diff core/lib/Drupal/Core/Render/Renderer.php @ 0:4c8ae668cc8c

Initial import (non-working)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/core/lib/Drupal/Core/Render/Renderer.php	Wed Nov 29 16:09:58 2017 +0000
@@ -0,0 +1,757 @@
+<?php
+
+namespace Drupal\Core\Render;
+
+use Drupal\Component\Render\MarkupInterface;
+use Drupal\Component\Utility\Html;
+use Drupal\Component\Utility\Xss;
+use Drupal\Core\Access\AccessResultInterface;
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Controller\ControllerResolverInterface;
+use Drupal\Core\Theme\ThemeManagerInterface;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * Turns a render array into a HTML string.
+ */
+class Renderer implements RendererInterface {
+
+  /**
+   * The theme manager.
+   *
+   * @var \Drupal\Core\Theme\ThemeManagerInterface
+   */
+  protected $theme;
+
+  /**
+   * The controller resolver.
+   *
+   * @var \Drupal\Core\Controller\ControllerResolverInterface
+   */
+  protected $controllerResolver;
+
+  /**
+   * The element info.
+   *
+   * @var \Drupal\Core\Render\ElementInfoManagerInterface
+   */
+  protected $elementInfo;
+
+  /**
+   * The placeholder generator.
+   *
+   * @var \Drupal\Core\Render\PlaceholderGeneratorInterface
+   */
+  protected $placeholderGenerator;
+
+  /**
+   * The render cache service.
+   *
+   * @var \Drupal\Core\Render\RenderCacheInterface
+   */
+  protected $renderCache;
+
+  /**
+   * The renderer configuration array.
+   *
+   * @var array
+   */
+  protected $rendererConfig;
+
+  /**
+   * Whether we're currently in a ::renderRoot() call.
+   *
+   * @var bool
+   */
+  protected $isRenderingRoot = FALSE;
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * The render context collection.
+   *
+   * An individual global render context is tied to the current request. We then
+   * need to maintain a different context for each request to correctly handle
+   * rendering in subrequests.
+   *
+   * This must be static as long as some controllers rebuild the container
+   * during a request. This causes multiple renderer instances to co-exist
+   * simultaneously, render state getting lost, and therefore causing pages to
+   * fail to render correctly. As soon as it is guaranteed that during a request
+   * the same container is used, it no longer needs to be static.
+   *
+   * @var \Drupal\Core\Render\RenderContext[]
+   */
+  protected static $contextCollection;
+
+  /**
+   * Constructs a new Renderer.
+   *
+   * @param \Drupal\Core\Controller\ControllerResolverInterface $controller_resolver
+   *   The controller resolver.
+   * @param \Drupal\Core\Theme\ThemeManagerInterface $theme
+   *   The theme manager.
+   * @param \Drupal\Core\Render\ElementInfoManagerInterface $element_info
+   *   The element info.
+   * @param \Drupal\Core\Render\PlaceholderGeneratorInterface $placeholder_generator
+   *   The placeholder generator.
+   * @param \Drupal\Core\Render\RenderCacheInterface $render_cache
+   *   The render cache service.
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *   The request stack.
+   * @param array $renderer_config
+   *   The renderer configuration array.
+   */
+  public function __construct(ControllerResolverInterface $controller_resolver, ThemeManagerInterface $theme, ElementInfoManagerInterface $element_info, PlaceholderGeneratorInterface $placeholder_generator, RenderCacheInterface $render_cache, RequestStack $request_stack, array $renderer_config) {
+    $this->controllerResolver = $controller_resolver;
+    $this->theme = $theme;
+    $this->elementInfo = $element_info;
+    $this->placeholderGenerator = $placeholder_generator;
+    $this->renderCache = $render_cache;
+    $this->rendererConfig = $renderer_config;
+    $this->requestStack = $request_stack;
+
+    // Initialize the context collection if needed.
+    if (!isset(static::$contextCollection)) {
+      static::$contextCollection = new \SplObjectStorage();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function renderRoot(&$elements) {
+    // Disallow calling ::renderRoot() from within another ::renderRoot() call.
+    if ($this->isRenderingRoot) {
+      $this->isRenderingRoot = FALSE;
+      throw new \LogicException('A stray renderRoot() invocation is causing bubbling of attached assets to break.');
+    }
+
+    // Render in its own render context.
+    $this->isRenderingRoot = TRUE;
+    $output = $this->executeInRenderContext(new RenderContext(), function () use (&$elements) {
+      return $this->render($elements, TRUE);
+    });
+    $this->isRenderingRoot = FALSE;
+
+    return $output;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function renderPlain(&$elements) {
+    return $this->executeInRenderContext(new RenderContext(), function () use (&$elements) {
+      return $this->render($elements, TRUE);
+    });
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function renderPlaceholder($placeholder, array $elements) {
+    // Get the render array for the given placeholder
+    $placeholder_elements = $elements['#attached']['placeholders'][$placeholder];
+
+    // Prevent the render array from being auto-placeholdered again.
+    $placeholder_elements['#create_placeholder'] = FALSE;
+
+    // Render the placeholder into markup.
+    $markup = $this->renderPlain($placeholder_elements);
+
+    // Replace the placeholder with its rendered markup, and merge its
+    // bubbleable metadata with the main elements'.
+    $elements['#markup'] = Markup::create(str_replace($placeholder, $markup, $elements['#markup']));
+    $elements = $this->mergeBubbleableMetadata($elements, $placeholder_elements);
+
+    // Remove the placeholder that we've just rendered.
+    unset($elements['#attached']['placeholders'][$placeholder]);
+
+    return $elements;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function render(&$elements, $is_root_call = FALSE) {
+    // Since #pre_render, #post_render, #lazy_builder callbacks and theme
+    // functions or templates may be used for generating a render array's
+    // content, and we might be rendering the main content for the page, it is
+    // possible that any of them throw an exception that will cause a different
+    // page to be rendered (e.g. throwing
+    // \Symfony\Component\HttpKernel\Exception\NotFoundHttpException will cause
+    // the 404 page to be rendered). That page might also use
+    // Renderer::renderRoot() but if exceptions aren't caught here, it will be
+    // impossible to call Renderer::renderRoot() again.
+    // Hence, catch all exceptions, reset the isRenderingRoot property and
+    // re-throw exceptions.
+    try {
+      return $this->doRender($elements, $is_root_call);
+    }
+    catch (\Exception $e) {
+      // Mark the ::rootRender() call finished due to this exception & re-throw.
+      $this->isRenderingRoot = FALSE;
+      throw $e;
+    }
+  }
+
+  /**
+   * See the docs for ::render().
+   */
+  protected function doRender(&$elements, $is_root_call = FALSE) {
+    if (empty($elements)) {
+      return '';
+    }
+
+    if (!isset($elements['#access']) && isset($elements['#access_callback'])) {
+      if (is_string($elements['#access_callback']) && strpos($elements['#access_callback'], '::') === FALSE) {
+        $elements['#access_callback'] = $this->controllerResolver->getControllerFromDefinition($elements['#access_callback']);
+      }
+      $elements['#access'] = call_user_func($elements['#access_callback'], $elements);
+    }
+
+    // Early-return nothing if user does not have access.
+    if (isset($elements['#access'])) {
+      // If #access is an AccessResultInterface object, we must apply it's
+      // cacheability metadata to the render array.
+      if ($elements['#access'] instanceof AccessResultInterface) {
+        $this->addCacheableDependency($elements, $elements['#access']);
+        if (!$elements['#access']->isAllowed()) {
+          return '';
+        }
+      }
+      elseif ($elements['#access'] === FALSE) {
+        return '';
+      }
+    }
+
+    // Do not print elements twice.
+    if (!empty($elements['#printed'])) {
+      return '';
+    }
+
+    $context = $this->getCurrentRenderContext();
+    if (!isset($context)) {
+      throw new \LogicException("Render context is empty, because render() was called outside of a renderRoot() or renderPlain() call. Use renderPlain()/renderRoot() or #lazy_builder/#pre_render instead.");
+    }
+    $context->push(new BubbleableMetadata());
+
+    // Set the bubbleable rendering metadata that has configurable defaults, if:
+    // - this is the root call, to ensure that the final render array definitely
+    //   has these configurable defaults, even when no subtree is render cached.
+    // - this is a render cacheable subtree, to ensure that the cached data has
+    //   the configurable defaults (which may affect the ID and invalidation).
+    if ($is_root_call || isset($elements['#cache']['keys'])) {
+      $required_cache_contexts = $this->rendererConfig['required_cache_contexts'];
+      if (isset($elements['#cache']['contexts'])) {
+        $elements['#cache']['contexts'] = Cache::mergeContexts($elements['#cache']['contexts'], $required_cache_contexts);
+      }
+      else {
+        $elements['#cache']['contexts'] = $required_cache_contexts;
+      }
+    }
+
+    // Try to fetch the prerendered element from cache, replace any placeholders
+    // and return the final markup.
+    if (isset($elements['#cache']['keys'])) {
+      $cached_element = $this->renderCache->get($elements);
+      if ($cached_element !== FALSE) {
+        $elements = $cached_element;
+        // Only when we're in a root (non-recursive) Renderer::render() call,
+        // placeholders must be processed, to prevent breaking the render cache
+        // in case of nested elements with #cache set.
+        if ($is_root_call) {
+          $this->replacePlaceholders($elements);
+        }
+        // Mark the element markup as safe if is it a string.
+        if (is_string($elements['#markup'])) {
+          $elements['#markup'] = Markup::create($elements['#markup']);
+        }
+        // The render cache item contains all the bubbleable rendering metadata
+        // for the subtree.
+        $context->update($elements);
+        // Render cache hit, so rendering is finished, all necessary info
+        // collected!
+        $context->bubble();
+        return $elements['#markup'];
+      }
+    }
+    // Two-tier caching: track pre-bubbling elements' #cache, #lazy_builder and
+    // #create_placeholder for later comparison.
+    // @see \Drupal\Core\Render\RenderCacheInterface::get()
+    // @see \Drupal\Core\Render\RenderCacheInterface::set()
+    $pre_bubbling_elements = array_intersect_key($elements, [
+      '#cache' => TRUE,
+      '#lazy_builder' => TRUE,
+      '#create_placeholder' => TRUE,
+    ]);
+
+    // If the default values for this element have not been loaded yet, populate
+    // them.
+    if (isset($elements['#type']) && empty($elements['#defaults_loaded'])) {
+      $elements += $this->elementInfo->getInfo($elements['#type']);
+    }
+
+    // First validate the usage of #lazy_builder; both of the next if-statements
+    // use it if available.
+    if (isset($elements['#lazy_builder'])) {
+      // @todo Convert to assertions once https://www.drupal.org/node/2408013
+      //   lands.
+      if (!is_array($elements['#lazy_builder'])) {
+        throw new \DomainException('The #lazy_builder property must have an array as a value.');
+      }
+      if (count($elements['#lazy_builder']) !== 2) {
+        throw new \DomainException('The #lazy_builder property must have an array as a value, containing two values: the callback, and the arguments for the callback.');
+      }
+      if (count($elements['#lazy_builder'][1]) !== count(array_filter($elements['#lazy_builder'][1], function ($v) {
+        return is_null($v) || is_scalar($v);
+      }))) {
+        throw new \DomainException("A #lazy_builder callback's context may only contain scalar values or NULL.");
+      }
+      $children = Element::children($elements);
+      if ($children) {
+        throw new \DomainException(sprintf('When a #lazy_builder callback is specified, no children can exist; all children must be generated by the #lazy_builder callback. You specified the following children: %s.', implode(', ', $children)));
+      }
+      $supported_keys = [
+        '#lazy_builder',
+        '#cache',
+        '#create_placeholder',
+        // The keys below are not actually supported, but these are added
+        // automatically by the Renderer. Adding them as though they are
+        // supported allows us to avoid throwing an exception 100% of the time.
+        '#weight',
+        '#printed'
+      ];
+      $unsupported_keys = array_diff(array_keys($elements), $supported_keys);
+      if (count($unsupported_keys)) {
+        throw new \DomainException(sprintf('When a #lazy_builder callback is specified, no properties can exist; all properties must be generated by the #lazy_builder callback. You specified the following properties: %s.', implode(', ', $unsupported_keys)));
+      }
+    }
+    // Determine whether to do auto-placeholdering.
+    if ($this->placeholderGenerator->canCreatePlaceholder($elements) && $this->placeholderGenerator->shouldAutomaticallyPlaceholder($elements)) {
+      $elements['#create_placeholder'] = TRUE;
+    }
+    // If instructed to create a placeholder, and a #lazy_builder callback is
+    // present (without such a callback, it would be impossible to replace the
+    // placeholder), replace the current element with a placeholder.
+    // @todo remove the isMethodCacheable() check when
+    //       https://www.drupal.org/node/2367555 lands.
+    if (isset($elements['#create_placeholder']) && $elements['#create_placeholder'] === TRUE && $this->requestStack->getCurrentRequest()->isMethodCacheable()) {
+      if (!isset($elements['#lazy_builder'])) {
+        throw new \LogicException('When #create_placeholder is set, a #lazy_builder callback must be present as well.');
+      }
+      $elements = $this->placeholderGenerator->createPlaceholder($elements);
+    }
+    // Build the element if it is still empty.
+    if (isset($elements['#lazy_builder'])) {
+      $callable = $elements['#lazy_builder'][0];
+      $args = $elements['#lazy_builder'][1];
+      if (is_string($callable) && strpos($callable, '::') === FALSE) {
+        $callable = $this->controllerResolver->getControllerFromDefinition($callable);
+      }
+      $new_elements = call_user_func_array($callable, $args);
+      // Retain the original cacheability metadata, plus cache keys.
+      CacheableMetadata::createFromRenderArray($elements)
+        ->merge(CacheableMetadata::createFromRenderArray($new_elements))
+        ->applyTo($new_elements);
+      if (isset($elements['#cache']['keys'])) {
+        $new_elements['#cache']['keys'] = $elements['#cache']['keys'];
+      }
+      $elements = $new_elements;
+      $elements['#lazy_builder_built'] = TRUE;
+    }
+
+    // Make any final changes to the element before it is rendered. This means
+    // that the $element or the children can be altered or corrected before the
+    // element is rendered into the final text.
+    if (isset($elements['#pre_render'])) {
+      foreach ($elements['#pre_render'] as $callable) {
+        if (is_string($callable) && strpos($callable, '::') === FALSE) {
+          $callable = $this->controllerResolver->getControllerFromDefinition($callable);
+        }
+        $elements = call_user_func($callable, $elements);
+      }
+    }
+
+    // All render elements support #markup and #plain_text.
+    if (!empty($elements['#markup']) || !empty($elements['#plain_text'])) {
+      $elements = $this->ensureMarkupIsSafe($elements);
+    }
+
+    // Defaults for bubbleable rendering metadata.
+    $elements['#cache']['tags'] = isset($elements['#cache']['tags']) ? $elements['#cache']['tags'] : [];
+    $elements['#cache']['max-age'] = isset($elements['#cache']['max-age']) ? $elements['#cache']['max-age'] : Cache::PERMANENT;
+    $elements['#attached'] = isset($elements['#attached']) ? $elements['#attached'] : [];
+
+    // Allow #pre_render to abort rendering.
+    if (!empty($elements['#printed'])) {
+      // The #printed element contains all the bubbleable rendering metadata for
+      // the subtree.
+      $context->update($elements);
+      // #printed, so rendering is finished, all necessary info collected!
+      $context->bubble();
+      return '';
+    }
+
+    // Add any JavaScript state information associated with the element.
+    if (!empty($elements['#states'])) {
+      drupal_process_states($elements);
+    }
+
+    // Get the children of the element, sorted by weight.
+    $children = Element::children($elements, TRUE);
+
+    // Initialize this element's #children, unless a #pre_render callback
+    // already preset #children.
+    if (!isset($elements['#children'])) {
+      $elements['#children'] = '';
+    }
+
+    // Assume that if #theme is set it represents an implemented hook.
+    $theme_is_implemented = isset($elements['#theme']);
+    // Check the elements for insecure HTML and pass through sanitization.
+    if (isset($elements)) {
+      $markup_keys = [
+        '#description',
+        '#field_prefix',
+        '#field_suffix',
+      ];
+      foreach ($markup_keys as $key) {
+        if (!empty($elements[$key]) && is_scalar($elements[$key])) {
+          $elements[$key] = $this->xssFilterAdminIfUnsafe($elements[$key]);
+        }
+      }
+    }
+
+    // Call the element's #theme function if it is set. Then any children of the
+    // element have to be rendered there. If the internal #render_children
+    // property is set, do not call the #theme function to prevent infinite
+    // recursion.
+    if ($theme_is_implemented && !isset($elements['#render_children'])) {
+      $elements['#children'] = $this->theme->render($elements['#theme'], $elements);
+
+      // If ThemeManagerInterface::render() returns FALSE this means that the
+      // hook in #theme was not found in the registry and so we need to update
+      // our flag accordingly. This is common for theme suggestions.
+      $theme_is_implemented = ($elements['#children'] !== FALSE);
+    }
+
+    // If #theme is not implemented or #render_children is set and the element
+    // has an empty #children attribute, render the children now. This is the
+    // same process as Renderer::render() but is inlined for speed.
+    if ((!$theme_is_implemented || isset($elements['#render_children'])) && empty($elements['#children'])) {
+      foreach ($children as $key) {
+        $elements['#children'] .= $this->doRender($elements[$key]);
+      }
+      $elements['#children'] = Markup::create($elements['#children']);
+    }
+
+    // If #theme is not implemented and the element has raw #markup as a
+    // fallback, prepend the content in #markup to #children. In this case
+    // #children will contain whatever is provided by #pre_render prepended to
+    // what is rendered recursively above. If #theme is implemented then it is
+    // the responsibility of that theme implementation to render #markup if
+    // required. Eventually #theme_wrappers will expect both #markup and
+    // #children to be a single string as #children.
+    if (!$theme_is_implemented && isset($elements['#markup'])) {
+      $elements['#children'] = Markup::create($elements['#markup'] . $elements['#children']);
+    }
+
+    // Let the theme functions in #theme_wrappers add markup around the rendered
+    // children.
+    // #states and #attached have to be processed before #theme_wrappers,
+    // because the #type 'page' render array from drupal_prepare_page() would
+    // render the $page and wrap it into the html.html.twig template without the
+    // attached assets otherwise.
+    // If the internal #render_children property is set, do not call the
+    // #theme_wrappers function(s) to prevent infinite recursion.
+    if (isset($elements['#theme_wrappers']) && !isset($elements['#render_children'])) {
+      foreach ($elements['#theme_wrappers'] as $key => $value) {
+        // If the value of a #theme_wrappers item is an array then the theme
+        // hook is found in the key of the item and the value contains attribute
+        // overrides. Attribute overrides replace key/value pairs in $elements
+        // for only this ThemeManagerInterface::render() call. This allows
+        // #theme hooks and #theme_wrappers hooks to share variable names
+        // without conflict or ambiguity.
+        $wrapper_elements = $elements;
+        if (is_string($key)) {
+          $wrapper_hook = $key;
+          foreach ($value as $attribute => $override) {
+            $wrapper_elements[$attribute] = $override;
+          }
+        }
+        else {
+          $wrapper_hook = $value;
+        }
+
+        $elements['#children'] = $this->theme->render($wrapper_hook, $wrapper_elements);
+      }
+    }
+
+    // Filter the outputted content and make any last changes before the content
+    // is sent to the browser. The changes are made on $content which allows the
+    // outputted text to be filtered.
+    if (isset($elements['#post_render'])) {
+      foreach ($elements['#post_render'] as $callable) {
+        if (is_string($callable) && strpos($callable, '::') === FALSE) {
+          $callable = $this->controllerResolver->getControllerFromDefinition($callable);
+        }
+        $elements['#children'] = call_user_func($callable, $elements['#children'], $elements);
+      }
+    }
+
+    // We store the resulting output in $elements['#markup'], to be consistent
+    // with how render cached output gets stored. This ensures that placeholder
+    // replacement logic gets the same data to work with, no matter if #cache is
+    // disabled, #cache is enabled, there is a cache hit or miss.
+    $prefix = isset($elements['#prefix']) ? $this->xssFilterAdminIfUnsafe($elements['#prefix']) : '';
+    $suffix = isset($elements['#suffix']) ? $this->xssFilterAdminIfUnsafe($elements['#suffix']) : '';
+
+    $elements['#markup'] = Markup::create($prefix . $elements['#children'] . $suffix);
+
+    // We've rendered this element (and its subtree!), now update the context.
+    $context->update($elements);
+
+    // Cache the processed element if both $pre_bubbling_elements and $elements
+    // have the metadata necessary to generate a cache ID.
+    if (isset($pre_bubbling_elements['#cache']['keys']) && isset($elements['#cache']['keys'])) {
+      if ($pre_bubbling_elements['#cache']['keys'] !== $elements['#cache']['keys']) {
+        throw new \LogicException('Cache keys may not be changed after initial setup. Use the contexts property instead to bubble additional metadata.');
+      }
+      $this->renderCache->set($elements, $pre_bubbling_elements);
+      // Update the render context; the render cache implementation may update
+      // the element, and it may have different bubbleable metadata now.
+      // @see \Drupal\Core\Render\PlaceholderingRenderCache::set()
+      $context->pop();
+      $context->push(new BubbleableMetadata());
+      $context->update($elements);
+    }
+
+    // Only when we're in a root (non-recursive) Renderer::render() call,
+    // placeholders must be processed, to prevent breaking the render cache in
+    // case of nested elements with #cache set.
+    //
+    // By running them here, we ensure that:
+    // - they run when #cache is disabled,
+    // - they run when #cache is enabled and there is a cache miss.
+    // Only the case of a cache hit when #cache is enabled, is not handled here,
+    // that is handled earlier in Renderer::render().
+    if ($is_root_call) {
+      $this->replacePlaceholders($elements);
+      // @todo remove as part of https://www.drupal.org/node/2511330.
+      if ($context->count() !== 1) {
+        throw new \LogicException('A stray drupal_render() invocation with $is_root_call = TRUE is causing bubbling of attached assets to break.');
+      }
+    }
+
+    // Rendering is finished, all necessary info collected!
+    $context->bubble();
+
+    $elements['#printed'] = TRUE;
+    return $elements['#markup'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function hasRenderContext() {
+    return (bool) $this->getCurrentRenderContext();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function executeInRenderContext(RenderContext $context, callable $callable) {
+    // Store the current render context.
+    $previous_context = $this->getCurrentRenderContext();
+
+    // Set the provided context and call the callable, it will use that context.
+    $this->setCurrentRenderContext($context);
+    $result = $callable();
+    // @todo Convert to an assertion in https://www.drupal.org/node/2408013
+    if ($context->count() > 1) {
+      throw new \LogicException('Bubbling failed.');
+    }
+
+    // Restore the original render context.
+    $this->setCurrentRenderContext($previous_context);
+
+    return $result;
+  }
+
+  /**
+   * Returns the current render context.
+   *
+   * @return \Drupal\Core\Render\RenderContext
+   *   The current render context.
+   */
+  protected function getCurrentRenderContext() {
+    $request = $this->requestStack->getCurrentRequest();
+    return isset(static::$contextCollection[$request]) ? static::$contextCollection[$request] : NULL;
+  }
+
+  /**
+   * Sets the current render context.
+   *
+   * @param \Drupal\Core\Render\RenderContext|null $context
+   *   The render context. This can be NULL for instance when restoring the
+   *   original render context, which is in fact NULL.
+   *
+   * @return $this
+   */
+  protected function setCurrentRenderContext(RenderContext $context = NULL) {
+    $request = $this->requestStack->getCurrentRequest();
+    static::$contextCollection[$request] = $context;
+    return $this;
+  }
+
+  /**
+   * Replaces placeholders.
+   *
+   * Placeholders may have:
+   * - #lazy_builder callback, to build a render array to be rendered into
+   *   markup that can replace the placeholder
+   * - #cache: to cache the result of the placeholder
+   *
+   * Also merges the bubbleable metadata resulting from the rendering of the
+   * contents of the placeholders. Hence $elements will be contain the entirety
+   * of bubbleable metadata.
+   *
+   * @param array &$elements
+   *   The structured array describing the data being rendered. Including the
+   *   bubbleable metadata associated with the markup that replaced the
+   *   placeholders.
+   *
+   * @returns bool
+   *   Whether placeholders were replaced.
+   *
+   * @see \Drupal\Core\Render\Renderer::renderPlaceholder()
+   */
+  protected function replacePlaceholders(array &$elements) {
+    if (!isset($elements['#attached']['placeholders']) || empty($elements['#attached']['placeholders'])) {
+      return FALSE;
+    }
+
+    // The 'status messages' placeholder needs to be special cased, because it
+    // depends on global state that can be modified when other placeholders are
+    // being rendered: any code can add messages to render.
+    // This violates the principle that each lazy builder must be able to render
+    // itself in isolation, and therefore in any order. However, we cannot
+    // change the way drupal_set_message() works in the Drupal 8 cycle. So we
+    // have to accommodate its special needs.
+    // Allowing placeholders to be rendered in a particular order (in this case:
+    // last) would violate this isolation principle. Thus a monopoly is granted
+    // to this one special case, with this hard-coded solution.
+    // @see \Drupal\Core\Render\Element\StatusMessages
+    // @see https://www.drupal.org/node/2712935#comment-11368923
+
+    // First render all placeholders except 'status messages' placeholders.
+    $message_placeholders = [];
+    foreach ($elements['#attached']['placeholders'] as $placeholder => $placeholder_element) {
+      if (isset($placeholder_element['#lazy_builder']) && $placeholder_element['#lazy_builder'][0] === 'Drupal\Core\Render\Element\StatusMessages::renderMessages') {
+        $message_placeholders[] = $placeholder;
+      }
+      else {
+        $elements = $this->renderPlaceholder($placeholder, $elements);
+      }
+    }
+
+    // Then render 'status messages' placeholders.
+    foreach ($message_placeholders as $message_placeholder) {
+      $elements = $this->renderPlaceholder($message_placeholder, $elements);
+    }
+
+    return TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function mergeBubbleableMetadata(array $a, array $b) {
+    $meta_a = BubbleableMetadata::createFromRenderArray($a);
+    $meta_b = BubbleableMetadata::createFromRenderArray($b);
+    $meta_a->merge($meta_b)->applyTo($a);
+    return $a;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addCacheableDependency(array &$elements, $dependency) {
+    $meta_a = CacheableMetadata::createFromRenderArray($elements);
+    $meta_b = CacheableMetadata::createFromObject($dependency);
+    $meta_a->merge($meta_b)->applyTo($elements);
+  }
+
+  /**
+   * Applies a very permissive XSS/HTML filter for admin-only use.
+   *
+   * Note: This method only filters if $string is not marked safe already. This
+   * ensures that HTML intended for display is not filtered.
+   *
+   * @param string|\Drupal\Core\Render\Markup $string
+   *   A string.
+   *
+   * @return \Drupal\Core\Render\Markup
+   *   The escaped string wrapped in a Markup object. If the string is an
+   *   instance of \Drupal\Component\Render\MarkupInterface, it won't be escaped
+   *   again.
+   */
+  protected function xssFilterAdminIfUnsafe($string) {
+    if (!($string instanceof MarkupInterface)) {
+      $string = Xss::filterAdmin($string);
+    }
+    return Markup::create($string);
+  }
+
+  /**
+   * Escapes #plain_text or filters #markup as required.
+   *
+   * Drupal uses Twig's auto-escape feature to improve security. This feature
+   * automatically escapes any HTML that is not known to be safe. Due to this
+   * the render system needs to ensure that all markup it generates is marked
+   * safe so that Twig does not do any additional escaping.
+   *
+   * By default all #markup is filtered to protect against XSS using the admin
+   * tag list. Render arrays can alter the list of tags allowed by the filter
+   * using the #allowed_tags property. This value should be an array of tags
+   * that Xss::filter() would accept. Render arrays can escape text instead
+   * of XSS filtering by setting the #plain_text property instead of #markup. If
+   * #plain_text is used #allowed_tags is ignored.
+   *
+   * @param array $elements
+   *   A render array with #markup set.
+   *
+   * @return \Drupal\Component\Render\MarkupInterface|string
+   *   The escaped markup wrapped in a Markup object. If $elements['#markup']
+   *   is an instance of \Drupal\Component\Render\MarkupInterface, it won't be
+   *   escaped or filtered again.
+   *
+   * @see \Drupal\Component\Utility\Html::escape()
+   * @see \Drupal\Component\Utility\Xss::filter()
+   * @see \Drupal\Component\Utility\Xss::filterAdmin()
+   */
+  protected function ensureMarkupIsSafe(array $elements) {
+    if (empty($elements['#markup']) && empty($elements['#plain_text'])) {
+      return $elements;
+    }
+
+    if (!empty($elements['#plain_text'])) {
+      $elements['#markup'] = Markup::create(Html::escape($elements['#plain_text']));
+    }
+    elseif (!($elements['#markup'] instanceof MarkupInterface)) {
+      // The default behaviour is to XSS filter using the admin tag list.
+      $tags = isset($elements['#allowed_tags']) ? $elements['#allowed_tags'] : Xss::getAdminTagList();
+      $elements['#markup'] = Markup::create(Xss::filter($elements['#markup'], $tags));
+    }
+
+    return $elements;
+  }
+
+}