class Renderer

class Renderer implements RendererInterface {
    use DoTrustedCallbackTrait;
    
    /**
     * 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'])) {
            $elements['#access'] = $this->doCallback('#access_callback', $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 its
            // 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'])) {
            $new_elements = $this->doCallback('#lazy_builder', $elements['#lazy_builder'][0], $elements['#lazy_builder'][1]);
            // Retain the original cacheability metadata, plus cache keys.
            CacheableMetadata::createFromRenderArray($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) {
                $elements = $this->doCallback('#pre_render', $callable, [
                    $elements,
                ]);
            }
        }
        // All render elements support #markup and #plain_text.
        if (isset($elements['#markup']) || isset($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'])) {
            FormHelper::processStates($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) {
                $elements['#children'] = $this->doCallback('#post_render', $callable, [
                    $elements['#children'],
                    $elements,
                ]);
            }
        }
        // We store the resulting output in $elements['#markup'], to be consistent
        // with how render cached output gets stored. This ensures that placeholder
        // replacement logic gets the same data to work with, no matter if #cache is
        // disabled, #cache is enabled, there is a cache hit or miss. If
        // #render_children is set the #prefix and #suffix will have already been
        // added.
        if (isset($elements['#render_children'])) {
            $elements['#markup'] = Markup::create($elements['#children']);
        }
        else {
            $prefix = isset($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\Core\Messenger\Messenger 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 $elements) {
        if (isset($elements['#plain_text'])) {
            $elements['#markup'] = Markup::create(Html::escape($elements['#plain_text']));
        }
        elseif (!$elements['#markup'] instanceof MarkupInterface) {
            // The default behavior 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;
    }
    
    /**
     * Performs a callback.
     *
     * @param string $callback_type
     *   The type of the callback. For example, '#post_render'.
     * @param string|callable $callback
     *   The callback to perform.
     * @param array $args
     *   The arguments to pass to the callback.
     *
     * @return mixed
     *   The callback's return value.
     *
     * @see \Drupal\Core\Security\TrustedCallbackInterface
     */
    protected function doCallback($callback_type, $callback, array $args) {
        if (is_string($callback)) {
            $double_colon = strpos($callback, '::');
            if ($double_colon === FALSE) {
                $callback = $this->controllerResolver
                    ->getControllerFromDefinition($callback);
            }
            elseif ($double_colon > 0) {
                $callback = explode('::', $callback, 2);
            }
        }
        $message = sprintf('Render %s callbacks must be methods of a class that implements \\Drupal\\Core\\Security\\TrustedCallbackInterface or be an anonymous function. The callback was %s. Support for this callback implementation is deprecated in 8.8.0 and will be removed in Drupal 9.0.0. See https://www.drupal.org/node/2966725', $callback_type, '%s');
        // Add \Drupal\Core\Render\Element\RenderCallbackInterface as an extra
        // trusted interface so that:
        // - All public methods on Render elements are considered trusted.
        // - Helper classes that contain only callback methods can implement this
        //   instead of TrustedCallbackInterface.
        return $this->doTrustedCallback($callback, $args, $message, TrustedCallbackInterface::TRIGGER_SILENCED_DEPRECATION, RenderCallbackInterface::class);
    }

}