class TimestampAgoFormatter

class TimestampAgoFormatter extends FormatterBase {
    
    /**
     * The date formatter service.
     *
     * @var \Drupal\Core\Datetime\DateFormatterInterface
     */
    protected $dateFormatter;
    
    /**
     * The current Request object.
     *
     * @var \Symfony\Component\HttpFoundation\Request
     */
    protected $request;
    
    /**
     * Constructs a TimestampAgoFormatter object.
     *
     * @param string $plugin_id
     *   The plugin ID for the formatter.
     * @param mixed $plugin_definition
     *   The plugin implementation definition.
     * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
     *   The definition of the field to which the formatter is associated.
     * @param array $settings
     *   The formatter settings.
     * @param string $label
     *   The formatter label display setting.
     * @param string $view_mode
     *   The view mode.
     * @param array $third_party_settings
     *   Any third party settings.
     * @param \Drupal\Core\Datetime\DateFormatterInterface $date_formatter
     *   The date formatter service.
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The current request.
     */
    public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, DateFormatterInterface $date_formatter, Request $request) {
        parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings);
        $this->dateFormatter = $date_formatter;
        $this->request = $request;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
        // @see \Drupal\Core\Field\FormatterPluginManager::createInstance().
        return new static($plugin_id, $plugin_definition, $configuration['field_definition'], $configuration['settings'], $configuration['label'], $configuration['view_mode'], $configuration['third_party_settings'], $container->get('date.formatter'), $container->get('request_stack')
            ->getCurrentRequest());
    }
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'future_format' => '@interval hence',
            'past_format' => '@interval ago',
            'granularity' => 2,
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $form = parent::settingsForm($form, $form_state);
        $form['future_format'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Future format'),
            '#default_value' => $this->getSetting('future_format'),
            '#required' => TRUE,
            '#description' => $this->t('Use <em>@interval</em> where you want the formatted interval text to appear.'),
        ];
        $form['past_format'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Past format'),
            '#default_value' => $this->getSetting('past_format'),
            '#required' => TRUE,
            '#description' => $this->t('Use <em>@interval</em> where you want the formatted interval text to appear.'),
        ];
        $form['granularity'] = [
            '#type' => 'number',
            '#title' => $this->t('Granularity'),
            '#description' => $this->t('How many time interval units should be shown in the formatted output.'),
            '#default_value' => $this->getSetting('granularity') ?: 2,
            '#min' => 1,
            '#max' => 7,
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = parent::settingsSummary();
        $future_date = new DrupalDateTime('1 year 1 month 1 week 1 day 1 hour 1 minute 1 second');
        $past_date = new DrupalDateTime('-1 year -1 month -1 week -1 day -1 hour -1 minute -1 second');
        $granularity = $this->getSetting('granularity');
        $options = [
            'granularity' => $granularity,
            'return_as_object' => FALSE,
        ];
        $future_date_interval = new FormattableMarkup($this->getSetting('future_format'), [
            '@interval' => $this->dateFormatter
                ->formatTimeDiffUntil($future_date->getTimestamp(), $options),
        ]);
        $past_date_interval = new FormattableMarkup($this->getSetting('past_format'), [
            '@interval' => $this->dateFormatter
                ->formatTimeDiffSince($past_date->getTimestamp(), $options),
        ]);
        $summary[] = $this->t('Future date: %display', [
            '%display' => $future_date_interval,
        ]);
        $summary[] = $this->t('Past date: %display', [
            '%display' => $past_date_interval,
        ]);
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            if ($item->value) {
                $updated = $this->formatTimestamp($item->value);
            }
            else {
                $updated = [
                    '#markup' => $this->t('never'),
                ];
            }
            $elements[$delta] = $updated;
        }
        return $elements;
    }
    
    /**
     * Formats a timestamp.
     *
     * @param int $timestamp
     *   A UNIX timestamp to format.
     *
     * @return array
     *   The formatted timestamp string using the past or future format setting.
     */
    protected function formatTimestamp($timestamp) {
        $granularity = $this->getSetting('granularity');
        $options = [
            'granularity' => $granularity,
            'return_as_object' => TRUE,
        ];
        if ($this->request->server
            ->get('REQUEST_TIME') > $timestamp) {
            $result = $this->dateFormatter
                ->formatTimeDiffSince($timestamp, $options);
            $build = [
                '#markup' => new FormattableMarkup($this->getSetting('past_format'), [
                    '@interval' => $result->getString(),
                ]),
            ];
        }
        else {
            $result = $this->dateFormatter
                ->formatTimeDiffUntil($timestamp, $options);
            $build = [
                '#markup' => new FormattableMarkup($this->getSetting('future_format'), [
                    '@interval' => $result->getString(),
                ]),
            ];
        }
        CacheableMetadata::createFromObject($result)->applyTo($build);
        return $build;
    }

}