class FormTestValidateForm

class FormTestValidateForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_validate_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $object = new Callbacks();
        $form['name'] = [
            '#type' => 'textfield',
            '#title' => 'Name',
            '#default_value' => '',
            '#element_validate' => [
                [
                    $object,
                    'validateName',
                ],
            ],
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => 'Save',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        if ($form_state->getValue('name') == 'validate') {
            // Alter the form element.
            $form['name']['#value'] = '#value changed by #validate';
            // Alter the submitted value in $form_state.
            $form_state->setValueForElement($form['name'], 'value changed by setValueForElement() in #validate');
            // Output the element's value from $form_state.
            $this->messenger()
                ->addStatus($this->t('@label value: @value', [
                '@label' => $form['name']['#title'],
                '@value' => $form_state->getValue('name'),
            ]));
            // Trigger a form validation error to see our changes.
            $form_state->setErrorByName('');
            // To simplify this test, enable form caching and use form storage to
            // remember our alteration.
            $form_state->setCached();
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }

}