Unity WebGL FileSystem Sync

FileSystem.cs

using AOT;
using System;
using System.Collections.Generic;

namespace WebGL
{
    public static class FileSystem
    {
#if UNITY_WEBGL && !UNITY_EDITOR
        [DllImport("__Internal")]
        static extern void FileSystemSyncfsAddEvent(Action<int, string> action);
        [DllImport("__Internal")]
        static extern void FileSystemSyncfs(int id);
#else

        static Action<int, string> _callback;
        static void FileSystemSyncfsAddEvent(Action<int, string> action) { _callback = action; }
        static void FileSystemSyncfs(int id) { _callback?.Invoke(id, ""); }
#endif


        static Dictionary<int, Action<string>> callbacks = new Dictionary<int, Action<string>>();

        /// <summary>
        /// 静的コンストラクタ
        /// 同期完了時に呼び出し内部コールバックを登録する
        /// </summary>
        static FileSystem()
        {
            FileSystemSyncfsAddEvent(Callback);
        }

        /// <summary>
        /// 同期完了時の内部コールバック
        /// </summary>
        /// <param name="id"></param>
        [MonoPInvokeCallback(typeof(Action<int, string>))]
        static void Callback(int id, string error)
        {
            var cb = callbacks[id];
            callbacks.Remove(id);
            cb?.Invoke(string.IsNullOrEmpty(error) ? null : error);
        }

        /// <summary>
        /// IndexedDBを同期させる
        /// </summary>
        /// <param name="cb">string : error</param>
        public static void Syncfs(Action<string> cb)
        {
            var id = callbacks.Count + 1;
            callbacks.Add(id, cb);
            FileSystemSyncfs(id);
        }
    }
}

FileSystem.jslib

//=====================================================
// JavaScript FileSystem Plugin for UnityWebGL
//=====================================================

var LibraryFileSystem = {
    $FileSystem: {
        callback: null,
    },

    // Add Event Callback
    FileSystemSyncfsAddEvent: function (cb) {
        FileSystem.callback = cb;
    },

	// Sync
	FileSystemSyncfs:function(id) {
		FS.syncfs(function (err) {
			var value = allocate(intArrayFromString(err?err:""), 'i8', ALLOC_NORMAL);
			Runtime.dynCall('vii', FileSystem.callback, [id, value]);
		});
	},
};
autoAddDeps(LibraryFileSystem, '$FileSystem');
mergeInto(LibraryManager.library, LibraryFileSystem);

Sample.cs

using UnityEngine;

public class Sample : MonoBehaviour
{
    public void Syncfs()
    {
        WebGL.FileSystem.Syncfs((err) =>
        {
            if (string.IsNullOrEmpty(err))
            {
                Debug.Log("OK!!");
            } else
            {
                Debug.Log("NG!!");
            }
        });
    }
}