class TextTrimmedFormatter

class TextTrimmedFormatter extends FormatterBase implements TrustedCallbackInterface {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'trim_length' => '600',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $element['trim_length'] = [
            '#title' => $this->t('Trimmed limit'),
            '#type' => 'number',
            '#field_suffix' => $this->t('characters'),
            '#default_value' => $this->getSetting('trim_length'),
            '#description' => $this->t('If the summary is not set, the trimmed %label field will end at the last full sentence before this character limit.', [
                '%label' => $this->fieldDefinition
                    ->getLabel(),
            ]),
            '#min' => 1,
            '#required' => TRUE,
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $summary[] = $this->t('Trimmed limit: @trim_length characters', [
            '@trim_length' => $this->getSetting('trim_length'),
        ]);
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        $render_as_summary = function (&$element) {
            // Make sure any default #pre_render callbacks are set on the element,
            // because text_pre_render_summary() must run last.
            $element += \Drupal::service('element_info')->getInfo($element['#type']);
            // Add the #pre_render callback that renders the text into a summary.
            $element['#pre_render'][] = [
                TextTrimmedFormatter::class,
                'preRenderSummary',
            ];
            // Pass on the trim length to the #pre_render callback via a property.
            $element['#text_summary_trim_length'] = $this->getSetting('trim_length');
        };
        // The ProcessedText element already handles cache context & tag bubbling.
        // @see \Drupal\filter\Element\ProcessedText::preRenderText()
        foreach ($items as $delta => $item) {
            $elements[$delta] = [
                '#type' => 'processed_text',
                '#text' => NULL,
                '#format' => $item->format,
                '#langcode' => $item->getLangcode(),
            ];
            if ($this->getPluginId() == 'text_summary_or_trimmed' && !empty($item->summary)) {
                $elements[$delta]['#text'] = $item->summary;
            }
            else {
                $elements[$delta]['#text'] = $item->value;
                $render_as_summary($elements[$delta]);
            }
        }
        return $elements;
    }
    
    /**
     * Pre-render callback: Renders a processed text element's #markup as a summary.
     *
     * @param array $element
     *   A structured array with the following key-value pairs:
     *   - #markup: the filtered text (as filtered by filter_pre_render_text())
     *   - #format: containing the machine name of the filter format to be used to
     *     filter the text. Defaults to the fallback format. See
     *     filter_fallback_format().
     *   - #text_summary_trim_length: the desired character length of the summary
     *     (used by text_summary())
     *
     * @return array
     *   The passed-in element with the filtered text in '#markup' trimmed.
     *
     * @see filter_pre_render_text()
     * @see text_summary()
     */
    public static function preRenderSummary(array $element) {
        $element['#markup'] = text_summary($element['#markup'], $element['#format'], $element['#text_summary_trim_length']);
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function trustedCallbacks() {
        return [
            'preRenderSummary',
        ];
    }

}