com.misaki.art-tools/Editor/AssetsHelpers/Implementation/Meshes/ExtractMaterials/ExtractMaterialsProcessor.cs

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

namespace Misaki.ArtToolEditor
{
    internal class ExtractMaterialsProcessor : IAssetsProcessor
    {
        private const string Material_Extension = ".mat";

        public bool useMaterialRemap = false;
        public ModelImporterMaterialLocation materialLocation = ModelImporterMaterialLocation.InPrefab;
        public ModelImporterMaterialName materialRemapNamingOption = ModelImporterMaterialName.BasedOnMaterialName;
        public ModelImporterMaterialSearch materialRemapSearchOption = ModelImporterMaterialSearch.RecursiveUp;

        public void OnPreProcess(AssetsProcessContext context)
        {
            context.userData = new HashSet<string>();
        }

        public void OnProcess(Object source, string outputDirectory, AssetsProcessContext context)
        {
            var assetsToReload = (HashSet<string>)context.userData;

            var assetImporter = AssetImporter.GetAtPath(AssetDatabase.GetAssetPath(source));
            if (assetImporter is not ModelImporter modelImporter)
            {
                return;
            }

            if (useMaterialRemap)
            {
                modelImporter.materialLocation = materialLocation;
                modelImporter.SearchAndRemapMaterials(materialRemapNamingOption, materialRemapSearchOption);
                modelImporter.SaveAndReimport();

                return;
            }
            else
            {
                var materials = AssetDatabase.LoadAllAssetsAtPath(modelImporter.assetPath).Where(x => x.GetType() == typeof(Material));

                foreach (var material in materials)
                {
                    var newAssetPath = Path.Combine(outputDirectory, material.name) + Material_Extension;
                    newAssetPath = AssetDatabase.GenerateUniqueAssetPath(newAssetPath);

                    var error = AssetDatabase.ExtractAsset(material, newAssetPath);
                    if (string.IsNullOrEmpty(error))
                    {
                        assetsToReload.Add(modelImporter.assetPath);
                    }
                }
            }
        }

        public void OnPostProcess(AssetsProcessContext context)
        {
            var assetsToReload = (HashSet<string>)context.userData;

            foreach (var assetPath in assetsToReload)
            {
                if (string.IsNullOrEmpty(assetPath))
                {
                    return;
                }

                AssetDatabase.WriteImportSettingsIfDirty(assetPath);
                AssetDatabase.ImportAsset(assetPath, ImportAssetOptions.ForceUpdate);
            }
        }
    }
}