Unity Group Selected GameObjects Command

GroupSelected.cs

/* GroupSelected.cs
 * © Eddie Cameron 2020
 * ----------------------------
 */
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

/// <summary>
/// Groups all selected scene transforms under a single parent.
/// Select GameObject/Group Selected Objects or use Ctrl/Cmd+Shift+G
/// </summary>
public static class GroupSelected {
    [MenuItem( "GameObject/Group Selected Objects %#G", isValidateFunction: true )]
    public static bool CreateGroupValidate() {
        return Selection.GetTransforms( SelectionMode.Editable | SelectionMode.ExcludePrefab | SelectionMode.OnlyUserModifiable | SelectionMode.TopLevel ).Length > 0;
    }

    [MenuItem( "GameObject/Group Selected Objects %#G")]
    public static void CreateGroup() {
        var transforms = Selection.GetTransforms( SelectionMode.Editable | SelectionMode.ExcludePrefab | SelectionMode.OnlyUserModifiable | SelectionMode.TopLevel );
        if ( transforms.Length == 0 )
            return;
        Vector3 avgPos = Vector3.zero;
        foreach ( var t in transforms ) {
            avgPos += t.position;
        }
        avgPos /= transforms.Length;

        Transform commonParent = FindLowestCommonAncestor( transforms );
        Transform newParent = new GameObject( "Group" ).transform;
        newParent.SetParent( commonParent );
        newParent.position = avgPos;
        Undo.RegisterCreatedObjectUndo( newParent.gameObject, "Create new parent" );
        foreach ( var t in transforms ) {
            Undo.SetTransformParent( t, newParent, "Set Parent" );
        }

        Selection.activeGameObject = newParent.gameObject;
    }

    public static Transform FindLowestCommonAncestor( IList<Transform> transforms ) {
        if ( transforms.Count == 0 )
            return null;
        if ( transforms.Count == 1 )
            return transforms[0];

        // get full heirarcy of first transform in list
        var fullHeirarchy = new List<Transform>();
        Transform parent = transforms[0].parent;
        while ( parent != null ) {
            fullHeirarchy.Add( parent );
            parent = parent.parent;
        }

        int highestCommonIdx = 0;
        for ( int i = 1; i < transforms.Count; i++ ) {
            parent = transforms[i].parent;
            bool foundCommonParent = false;
            while ( parent != null ) {
                int commonIdx = fullHeirarchy.IndexOf( parent );
                if ( commonIdx >= 0 ) {
                    // found a common parent
                    highestCommonIdx = Mathf.Max( commonIdx, highestCommonIdx );
                    foundCommonParent = true;
                    break;
                }
                parent = parent.parent;
            }

            if ( !foundCommonParent ) {
                // no common parent
                return null;
            }
        }

        return fullHeirarchy[highestCommonIdx];
    }
}