Simple navigator + bottom nav bar example

login.dart

import 'package:flutter/material.dart';

class LoginScreen extends StatefulWidget {
  @override
  _LoginScreenState createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: RaisedButton(
          child: Text("login"),
          onPressed: () => Navigator.of(context).pushNamed("home"),
        ),
      ),
    );
  }
}

main.dart

import 'package:flutter/material.dart';
import 'login.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: LoginScreen(),
      routes: {
        "home": (_) => MyHomePage(),
      },
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _currentIndex = 0;

  _handleTap(int index) => setState(() => _currentIndex = index);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Home"),
      ),
      body: Center(
        child: HomeView(currentView: CurrentView.values[_currentIndex]),
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        onTap: _handleTap,
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.access_alarm),
            title: Text("A"),
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.backspace),
            title: Text("B"),
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.cached),
            title: Text("C"),
          ),
        ],
      ),
    );
  }
}

enum CurrentView {
  a,
  b,
  c,
}

class HomeView extends StatefulWidget {
  final CurrentView currentView;
  HomeView({this.currentView});

  @override
  _HomeViewState createState() => _HomeViewState();
}

class _HomeViewState extends State<HomeView> {
  @override
  Widget build(BuildContext context) {
    switch (widget.currentView) {
      case CurrentView.a:
        return Text("View A!");
      case CurrentView.b:
        return Text("View B!");
        break;
      case CurrentView.c:
        return Text("View C!");
        break;
      default:
        return Text("Default!");
    }
  }
}