TextField validation made easy with TextEditingController and AnimatedBuilder

text-editing-controller-animated-builder.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.indigo,
      ),
      home: const SubmitPage(),
    );
  }
}

class SubmitPage extends StatelessWidget {
  const SubmitPage({Key? key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Validation')),
      body: Padding(
        padding: const EdgeInsets.all(24),
        child: TextSubmitWidget(onSubmit: (value) => print(value)),
      ),
    );
  }
}

class TextSubmitWidget extends StatefulWidget {
  const TextSubmitWidget({Key? key, required this.onSubmit}) : super(key: key);
  final ValueChanged<String> onSubmit;

  @override
  State<TextSubmitWidget> createState() => _TextSubmitWidgetState();
}

class _TextSubmitWidgetState extends State<TextSubmitWidget> {
  final _controller = TextEditingController();

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  String? get errorText {
    final text = _controller.value.text;
    if (text.isEmpty) {
      return 'Can\'t be empty';
    }
    if (text.length < 4) {
      return 'Too short';
    }
    return null;
  }

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: _controller,
      builder: (context, _) {
        return Column(
          mainAxisSize: MainAxisSize.min,
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: [
            TextField(
              controller: _controller,
              decoration: InputDecoration(
                labelText: 'Enter your name',
                errorText: errorText,
              ),
            ),
            ElevatedButton(
              onPressed: errorText == null
                  ? () => widget.onSubmit(_controller.value.text)
                  : null,
              child: Text(
                'Submit',
                style: Theme.of(context)
                    .textTheme
                    .headline6!
                    .copyWith(color: Colors.white),
              ),
            )
          ],
        );
      },
    );
  }
}