Dart Learning: Simple Login Screen with StatefulWidget (+StatelessWidget for static)

app.dart

//D:\Projetos\CursoFlutter\FlutterApps\Login\login_stateful\lib\src\app.dart

import 'package:flutter/material.dart';
import 'screens/login_screen.dart';

class App extends StatelessWidget {
  Widget build(context) {
    return MaterialApp(
      title: 'Log Me In!',
      home: Scaffold(
        body: LoginScreen(),
      ),
    );
  }
}

login_screen.dart

//D:\Projetos\CursoFlutter\FlutterApps\Login\login_stateful\lib\main.dart

import 'package:flutter/material.dart';
import '../mixins/validation_mixin.dart';

class LoginScreen extends StatefulWidget {
  createState() {
    return LoginScreenState();
  }
}

class LoginScreenState extends State<LoginScreen> with ValidationMixin {
  final formKey = GlobalKey<FormState>();

  String email = '';
  String password = '';

  Widget build(context) {
    return Container(
      margin: EdgeInsets.all(20.0),
      child: Form(
        key: formKey,
        child: Column(
          children: [
            emailField(),
            passwordField(),
            Container(margin: EdgeInsets.only(top: 25.00)),
            submitButton(),
          ],
        ),
      ),
    );
  }

  Widget emailField() {
    return TextFormField(
      keyboardType: TextInputType.emailAddress,
      decoration: InputDecoration(
        labelText: 'E-mail Adress',
        hintText: 'you@example.com',
      ),
      validator: validateEmail,
      onSaved: (String value) {
        email = value;
      },
    );
  }

  Widget passwordField() {
    return TextFormField(
      obscureText: true,
      decoration: InputDecoration(
        labelText: 'Password',
        hintText: 'Password',
      ),
      validator: validatePassword,
      onSaved: (String value) {
        password = value;
      },
    );
  }

  Widget submitButton() {
    return RaisedButton(
      color: Colors.blue,
      child: Text('Sutmit'),
      onPressed: () {
        if (formKey.currentState.validate()) {
          formKey.currentState.save();
          print('Time to post $email and $password to my API');
        }
      },
    );
  }
}

main.dart

//D:\Projetos\CursoFlutter\FlutterApps\Login\login_stateful\lib\main.dart

import 'package:flutter/material.dart';
import 'src/app.dart';

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

validation_mixin.dart

//D:\Projetos\CursoFlutter\FlutterApps\Login\login_stateful\lib\src\mixins\validation_mixin.dart

import 'package:flutter/material.dart';
import 'src/app.dart';

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