Form validation

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    debugShowCheckedModeBanner: false,
    home: FormValidationExample(),
  ));
}

class FormValidationExample extends StatelessWidget with InputValidationMixin {
  final formGlobalKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Form validation example'),
      ),
      body:
      Padding(
        padding: const EdgeInsets.symmetric(horizontal: 24),
        child:Form(
        key: formGlobalKey,
        child: Column(
          children: [
            const SizedBox(height: 50),
            TextFormField(
              decoration: InputDecoration(
                labelText: "Email"
              ),
              validator: (email) {
                if (isEmailValid(email)) return null;
                else
                  return 'Enter a valid email address';
              },
            ),
            const SizedBox(height: 24),
            TextFormField(
              decoration: InputDecoration(
                  labelText: "Password",
              ),
              maxLength: 6,
              obscureText: true,
              validator: (password) {
                if (isPasswordValid(password)) return null;
                else
                  return 'Enter a valid password';
              },
            ),
            const SizedBox(height: 50),
            ElevatedButton(
                onPressed: () {
                  if (formGlobalKey.currentState.validate()) {
                    formGlobalKey.currentState.save();
                    // use the email provided here
                  }
                },
                child: Text("Submit"))
          ],
        ),
      ),
    ));
  }
}

mixin InputValidationMixin {
  bool isPasswordValid(String password) => password.length == 6;

  bool isEmailValid(String email) {
    Pattern pattern =
        r'^(([^<>()[\]\\.,;:\s@"]+(\.[^<>()[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$';
    RegExp regex = new RegExp(pattern);
    return regex.hasMatch(email);
  }
}