bottomNavigationBar: BottomNavigationBar(
        selectedItemColor: Colors.blue,
        unselectedItemColor: Colors.grey,
        onTap: (index) {
          pageController.jumpToPage(index);
          setState(() {
            _curIndex = index;
          });
        },
	  }

BottomNavigationBar的onTap入参是有index的，这个是从哪里看出来的呢？我点进去看源码也没有相关信息，以后用到不熟悉的方法怎么知道入参呢？

  /// Called when one of the [items] is tapped.
  ///
  /// The stateful widget that creates the bottom navigation bar needs to keep
  /// track of the index of the selected [BottomNavigationBarItem] and call
  /// `setState` to rebuild the bottom navigation bar with the new [currentIndex].
  final ValueChanged<int> onTap;