How to modify parent FloatingActionButton on flutter?

Question

I am making an app in which I have to pages, routed by a BottomNavigationBar, and each page has its own nested routes. The app contains a main FloatingActionButton centered and docked into the BottomNavigationBar. The problem is that I wanted to control what the FAB does from inside each nested route. The problem is that if I create a FAB inside each nested route the button will not be docked into the BottomNavigationBar. Bellow are some pictures. Anyone can help? Thanks in advance.

What I have :

What I wanted :

Answer 1

Is this what you are asking? If not please share some code which casing the problem

UPDATE

 Scaffold(
      appBar: AppBar(
        title: Text('Test'),
      ),
      body: _screenList[_screenIndex],
      bottomNavigationBar: BottomNavigationBar(
          type: BottomNavigationBarType.fixed,
          currentIndex: _screenIndex,
          onTap: (index) {
            print(index);
            setState(() {
              _screenIndex = index;
            });
          },
          items: [
            BottomNavigationBarItem(
              icon: Icon(Icons.inbox),
              title: Text('Screen1'),
            ),
            BottomNavigationBarItem(
              icon: Icon(Icons.search),
              title: Text('Screen2'),
            ),
          ]),
      floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
      floatingActionButton: FloatingActionButton(
        child: Icon(
          Icons.android,
        ),
        onPressed: () {
          _screenIndex == 0
              ? print('hello from screen1')
              : print('hello from screen2');
        },
      ),
    );

OUTPUT