MyAssemblyWindow.cs

using UnityEngine;
using UnityEditor;
using UnityEngine.Timeline;
using UnityEngine.Playables;
using System.Collections.Generic;
using UnityEditor.Timeline;

public class MyAssemblyWindow : EditorWindow
{
    [MenuItem("Window/Alan Tools/Assembly")]
    public static void ShowWindow()
    {
        // Find or create window so it appears on the screen.
        EditorWindow.GetWindow<MyAssemblyWindow>("Assembly");
    }

    private enum Positioning { InScene, AtCamera }
    private enum Rotation { FacingCamera, SameAsCamera, AsIs }
    private enum Gravity { Float, OnGround }

    void OnGUI()
    {
        AddCategory("Characters", Positioning.InScene, Rotation.FacingCamera, Gravity.OnGround);
        AddCategory("Props", Positioning.InScene, Rotation.AsIs, Gravity.OnGround);
        AddCategory("Lighting", Positioning.InScene, Rotation.AsIs, Gravity.Float);
        AddCategory("Photography", Positioning.AtCamera, Rotation.SameAsCamera, Gravity.Float);
    }

    private void AddCategory(string name, Positioning positioning, Rotation rotation, Gravity gravity)
    {
        var assets = AssetDatabase.FindAssets("t:Prefab", new[] { "Assets/_SHARED/Assembly Prefabs/" + name, "Assets/_LOCAL/Assembly Prefabs/" + name });
        if (assets.Length == 0)
        {
            return;
        }

        var buttonStyle = EditorStyles.miniButton;

        GUILayout.Label(name, EditorStyles.boldLabel);

        var inHoriz = false;
        int i = 0;

        foreach (var guid in assets)
        {
            var path = AssetDatabase.GUIDToAssetPath(guid);

            // Get a clean name for the button (removing "Prefab Variant" style endings).
            var prefabName = path.Substring(path.LastIndexOf('/') + 1);
            prefabName = prefabName.Remove(prefabName.LastIndexOf('.'));
            if (prefabName.EndsWith(" Variant"))
            {
                prefabName = prefabName.Remove(prefabName.LastIndexOf(' '));
            }
            if (prefabName.EndsWith(" Prefab"))
            {
                prefabName = prefabName.Remove(prefabName.LastIndexOf(' '));
            }

            // Putting 4 per line because cannot work out how to flow them nicely to pack as many as will fit per line.
            // TODO: try https://docs.unity3d.com/ScriptReference/EditorGUIUtility.GetFlowLayoutedRects.html
            if (!inHoriz)
            {
                GUILayout.BeginHorizontal("box");
                inHoriz = true;
            }
            if (GUILayout.Button(prefabName, buttonStyle))
            {
                AddPrefabInstance(prefabName, path, positioning, rotation, gravity);
            }
            if ((i % 4) == 3)
            {
                GUILayout.EndHorizontal();
                inHoriz = false;
            }
            i++;
        }
        if (inHoriz)
        {
            GUILayout.EndHorizontal();
        }
    }

    /*
     * Add prefab instance 
     */
    private void AddPrefabInstance(string name, string path, Positioning positioning, Rotation rotation, Gravity gravity)
    {
        bool refreshNeeded = false;

        // Normally only one sequence would be selected, but just in case...
        foreach (GameObject obj in Selection.gameObjects)
        {
            // Don't add if a sequence not currently selected
            // (User can end up dropping things in wrong spots by accident)
            PlayableDirector director = obj.GetComponent<PlayableDirector>();
            if (director == null)
            {
                Debug.LogWarning("Not dropping instance as current selection is not a sequence node");
            }
            else
            {
                // Don't add the same character multiple times
                // (This might be a bad idea - e.g. might want multiple instances of a prop)
                //var child = obj.transform.Find(name);
                //if (child == null)
                {
                    // Does not already exist, lets try and add it.
                    var prefab = (GameObject)AssetDatabase.LoadAssetAtPath(path, typeof(GameObject));
                    if (prefab == null)
                    {
                        Debug.LogError("Could not load '" + name + "' at path '" + path + "'.");
                    }
                    else
                    {
                        // Add prefab game object as child.
                        var inst = PrefabUtility.InstantiatePrefab(prefab) as GameObject;
                        Undo.RecordObject(obj.transform, "Add " + name + " to sequence");
                        inst.transform.parent = obj.transform;
                        refreshNeeded = true;

                        // Move new instance to where camera is (rather than starting at 0,0)
                        var sceneView = UnityEditor.SceneView.lastActiveSceneView;
                        if (sceneView != null)
                        {
                            if (positioning == Positioning.InScene)
                            {
                                //inst.transform.position = sceneView.pivot;
                                inst.transform.position = sceneView.camera.transform.position + 4f * sceneView.camera.transform.forward;
                            }
                            else
                            {
                                inst.transform.position = sceneView.camera.transform.position;
                            }

                            if (rotation != Rotation.AsIs)
                            {
                                // Default it to look at the scene camera
                                var offset = (rotation == Rotation.FacingCamera) ? 180f : 0f;
                                inst.transform.rotation = Quaternion.Euler(0f, sceneView.rotation.eulerAngles.y + offset, 0f);
                            }

                            // Try to make it land on ground so don't have to get the Y value correct manually
                            // https://answers.unity.com/questions/39203/instantiate-object-and-align-with-object-surface.html
                            if (gravity == Gravity.OnGround)
                            {
                                RaycastHit hit;
                                var down = new Vector3(0, -1, 0);
                                if (Physics.Raycast(inst.transform.position, down, out hit))
                                {
                                    // if too far away, its probably best to leave at middle of scene view
                                    var distanceToGround = hit.distance;
                                    if (distanceToGround < 3f)
                                    {
                                        var currentPos = inst.transform.position;
                                        var newY = currentPos.y - distanceToGround;
                                        inst.transform.position = new Vector3(currentPos.x, newY, currentPos.z);
                                    }
                                }
                            }
                        }

                        // If we find the Timeline, add an animation track to the timeline as well
                        // if the new object has an Animator component (otherwise we cannot animate it)
                        TimelineAsset timeline = (TimelineAsset) director.playableAsset;
                        if (timeline != null && inst.GetComponent<Animator>() != null)
                        {
                            AnimationTrack track = timeline.CreateTrack<AnimationTrack>(null, name);
                            director.SetGenericBinding(track, inst); 
                        }
                    }
                }
            }
        }
        if (refreshNeeded)
        {
            TimelineEditor.Refresh(RefreshReason.ContentsAddedOrRemoved);
        }
    }
}