private void drawImportExportGUI()
    {
        GUIStyle btnStyle = new GUIStyle(GUI.skin.GetStyle("Button"))
        {
            fixedWidth = 100,
            alignment  = TextAnchor.MiddleCenter
        };

        EditorGUILayout.BeginHorizontal();
        GUILayout.Label("Import from: ", EditorStyles.label);
        drawBtn("Google Sheet", btnStyle, ImportToggle.GoogleSheet, () =>
        {
            // download excel from google sheet

            Action <string> callback = (string path) =>
            {
                // parse excel file
                ImportFromExcel(path);
            };

            WebTool.DownloadXLSX(GameConstant.GoogleSheetId, callback, fileName);
        });
        drawBtn("Excel", btnStyle, ImportToggle.Excel, () => {
            string path = EditorUtility.OpenFilePanel("Open Excel File", GameConstant.ExcelSourceFolder, "xlsx");
            if (path.Length != 0)
            {
                ImportFromExcel(path);
            }
        });
        drawBtn("Json", btnStyle, ImportToggle.Json, () =>
        {
            string path = EditorUtility.OpenFilePanel("Open Json File", GameConstant.JsonSourceFolder, "json");
            if (path.Length != 0)
            {
                ImportFromJson(path);
            }
        });
        EditorGUILayout.EndHorizontal();

        EditorGUILayout.BeginHorizontal();
        GUILayout.Label("Export to: ", EditorStyles.label);
        drawBtn("Google Sheet", btnStyle, ExportToggle.GoogleSheet);
        drawBtn("Excel", btnStyle, ExportToggle.Excel, () => {
            var path = EditorUtility.SaveFilePanel("Save Excel File", GameConstant.ExcelSourceFolder, string.Format("{0}.{1}", fileName, "xlsx"), "xlsx");
            if (path.Length != 0)
            {
                ExportToExcel(path);
            }
        });
        drawBtn("Json", btnStyle, ExportToggle.Json, () =>
        {
            var path = EditorUtility.SaveFilePanel("Save Json File", GameConstant.JsonSourceFolder, string.Format("{0}.{1}", fileName, "json"), "json");
            if (path.Length != 0)
            {
                ExportToJson(path);
            }
        });
        EditorGUILayout.EndHorizontal();
    }