How to use FilterProcessResult::createPlaceholder (#lazy_builder) in an input filter?

Question

The FilterProcessResult::createPlaceholder method suggests that I can set a PHP callable that will allow me to return a render array. However my callable is not being called when visiting a node using Drupal 8.0.x.

I do not see any error messages in watchdog or in the PHP log.

What should I be doing in order to get FilterProcessResult::createPlaceholder() to call my callable?

Code

  public function process($text, $langcode) {
    $matches = [];
    $result = new FilterProcessResult($text);

    $count = preg_match_all('/\[processing\](?P<code>.*)\[\/processing\]/is', $text, $matches);

    if ($count) {
      $i = 0;
      foreach ($matches['code'] as $i => $code) {
        $args = [$code, $this->settings['render_mode']];
        $markup = $result->createPlaceholder('\Drupal\processing\Plugin\Filter\FilterProcessing::build', $args);
        $text = preg_replace('/\[processing\](?P<code>.*)\[\/processing\]/is', $markup, $text);
      }
      $result
        ->setProcessedText($text)
        ->setAttachments(['library' => ['processing/drupalprocessing']])
        ->setCacheTags(['processing']);
    }

        return $result;
      }

  static public function build($script, $mode) {
    $id = uniqid('processing-');
    $canvas_classes = ['processing-js-canvas', 'processing__canvas'];
    $render = [
      '#type' => 'container',
      '#attributes' => [
        'class' => ['processing-wrapper']
      ],
      'canvas' => [
        '#theme' => 'processing_display',
        '#rendermode' => $mode,
        '#unique' => $id,
        '#code' => $script,
        '#attributes' => [
          'class' => $canvas_classes,
          'id' => $id,
        ],
        '#attached' => [
          'drupalSettings' => [
            'processing' => [
              'drupal.processing' => [
                'elementId' => $id,
                'renderMode' => $mode,
              ]
            ]
          ]
        ],
      ],
    ];
    return $render;
  }

See commit 9d84e9477515bce1b7dd15f9724da1c6c0bd0fc2

HTML Output

<drupal-filter-placeholder callback="\Drupal\processing\Plugin\Filter\FilterProcessing::build" arguments="0=%0A%20%20%20%20%20%20%20%20size%28200%2C%20200%29%3B%0A%20%20%20%20%20%20%20%20background%28200%29%3B%0A%20%20%20%20%20%20&amp;1=source" token="bd5dbeba"></drupal-filter-placeholder>

I am running a settings.local.php with cache disabled on a local VM, but I got the same result without it.

Answer 1

After several months, I looked at the code again and found the issue. The createPlaceholder method calls addAttachments, but then I call setAttachments, which removes the previous attachment.

I should always use addAttachments as that will merge instead of overwrite.

foreach ($matches['code'] as $i => $code) {
  $args = [$code, $this->settings['render_mode']];
  $markup = $result->createPlaceholder('\Drupal\processing\Plugin\Filter\FilterProcessing::build', $args);
  $text = preg_replace('/\[processing\](?P<code>.*)\[\/processing\]/is', $markup, $text);
}
$result
  ->setProcessedText($text)
  ->addAttachments(['library' => ['processing/drupalprocessing']])
  ->setCacheTags(['processing']);