BlockViewBuilder::preRender

public static BlockViewBuilder::preRender($build)

public static function preRender($build) {
  $content = $build['#block']->getPlugin()->build();
  // Remove the block entity from the render array, to ensure that blocks
  // can be rendered without the block config entity.
  unset($build['#block']);
  if ($content !== NULL && !Element::isEmpty($content)) {
    // Place the $content returned by the block plugin into a 'content' child
    // element, as a way to allow the plugin to have complete control of its
    // properties and rendering (for instance, its own #theme) without
    // conflicting with the properties used above, or alternate ones used by
    // alternate block rendering approaches in contrib (for instance, Panels).
    // However, the use of a child element is an implementation detail of this
    // particular block rendering approach. Semantically, the content returned
    // by the plugin "is the" block, and in particular, #attributes and
    // #contextual_links is information about the *entire* block. Therefore,
    // we must move these properties from $content and merge them into the
    // top-level element.
    foreach (array('#attributes', '#contextual_links') as $property) {
      if (isset($content[$property])) {
        $build[$property] += $content[$property];
        unset($content[$property]);
      }
    }
    $build['content'] = $content;
  }
  // Either the block's content is completely empty, or it consists only of
  // cacheability metadata.
  else {
    // Abort rendering: render as the empty string and ensure this block is
    // render cached, so we can avoid the work of having to repeatedly
    // determine whether the block is empty. For instance, modifying or adding
    // entities could cause the block to no longer be empty.
    $build = array(
      '#markup' => '',
      '#cache' => $build['#cache'],
    );
    // If $content is not empty, then it contains cacheability metadata, and
    // we must merge it with the existing cacheability metadata. This allows
    // blocks to be empty, yet still bubble cacheability metadata, to indicate
    // why they are empty.
    if (!empty($content)) {
      CacheableMetadata::createFromRenderArray($build)
        ->merge(CacheableMetadata::createFromRenderArray($content))
        ->applyTo($build);
    }
  }
  return $build;
}