class _MyNavigationBarState extends State<MyNavigationBar > {
  int _currentTabIndex = 0;

  @override
  Widget build(BuildContext context) {
    final _kTabPages = <Widget>[
      Page1(),
      Page2(),
      Page3(),
      Container(),
    ];
    final _kBottmonNavBarItems = <BottomNavigationBarItem>[
      const BottomNavigationBarItem(icon: Icon(Icons.home), label: 'Home'),
      const BottomNavigationBarItem(icon: Icon(Icons.network_cell), label: 'Prices'),
      const BottomNavigationBarItem(icon: Icon(Icons.add_circle), label: 'Trade'),
      const BottomNavigationBarItem(icon: Icon(Icons.account_balance_wallet), label: 'Wallet'),
    ];
    assert(_kTabPages.length == _kBottmonNavBarItems.length);
    final bottomNavBar = BottomNavigationBar(
      items: _kBottmonNavBarItems,
      currentIndex: _currentTabIndex,
      type: BottomNavigationBarType.fixed,
      onTap: (int index) {
        if(index == 3){
          showBottomSheet();
          return;
        }

        setState(() {
          _currentTabIndex = index;
        });
      },
    );
    return Scaffold(
      body: _kTabPages[_currentTabIndex],
      bottomNavigationBar: bottomNavBar,
      ),
    );
  }
}

showBottomSheet(){
      Container _buildBottomSheet(BuildContext context) {
    return Container(
      height: 300,
      padding: const EdgeInsets.all(8.0),
      decoration: BoxDecoration(
        border: Border.all(color: Colors.blue, width: 2.0),
        borderRadius: BorderRadius.circular(8.0),
      ),
      child: ListView(
        children: <Widget>[
          const ListTile(title: Text('Bottom sheet')),
          const TextField(
            keyboardType: TextInputType.number,
            decoration: InputDecoration(
              border: OutlineInputBorder(),
              icon: Icon(Icons.attach_money),
              labelText: 'Enter an integer',
            ),
          ),
          Container(
            alignment: Alignment.center,
            child: ElevatedButton.icon(
              icon: const Icon(Icons.save),
              label: const Text('Save and close'),
              onPressed: () => Navigator.pop(context),
            ),
          )
        ],
      ),
    );
  }
}