A example of using firebase authentication in a flutter application.

firebase_authentication_example.dart

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart';

void main() => runApp(new MaterialApp(
      title: "Auth Demo",
      home: Home(),
    ));

final FirebaseAuth _auth = FirebaseAuth.instance;

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  TextEditingController emailController = new TextEditingController();
  TextEditingController passwordController = new TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Container();
  }

  @override
  void initState() {
    super.initState();
    getUser().then((user) {
      if (user != null) {
        // send the user to the home page
        // homePage();
      }
    });
  }

  Future<FirebaseUser> getUser() async {
    return await _auth.currentUser();
  }

  void signUpWithEmail() async {
    // marked async
    FirebaseUser user;
    try {
      user = await _auth.createUserWithEmailAndPassword(
        email: emailController.text,
        password: passwordController.text,
      );
    } catch (e) {
      print(e.toString());
    } finally {
      if (user != null) {
        // sign in successful!
        // ex: bring the user to the home page
      } else {
        // sign in unsuccessful
        // ex: prompt the user to try again
      }
    }
  }

  void signInWithEmail() async {
    // marked async
    FirebaseUser user;
    try {
      user = await _auth.signInWithEmailAndPassword(
          email: emailController.text, password: passwordController.text);
    } catch (e) {
      print(e.toString());
    } finally {
      if (user != null) {
        // sign in successful!
        // ex: bring the user to the home page
      } else {
        // sign in unsuccessful
        // ex: prompt the user to try again
      }
    }
  }
}