <see cref="ConstructionScript" /> gameobject or Child,
    ///     and this script will be run whenever the generated object is updated.
    /// </summary>
    public interface IOnConstruct
    {
        /// <summary>Called when the generated object is updated.</summary>
        void OnConstruct();
    }
}