Build a form with validation

form_validation.dart

// TextFormField & Form Validation.
// https://flutter.dev/docs/cookbook/forms/validation
class FormValidation extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final appTitle = 'Form Validation Demo';

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

// Create a Form widget.
class _FormValidationForm extends StatefulWidget {
  @override
  State createState() => _FormValidationState();
}

// Create a corresponding State class.
// This class holds data related to the form.
class _FormValidationState extends State<_FormValidationForm> {
  // 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: <Widget>[
          TextFormField(
            validator: (value) {
              if (value.isEmpty) {
                return 'Please enter some text';
              }
              return null;
            },
          ),
          Padding(
            padding: const EdgeInsets.symmetric(vertical: 16.0),
            child: RaisedButton(
              onPressed: () {
                // Validate returns true if the form is valid, or false
                // otherwise.
                final FormState formState = _formKey.currentState;
                if (formState.validate()) {
                  formState.save();
                  // If the form is valid, display a Snackbar.
                  Scaffold.of(context)
                      .showSnackBar(SnackBar(content: const Text('Processing Data')));
                }
              },
              child: const Text('Submit'),
            ),
          ),
        ],
      ),
    );
  }
}