class TestFieldWidgetMultiple

class TestFieldWidgetMultiple extends WidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'test_widget_setting_multiple' => 'dummy test string',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $element['test_widget_setting_multiple'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Field test field widget setting'),
            '#description' => $this->t('A dummy form element to simulate field widget setting.'),
            '#default_value' => $this->getSetting('test_widget_setting_multiple'),
            '#required' => FALSE,
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $summary[] = $this->t('@setting: @value', [
            '@setting' => 'test_widget_setting_multiple',
            '@value' => $this->getSetting('test_widget_setting_multiple'),
        ]);
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $values = [];
        foreach ($items as $item) {
            $values[] = $item->value;
        }
        $element += [
            '#type' => 'textfield',
            '#default_value' => implode(', ', $values),
            '#element_validate' => [
                [
                    static::class,
                    'multipleValidate',
                ],
            ],
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function errorElement(array $element, ConstraintViolationInterface $violation, array $form, FormStateInterface $form_state) {
        return $element;
    }
    
    /**
     * Element validation helper.
     */
    public static function multipleValidate($element, FormStateInterface $form_state) {
        $values = array_map('trim', explode(',', $element['#value']));
        $items = [];
        foreach ($values as $value) {
            $items[] = [
                'value' => $value,
            ];
        }
        $form_state->setValueForElement($element, $items);
    }
    
    /**
     * Test is the widget is applicable to the field definition.
     *
     * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
     *   The field definition that should be checked.
     *
     * @return bool
     *   TRUE if the machine name of the field is not equals to
     *   field_onewidgetfield, FALSE otherwise.
     *
     * @see \Drupal\Tests\field\Functional\EntityReference\EntityReferenceAdminTest::testAvailableFormatters
     */
    public static function isApplicable(FieldDefinitionInterface $field_definition) {
        // Returns FALSE if machine name of the field equals field_onewidgetfield.
        return $field_definition->getName() != "field_onewidgetfield";
    }

}