Simple Firebase Login Flow in Flutter

We will create a simple application with the following components

• Default Main App Entry Point
  • Use of FutureBuilder Widget to wait for data before rendering UI, concept used throughout the app
• Login Page
• Home Page
• Authentication Service
  • Demonstrate the use of the Provider as discussed here in the Flutter Documentation Simple App State Management

There are plenty of examples online about setting up Firebase for Flutter so I will jump right into the code instead of walking thru the basics.

See Google CodeLabs Flutter for Firebase for step by step instructions for setting up you project on iOS or Android

Create a Test User in Firebase

Since we are just building the application and there is no functionalty to create users in the application right now, please login to you Firebase Console and add an user to your project. Please be sure to enable email authentication when updating the project in your Firebase Console.

Cleaning Up the Default Flutter Project

first lets create the project

flutter create simple_firebase_auth

Now lets do some project cleanup, open up the project and delete the existing HomePage and HomePageState widget from the file main.dart.

Change the home property of the MaterialApp widget to point to the LoginPage widget we are about to create in the next section

The file should look similar to this when completed

import 'package:flutter/material.dart';
import 'package:simple_firebase_auth/login_page.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: LoginPage(),
    );
  }
}

Create the LoginPage Widget

Lets walk through the creation of the LoginPage for the application. We need capture an email and a password to pass to the AuthService to call the login function.

We are going to create a simple page with the required TextFormField widgets and one RaisedButton to click to make the login happen.

1. Open your editor and create a new file in the lib directory named login_page.dart
2. Paste the contents below into the file login_page.dart

import 'package:flutter/material.dart';

class LoginPage extends StatefulWidget {
  @override
  _LoginPageState createState() => _LoginPageState();
}

class _LoginPageState extends State<LoginPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Login Page Flutter Firebase"),
      ),
      body: Center(
        child: Text('Login Page Flutter Firebase  Content'),
      ),
    );
  }
}

you should be able to run the code to see what the screen looks like now. Be sure to change the default route or home property in main.dart widget to LoginPage while we work through the UI so you can see the changes with live reload

Style and Adding Text Fields

Lets make the body of the page a centered Column with the childre of the column being primarily the TextFormFields and the RaisedButton

the centered container to hold the form fields and buttons

    body: Container(
      padding: EdgeInsets.all(20.0),
      child: Column()
    )

Next add the actual form field widgets and the buttons as children of the Column widget. We will do some basic styling of the form fields so that this looks presentable. See the Flutter documentation for more information on TextFormFields

  body: Container(
    padding: EdgeInsets.all(20.0),
    child: Column(
      children: <Widget>[
        Text(
          'Login Information',
          style: TextStyle(fontSize: 20),
        ),
        TextFormField(
            keyboardType: TextInputType.emailAddress,
            decoration: InputDecoration(labelText: "Email Address")),
        TextFormField(
            obscureText: true,
            decoration: InputDecoration(labelText: "Password")),
        RaisedButton(child: Text("LOGIN"), onPressed: () {}),
      ],
    ),
  ),

Lets add some spacing between the fields in the column so it is more presentable. We are going to use the SizedBox widget and set the height property to get some spacing in the application. Replace the children property of the Column widget to get the desired spacing

  children: <Widget>[
    SizedBox(height: 20.0),    // <= NEW
    Text(
      'Login Information',
      style: TextStyle(fontSize: 20),
    ),
    SizedBox(height: 20.0),   // <= NEW
    TextFormField(
        keyboardType: TextInputType.emailAddress,
        decoration: InputDecoration(labelText: "Email Address")),
    TextFormField(
        obscureText: true,
        decoration: InputDecoration(labelText: "Password")),
    SizedBox(height: 20.0),  // <= NEW
    RaisedButton(child: Text("LOGIN"), onPressed: () {}),
  ],

Getting Text Values from Form Fields

We are going to be using a Form widget and a GlobalKey, additional information on these concepts can be found in the flutter cookbook section Building a form with validation

Add the formKey in the LoginPage widget

class _LoginPageState extends State<LoginPage> {
  final _formKey = GlobalKey<FormState>();

Then add two new fields to hold the email address and password values we will need to send to Firebase for authentication

class _LoginPageState extends State<LoginPage> {
  final _formKey = GlobalKey<FormState>();
  String _password;
  String _email;

Next we add a property onSaved to the TextFormFields we have for email and password, when the save method is called on the form, all of the widgets onSaved methods will be called to update the local variables.

  TextFormField(
      onSaved: (value) => _email = value,    // <= NEW
      keyboardType: TextInputType.emailAddress,
      decoration: InputDecoration(labelText: "Email Address")),
  TextFormField(
      onSaved: (value) => _password = value, // <= NEW
      obscureText: true,
      decoration: InputDecoration(labelText: "Password")),

Wrap the Column Widget with a new Form Widget, the code should look similar to this

      body: Container(
        padding: EdgeInsets.all(20.0),
        child: Form(          // <= NEW
          key: _formKey,      // <= NEW
          child: Column(
            children: <Widget>[
            ....
            ],
          ),
        ),
      ),            

Now that the fields are set, the TextFormField are updated, we can using the _formKey to not only validate the fields provided, but to also get the values locally by calling the save method.

Replace the code in the RaisedButton onPressed method to the following, and you will see that we are getting the values for email and password set in out widget. We can now pass these values to the AuthService that wraps the Firebase signin functionality.

    // save the fields..
    final form = _formKey.currentState;
    form.save();

    // Validate will return true if is valid, or false if invalid.
    if (form.validate()) {
      print("$_email $_password");
    }

Create the HomePage Widget

For now, we will keep the home page simple since we are just trying to demonstrate how the flow works. Ignore the commented out LogoutButton widget, we will discuss that in a later section of the tutorial.

1. Open your editor and create a new file in the lib directory named home_page.dart
2. Paste the contents below into the file home_page.dart

import 'package:flutter/material.dart';

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return  Scaffold(
      appBar: AppBar(
        title: Text("Home Flutter Firebase"),
        //actions: <Widget>[LogoutButton()],
      ),
      body: Center(
        child: Text('Home Page Flutter Firebase  Content'),
      ),
    );
  }
}

1. Open main.dart and add the following import statement

import 'home_page.dart';

1. Change the home property from this:

home: HomePage(title: 'Flutter Demo Home Page'),

       to this so you can verify that the page is working properly

home: HomePage(),

Creating a Template for An Authentication Service

Here we will build out the authentication service separate from Firebase, validate that everything works and then integrate Firebase.

In this service, we are using a mixin called ChangeNotifier and a method notifyListeners this will allow for the widgets that are using this Service to be updated when the method is called. We are calling notifyListeners when we update the currentUser property because that means that the user has either logged in or logged out and we want the application to update based on the users state.

More information on Provider and State Management can be found here in the Flutter Documentation

What we need as a baseline is the following:

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

class AuthService with ChangeNotifier {
  var currentUser;

  AuthService() {
    print("new AuthService");
  }

  Future getUser() {
    return Future.value(currentUser);
  }

  // wrappinhg the firebase calls
  Future logout() {
    this.currentUser = null;
    notifyListeners();
    return Future.value(currentUser);
  }

  // wrapping the firebase calls
  Future createUser(
      {String firstName,
      String lastName,
      String email,
      String password}) async {}

  // logs in the user if password matches
  Future loginUser({String email, String password}) {
    if (password == 'password123') {
      this.currentUser = {'email': email};
      notifyListeners();
      return Future.value(currentUser);
    } else {
      this.currentUser = null;
      return Future.value(null);
    }
  }
}

We will keep a local property in the service call currentUser which is the object storing the user, when the user calls the login method, if the password matches we will set currentUser and the user will be logged in. This will now provide a user when the call is made to getUser method. For logging the user out, we will set the currentUserproperty to null indicating that we are no longer logged into the system.

Determining User State On Application Launch

The first challenge when working with the application is to determine which page to open when the application starts up. What we want to do here is determine if we have a user or not. We will be using an AuthService we will created above combined with the FutureBuilder widget from flutter to render the correct first page of either a HomePage or a LoginPage

Using the Provider

In main.dart we will need to update the default main method to look like this; we are wrapping the whole application with the ChangeNotifierProvider to get the ability to scan up the widget tree and find an object of type AuthService.

void main() => runApp(
      ChangeNotifierProvider<AuthService>(
        child: MyApp(),
        builder: (BuildContext context) {
          return AuthService();
        },
      ),
    );

Modifying the MyApp Widget

Go into the main.dart and make the following changes that will allow the MyApp Widget to set the route. This widget will determine if the application should navigate to the HomePage widget or LoginPage widget when the app is launched.

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(primarySwatch: Colors.blue),
      home: FutureBuilder(
        // get the Provider, and call the getUser method
        future: Provider.of<AuthService>(context).getUser(),
        // wait for the future to resolve and render the appropriate
        // widget for HomePage or LoginPage
        builder: (context, AsyncSnapshot snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            return snapshot.hasData ? HomePage() : LoginPage();
          } else {
            return Container(color: Colors.white);
          }
        },
      ),
    );
  }
}

End of Part One

At this point you should have a functioning application with the basic login flow where the user will be logged in successfully if you provide the password as password123

I added in a little extra functionality of displaying an error message in a dialog box if some sort of error occurs.

Aaron Saunders — CEO Clearly Innovative Inc, #Entrepreneur, #Developer #EcosystemBuilder #TheFutureIsWrittenInCode