class BooleanCheckboxWidget

class BooleanCheckboxWidget extends WidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'display_label' => TRUE,
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $element['display_label'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Use field label instead of the "On" label as the label.'),
            '#default_value' => $this->getSetting('display_label'),
            '#weight' => -1,
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $display_label = $this->getSetting('display_label');
        $summary[] = $this->t('Use field label: @display_label', [
            '@display_label' => $display_label ? $this->t('Yes') : $this->t('No'),
        ]);
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $element['value'] = $element + [
            '#type' => 'checkbox',
            '#default_value' => !empty($items[0]->value),
        ];
        // Override the title from the incoming $element.
        if ($this->getSetting('display_label')) {
            $element['value']['#title'] = $this->fieldDefinition
                ->getLabel();
        }
        else {
            $element['value']['#title'] = $this->fieldDefinition
                ->getSetting('on_label');
        }
        return $element;
    }

}