Flutter DefaultTabController 获取当前索引

final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
List<TabModel> _tabs = [];  // 动态选项卡

  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
      length: _tabs.length,
      child: Scaffold(
          key: _scaffoldKey,
          ...
      )
  )}

DefaultTabController.of(_scaffoldKey.currentState.context).index;