flutter 双击返回退出APP

DateTime mLastDateTime;

Future<bool> _doubleExit() {
    var nowDateTime = DateTime.now();
    if (mLastDateTime != null &&
        nowDateTime.millisecondsSinceEpoch -
                mLastDateTime.millisecondsSinceEpoch <
            2000) {
        return Future.value(true);
    } else {
        mLastDateTime = DateTime.now();
        Utils.toast.info('再按一次退出');
        return Future.value(false);
    }
}

@override
Widget build(BuildContext context) {
return WillPopScope(
    onWillPop: _doubleExit,
    child: Scaffold(
        body: renderBody(),
        bottomNavigationBar: renderBottomNavigationBar(),
    ));
}