再按一次退出页面

    var _first = 0;

    // 双击退出页面
    Future<bool> _requestPop() {
      if (_first != 0 && new DateTime.now().millisecondsSinceEpoch - _first < 1000) {
        return new Future<bool>.value(true);
      } else {
        _first = new DateTime.now().millisecondsSinceEpoch;
        Fluttertoast.showToast(
            msg: "再按一次退出页面",
            toastLength: Toast.LENGTH_SHORT,
            timeInSecForIos: 1,
            gravity: ToastGravity.BOTTOM,
            bgcolor: "#99000000",
            textcolor: '#ffffff');
        new Timer(new Duration(milliseconds: 1000), () {
          _first = 0;
        });
        return new Future<bool>.value(false);
      }
    }