class FormTestObject

class FormTestObject extends ConfigFormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_form_test_object';
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'form_test.object',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['element'] = [
            '#markup' => 'The FormTestObject::buildForm() method was used for this form.',
        ];
        $form['bananas'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Bananas'),
        ];
        $form['strawberry'] = [
            '#type' => 'hidden',
            '#value' => 'red',
            '#attributes' => [
                'id' => 'red-strawberry-hidden-field',
            ],
        ];
        $form['actions']['#type'] = 'actions';
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Save'),
        ];
        $form['#title'] = 'Test dynamic title';
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addStatus($this->t('The FormTestObject::validateForm() method was used for this form.'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $this->messenger()
            ->addStatus($this->t('The FormTestObject::submitForm() method was used for this form.'));
        $this->config('form_test.object')
            ->set('bananas', $form_state->getValue('bananas'))
            ->save();
    }

}