bottomNavigationBar: BottomNavigationBar(
  type: BottomNavigationBarType.fixed,
  currentIndex: _currentIndex,
  onTap: (index) {
    setState(() {
      _contentPagerController.jumpToPage(index);
      _currentIndex = index;
    });
  },

为什么 _contentPagerController.jumpToPage(index); 必须要写在setState方法外面， 写在里就变得无效了呢