Primeiro App Flutter com DartPad

main.dart

import 'package:flutter/material.dart';
final _form = GlobalKey<FormState>();
void _realizarLogin() {
    final isValid = _form.currentState.validate();

    if (isValid) {
      _form.currentState.save();
    }
  }


void main() {
  runApp(
    MaterialApp(
    home: Scaffold(
      appBar: AppBar(
        title: Text(
          'APP Test - Login',
        ),
      ),
      body: Center(
        child: Padding(
          padding: EdgeInsets.all(20),
          child: Form(
            key: _form,
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Container(
                  child: TextFormField(
                    decoration: InputDecoration(labelText: 'Usuário'),
                    validator: (value) {
                      if (value.isEmpty || value == null) {
                        return 'Informar Usuário';
                      }
                      return null;
                    },
                  ),
                ),
                Container(
                  child: TextFormField(
                    decoration: InputDecoration(labelText: 'Senha'),
                    validator: (value) {
                      if (value.isEmpty || value == null) {
                        return 'É necessário informar a senha';
                      } else {
                        if (value.length < 6) {
                          return 'A senha deve conter mais de 6 caracteres';
                        }
                      }
                      return null;
                    },
                  ),
                ),
                Container(
                    child: FlatButton(
                  child: Text('Login'),
                  onPressed: () {
                    _realizarLogin();
                  },
                )),
              ],
            ),
          ),
        ),
      ),
    ),
  ),
  );
  
  }