Validate the input

ieee_validate_input.dart

import 'dart:async';

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    home: Page1(),
  ));
}

class Page1 extends StatefulWidget {
  @override
  _Page1State createState() => _Page1State();
}

class _Page1State extends State<Page1> {
  TextEditingController usernameController = TextEditingController();

  TextEditingController passwordController = TextEditingController();

  bool isBusy = false;

  bool hasError = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Page 1"),
      ),
      body: Center(
        child: Padding(
          padding: const EdgeInsets.all(8.0),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              TextField(
                controller: usernameController,
                decoration: InputDecoration(
                  labelText: "Username",
                  hintText: "Your Username",
                ),
              ),
              TextField(
                controller: passwordController,
                obscureText: true,
                obscuringCharacter: '*',
                decoration: InputDecoration(
                  labelText: "Password",
                  hintText: "Your password",
                ),
              ),
              RaisedButton(
                child: Text("Go to page 2"),
                onPressed: () {
                  setState(() {
                    isBusy = true;
                    hasError = false;
                  });
                  Timer(Duration(seconds: 2), () {
                    var username = usernameController.text;
                    var password = passwordController.text;
                    if (username.isEmpty || password.isEmpty) {
                      setState(() {
                        hasError = true;
                      });
                    }
                    setState(() {
                      isBusy = false;
                    });
                    if (!hasError) {
                      Navigator.of(context).push(
                        MaterialPageRoute(
                          builder: (context) {
                            return Page2();
                          },
                        ),
                      );
                    }
                  });
                },
              ),
              if (isBusy) CircularProgressIndicator(),
              if (hasError) Text('There is an error'),
            ],
          ),
        ),
      ),
    );
  }
}

class Page2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Page 2"),
      ),
      body: Center(
        child: Container(
          child: Column(
            children: [
              RaisedButton(
                child: Text("Go Back"),
                onPressed: () {
                  Navigator.of(context).pop();
                },
              )
            ],
          ),
        ),
      ),
    );
  }
}