Flutter app for JWT tutorial

baseConfig.dart

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'package:flutter_secure_storage/flutter_secure_storage.dart';
import 'dart:convert' show json, base64, ascii;

const SERVER_IP = 'http://192.168.1.167:5000';
final storage = FlutterSecureStorage();

void main() {
  runApp(MyApp());
}

build.gradle

// ...
android {
    // ...
    defaultConfig {
        // ...
        minSdkVersion 18
        // ...
    }
    // ...
}
// ...

HomePage.dart

class HomePage extends StatelessWidget {
  HomePage(this.jwt, this.payload);
  
  factory HomePage.fromBase64(String jwt) =>
    HomePage(
      jwt,
      json.decode(
        ascii.decode(
          base64.decode(base64.normalize(jwt.split(".")[1]))
        )
      )
    );

  final String jwt;
  final Map<String, dynamic> payload;

  @override
  Widget build(BuildContext context) =>
    Scaffold(
      appBar: AppBar(title: Text("Secret Data Screen")),
      body: Center(
        child: FutureBuilder(
          future: http.read('$SERVER_IP/data', headers: {"Authorization": jwt}),
          builder: (context, snapshot) =>
            snapshot.hasData ?
            Column(children: <Widget>[
              Text("${payload['username']}, here's the data:"),
              Text(snapshot.data, style: Theme.of(context).textTheme.display1)
            ],)
            :
            snapshot.hasError ? Text("An error occurred") : CircularProgressIndicator()
        ),
      ),
    );
}

LoginPage.dart

class LoginPage extends StatelessWidget {
  final TextEditingController _usernameController = TextEditingController();
  final TextEditingController _passwordController = TextEditingController();

  void displayDialog(context, title, text) => showDialog(
      context: context,
      builder: (context) =>
        AlertDialog(
          title: Text(title),
          content: Text(text)
        ),
    );

  Future<String> attemptLogIn(String username, String password) async {
    var res = await http.post(
      "$SERVER_IP/login",
      body: {
        "username": username,
        "password": password
      }
    );
    if(res.statusCode == 200) return res.body;
    return null;
  }

  Future<int> attemptSignUp(String username, String password) async {
    var res = await http.post(
      '$SERVER_IP/signup',
      body: {
        "username": username,
        "password": password
      }
    );
    return res.statusCode;
    
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Log In"),),
      body: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Column(
          children: <Widget>[
            TextField(
              controller: _usernameController,
              decoration: InputDecoration(
                labelText: 'Username'
              ),
            ),
            TextField(
              controller: _passwordController,
              obscureText: true,
              decoration: InputDecoration(
                labelText: 'Password'
              ),
            ),
            FlatButton(
              onPressed: () async {
                var username = _usernameController.text;
                var password = _passwordController.text;
                var jwt = await attemptLogIn(username, password);
                if(jwt != null) {
                  storage.write(key: "jwt", value: jwt);
                  Navigator.push(
                    context,
                    MaterialPageRoute(
                      builder: (context) => HomePage.fromBase64(jwt)
                    )
                  );
                } else {
                  displayDialog(context, "An Error Occurred", "No account was found matching that username and password");
                }
              },
              child: Text("Log In")
            ),
            FlatButton(
              onPressed: () async {
                var username = _usernameController.text;
                var password = _passwordController.text;

                if(username.length < 4) 
                  displayDialog(context, "Invalid Username", "The username should be at least 4 characters long");
                else if(password.length < 4) 
                  displayDialog(context, "Invalid Password", "The password should be at least 4 characters long");
                else{
                  var res = await attemptSignUp(username, password);
                  if(res == 201)
                    displayDialog(context, "Success", "The user was created. Log in now.");
                  else if(res == 409)
                    displayDialog(context, "That username is already registered", "Please try to sign up using another username or log in if you already have an account.");  
                  else {
                    displayDialog(context, "Error", "An unknown error occurred.");
                  }
                }
              },
              child: Text("Sign Up")
            )
          ],
        ),
      )
    );
  }
}

MyApp.dart

class MyApp extends StatelessWidget {
  Future<String> get jwtOrEmpty async {
    var jwt = await storage.read(key: "jwt");
    if(jwt == null) return "";
    return jwt;
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Authentication Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: FutureBuilder(
        future: jwtOrEmpty,            
        builder: (context, snapshot) {
          if(!snapshot.hasData) return CircularProgressIndicator();
          if(snapshot.data != "") {
            var str = snapshot.data;
            var jwt = str.split(".");

            if(jwt.length !=3) {
              return LoginPage();
            } else {
              var payload = json.decode(ascii.decode(base64.decode(base64.normalize(jwt[1]))));
              if(DateTime.fromMillisecondsSinceEpoch(payload["exp"]*1000).isAfter(DateTime.now())) {
                return HomePage(str, payload);
              } else {
                return LoginPage();
              }
            }
          } else {
            return LoginPage();
          }
        }
      ),
    );
  }
}

pubspec.yaml

dependencies:
  flutter:
    sdk: flutter
  http: ^0.12.0+4
  flutter_secure_storage: ^3.3.1+1
  cupertino_icons: ^0.1.2

how user will logout its not mentioned