SpriteImporterAtlasRebuild.cs

using System.Collections.Generic;
using System.Linq;
using UnityEditor;
using UnityEditor.U2D;
using UnityEngine;

namespace Editor.Assets
{
    public class SpriteImporterAtlasRebuild : AssetPostprocessor
    {
        static void OnPostprocessAllAssets(string[] importedAssets, string[] deletedAssets, string[] movedAssets, string[] movedFromAssetPaths)
        {
            foreach (var i in importedAssets)
            {
                if (IsSprite(i, out var sprite))
                {
                    UnityEditor.Sprites.Packer.GetAtlasDataForSprite(sprite, out var atlasName, out var tex);
                    if (IsAtlased(sprite, out var atlas))
                    {
                        UnityEditor.U2D.SpriteAtlasUtility.PackAtlases(new[] { atlas }, BuildTarget.StandaloneWindows);
                    }
                }
            }
        }

        static bool IsSprite(string path, out Sprite sprite)
        {
            sprite = AssetDatabase.LoadAssetAtPath<Sprite>(path);
            return sprite != null;
        }

        static bool IsAtlased(Sprite sprite, out UnityEngine.U2D.SpriteAtlas atlas)
        {
            var spritePath = AssetDatabase.GetAssetPath(sprite.GetInstanceID());
            foreach (var assetPath in AssetDatabase.FindAssets("t:SpriteAtlas").Select(p => AssetDatabase.GUIDToAssetPath(p)))
            {
                var at = AssetDatabase.LoadAssetAtPath<UnityEngine.U2D.SpriteAtlas>(assetPath);
                var packed = new List<Object>(at.GetPackables());
                
                foreach( var p in packed)
                {
                    if(p == sprite)
                    {
                        atlas = at;
                        return true;
                    } else if (p is DefaultAsset da)
                    {
                        //look for folder refs!
                        var folderPath = AssetDatabase.GetAssetPath(p);
                        if(spritePath.Contains(folderPath))
                        {
                            atlas = at;
                            return true;
                        }
                    }
                }
            }

            atlas = null;
            return false;
        }

    }
}