1. Create a Form with a GlobalKey.

2. Add a TextFormField with validation logic.

3. Create a button to validate and submit the form.

1. Create a Form with a GlobalKey

                final _formKey = GlobalKey<FormState>();

2. Add a TextFormField with validation logic

                TextFormField(
                  // The validator receives the text that the user has entered.
                  validator: (value) {
                    if (value == null || value.isEmpty) {
                      return 'Please enter some text';
                    }
                    return null;
                  },
                ),

                

3. Create a button to validate and submit the form

                ElevatedButton(
                    onPressed: () {
                      // Validate returns true if the form is valid, or false otherwise.
                      if (_formKey.currentState!.validate()) {
                        // If the form is valid, display a snackbar. In the real world,
                        // you'd often call a server or save the information in a database.
                        ScaffoldMessenger.of(context).showSnackBar(
                          const SnackBar(content: Text('Processing Data')),
                        );
                      }
                    },
                    child: const Text('Submit'),
                  ),

Complete code

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    const appTitle = 'Form Validation Demo';

    return MaterialApp(
      title: appTitle,
      home: Scaffold(
        appBar: AppBar(
          title: const Text(appTitle),
        ),
        body: const MyCustomForm(),
      ),
    );
  }
}

// Create a Form widget.
class MyCustomForm extends StatefulWidget {
  const MyCustomForm({super.key});

  @override
  MyCustomFormState createState() {
    return MyCustomFormState();
  }
}

// Create a corresponding State class.
// This class holds data related to the form.
class MyCustomFormState extends State<MyCustomForm> {
  // Create a global key that uniquely identifies the Form widget
  // and allows validation of the form.
  //
  // Note: This is a GlobalKey<FormState>,
  // not a GlobalKey<MyCustomFormState>.
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    // Build a Form widget using the _formKey created above.
    return Form(
      key: _formKey,
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          TextFormField(
            // The validator receives the text that the user has entered.
            validator: (value) {
              if (value == null || value.isEmpty) {
                return 'Please enter some text';
              }
              return null;
            },
          ),
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 16),
            child: ElevatedButton(
              onPressed: () {
                // Validate returns true if the form is valid, or false otherwise.
                if (_formKey.currentState!.validate()) {
                  // If the form is valid, display a snackbar. In the real world,
                  // you'd often call a server or save the information in a database.
                  ScaffoldMessenger.of(context).showSnackBar(
                    const SnackBar(content: Text('Processing Data')),
                  );
                }
              },
              child: const Text('Submit'),
            ),
          ),
        ],
      ),
    );
  }
}