Issue #2938969 by msankhala, dhirendra.mishra, paulocs, TR, MerryHamster,...

 * @code

 * array('#type' => 'status_messages')

 * @endcode

 * to a render array, use drupal_render() to render it, and add a command to

 * place the messages in an appropriate location.

 * to a render array, use \Drupal::service('renderer')->render() to render it,

 * and add a command to place the messages in an appropriate location.

 *

 * @section sec_other Other methods for triggering Ajax

 * Here are some additional methods you can use to trigger Ajax responses in

/**

 * Hides an element from later rendering.

 *

 * The first time render() or drupal_render() is called on an element tree,

 * as each element in the tree is rendered, it is marked with a #printed flag

 * and the rendered children of the element are cached. Subsequent calls to

 * render() or drupal_render() will not traverse the child tree of this element

 * again: they will just use the cached children. So if you want to hide an

 * element, be sure to call hide() on the element before its parent tree is

 * rendered for the first time, as it will have no effect on subsequent

 * The first time render() or RenderInterface::render() is called on an element

 * tree, as each element in the tree is rendered, it is marked with a #printed

 * flag and the rendered children of the element are cached. Subsequent calls to

 * render() or RenderInterface::render() will not traverse the child tree of

 * this element again: they will just use the cached children. So if you want to

 * hide an element, be sure to call hide() on the element before its parent tree

 * is rendered for the first time, as it will have no effect on subsequent

 * renderings of the parent tree.

 *

 * @param $element

 * @return

 *   The element.

 *

 * @see \Drupal\Core\Render\RendererInterface

 * @see render()

 * @see show()

 */

 * You can also use render($element), which shows the element while rendering

 * it.

 *

 * The first time render() or drupal_render() is called on an element tree,

 * as each element in the tree is rendered, it is marked with a #printed flag

 * and the rendered children of the element are cached. Subsequent calls to

 * render() or drupal_render() will not traverse the child tree of this element

 * again: they will just use the cached children. So if you want to show an

 * element, be sure to call show() on the element before its parent tree is

 * rendered for the first time, as it will have no effect on subsequent

 * The first time render() or RenderInterface::render() is called on an element

 * tree, as each element in the tree is rendered, it is marked with a #printed

 * flag and the rendered children of the element are cached. Subsequent calls to

 * render() or RenderInterface::render() will not traverse the child tree of

 * this element again: they will just use the cached children. So if you want to

 * show an element, be sure to call show() on the element before its parent tree

 * is rendered for the first time, as it will have no effect on subsequent

 * renderings of the parent tree.

 *

 * @param $element

 * @return

 *   The element.

 *

 * @see \Drupal\Core\Render\RendererInterface

 * @see render()

 * @see hide()

 */

   * This function is assigned as a #pre_render callback in ::viewMultiple().

   *

   * By delaying the building of an entity until the #pre_render processing in

   * drupal_render(), the processing cost of assembling an entity's renderable

   * array is saved on cache-hit requests.

   * \Drupal::service('renderer')->render(), the processing cost of assembling

   * an entity's renderable array is saved on cache-hit requests.

   *

   * @param array $build_list

   *   A renderable  array containing build information and context for an

 * the particular entity type template, if there is one (e.g., node.html.twig).

 *

 * See the @link themeable Default theme implementations topic @endlink and

 * drupal_render() for details.

 * \Drupal\Core\Render\RendererInterface::render() for details.

 *

 * @param array &$build

 *   A renderable array representing the entity content.

 * the particular entity type template, if there is one (e.g., node.html.twig).

 *

 * See the @link themeable Default theme implementations topic @endlink and

 * drupal_render() for details.

 * \Drupal\Core\Render\RendererInterface::render() for details.

 *

 * @param array &$build

 *   A renderable array representing the entity content.

}

/**

 * Alter entity renderable values before cache checking in drupal_render().

 * Alter entity renderable values before cache checking during rendering.

 *

 * Invoked for a specific entity type.

 *

}

/**

 * Alter entity renderable values before cache checking in drupal_render().

 * Alter entity renderable values before cache checking during rendering.

 *

 * The values in the #cache key of the renderable array are used to determine if

 * a cache entry exists for the entity's rendered output. Ideally only values

/**

 * Subscriber that wraps controllers, to handle early rendering.

 *

 * When controllers call drupal_render() (RendererInterface::render()) outside

 * of a render context, we call that "early rendering". Controllers should

 * return only render arrays, but we cannot prevent controllers from doing early

 * rendering. The problem with early rendering is that the bubbleable metadata

 * (cacheability & attachments) are lost.

 * When controllers call RendererInterface::render() outside of a render

 * context, we call that "early rendering". Controllers should return

 * only render arrays, but we cannot prevent controllers from doing

 * early rendering. The problem with early rendering is that the

 * bubbleable metadata (cacheability & attachments) are lost.

 *

 * This can lead to broken pages (missing assets), stale pages (missing cache

 * tags causing a page not to be invalidated) or even security problems (missing

 * ::renderPlain() methods. In that case, no bubbleable metadata is lost.

 *

 * If the render context is not empty, then the controller did use

 * drupal_render(), and bubbleable metadata was collected. This bubbleable

 * metadata is then merged onto the render array.

 * RendererInterface::render(), and bubbleable metadata was collected.

 * This bubbleable metadata is then merged onto the render array.

 *

 * In other words: this just exists to ease the transition to Drupal 8: it

 * allows controllers that return render arrays (the majority) and

    });

    // If early rendering happened, i.e. if code in the controller called

    // drupal_render() outside of a render context, then the bubbleable metadata

    // for that is stored in the current render context.

    // RendererInterface::render() outside of a render context, then the

    // bubbleable metadata for that is stored in the current render context.

    if (!$context->isEmpty()) {

      /** @var \Drupal\Core\Render\BubbleableMetadata $early_rendering_bubbleable_metadata */

      $early_rendering_bubbleable_metadata = $context->pop();