TextForm.php

<?php

namespace Drupal\theming_example\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * A simple form that displays a textfield and submit button.
 *
 * This form will be rendered by theme('form') (theme_form() by default)
 * because we do not provide a theme function for it here.
 */
class TextForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'theming_example_form_text';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['text'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Please input something!'),
            '#required' => TRUE,
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Go'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addMessage($this->t('You chose %input', [
            '%input' => $form_state->getValue('text'),
        ]));
    }

}