Login Form demo - https://dartpad.dartlang.org/8d7dd37aaab21bf827787fc88890aefa

flutter_login_form.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Login Form Demo'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  FocusNode _emailFieldFocusNode = FocusNode();
  FocusNode _passwordFieldFocusNode = FocusNode();
  GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  String _email;
  String _password;
  bool _tryLogin = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Container(
              width: 300,
              child: _buildLoginForm(),
            ),
          ],
        ),
      ),
    );
  }

  Form _buildLoginForm() {
    return Form(
      key: _formKey,
      autovalidate: true,
      child: Column(
        children: <Widget>[
          _buildEmailFormField(),
          _buildPasswordFormField(),
          _tryLogin
              ? Container(
                  padding: EdgeInsets.symmetric(vertical: 6.0),
                  child: CircularProgressIndicator())
              : Container(
                  child: RaisedButton(
                    child: Text('Login', style: TextStyle(color: Colors.white)),
                    onPressed: _login,
                    color: Theme.of(context).primaryColor,
                  ),
                ),
        ],
      ),
    );
  }

  _buildEmailFormField() {
    return TextFormField(
      focusNode: _emailFieldFocusNode,
      textInputAction: TextInputAction.next,
      decoration: InputDecoration(hintText: 'Email'),
      validator: (value) {
        if (value.isEmpty) {
          return 'Please insert email';
        } else
          return null;
      },
      onFieldSubmitted: (String value) {
        _emailFieldFocusNode.unfocus();
        FocusScope.of(context).requestFocus(_passwordFieldFocusNode);
      },
      onSaved: (String value) {
        setState(() {
          this._email = value;
        });
      },
    );
  }

  _buildPasswordFormField() {
    return TextFormField(
      focusNode: _passwordFieldFocusNode,
      textInputAction: TextInputAction.done,
      obscureText: true,
      decoration: InputDecoration(hintText: 'Password'),
      validator: (value) {
        if (value.isEmpty) {
          return 'Please insert password';
        } else
          return null;
      },
      onFieldSubmitted: (String value) {
        _passwordFieldFocusNode.unfocus();
      },
      onSaved: (String value) {
        setState(() {
          this._password = value;
        });
      },
    );
  }

  _login() async {
    if (!_formKey.currentState.validate()) {
      return;
    }

    _formKey.currentState.save();
    setState(() {
      this._tryLogin = true;
    });
    await _requestLogin(
      email: this._email,
      password: this._password,
    );
    setState(() {
      this._tryLogin = false;
    });
  }

  Future<bool> _requestLogin({String email, String password}) {
    return Future.delayed(Duration(seconds: 2), () {
      return true;
    });
  }
}