interface RendererInterface

interface RendererInterface {
    
    /**
     * Renders final HTML given a structured array tree.
     *
     * Calls ::render() in such a way that placeholders are replaced.
     *
     * Should therefore only be used in occasions where the final rendering is
     * happening, just before sending a Response:
     * - system internals that are responsible for rendering the final HTML
     * - render arrays for non-HTML responses, such as feeds
     *
     * (Cannot be executed within another render context.)
     *
     * @param array $elements
     *   The structured array describing the data to be rendered.
     *
     * @return \Drupal\Component\Render\MarkupInterface
     *   The rendered HTML.
     *
     * @throws \LogicException
     *   When called from inside another renderRoot() call.
     *
     * @see \Drupal\Core\Render\RendererInterface::render()
     */
    public function renderRoot(&$elements);
    
    /**
     * Renders final HTML in situations where no assets are needed.
     *
     * Calls ::render() in such a way that placeholders are replaced.
     *
     * Useful for instance when rendering the values of tokens or emails, which
     * need a render array being turned into a string, but do not need any of the
     * bubbleable metadata (the attached assets and cache tags).
     *
     * Some of these are a relatively common use case and happen *within* a
     * ::renderRoot() call, but that is generally highly problematic (and hence an
     * exception is thrown when a ::renderRoot() call happens within another
     * ::renderRoot() call). However, in this case, we only care about the output,
     * not about the bubbling. Hence this uses a separate render context, to not
     * affect the parent ::renderRoot() call.
     *
     * (Can be executed within another render context: it runs in isolation.)
     *
     * @param array $elements
     *   The structured array describing the data to be rendered.
     *
     * @return \Drupal\Component\Render\MarkupInterface
     *   The rendered HTML.
     *
     * @see \Drupal\Core\Render\RendererInterface::renderRoot()
     * @see \Drupal\Core\Render\RendererInterface::render()
     */
    public function renderPlain(&$elements);
    
    /**
     * Renders final HTML for a placeholder.
     *
     * Renders the placeholder in isolation.
     *
     * @param string $placeholder
     *   An attached placeholder to render. (This must be a key of one of the
     *   values of $elements['#attached']['placeholders'].)
     * @param array $elements
     *   The structured array describing the data to be rendered.
     *
     * @return array
     *   The updated $elements.
     *
     * @see \Drupal\Core\Render\RendererInterface::render()
     */
    public function renderPlaceholder($placeholder, array $elements);
    
    /**
     * Renders HTML given a structured array tree.
     *
     * Renderable arrays have two kinds of key/value pairs: properties and
     * children. Properties have keys starting with '#' and their values influence
     * how the array will be rendered. Children are all elements whose keys do not
     * start with a '#'. Their values should be renderable arrays themselves,
     * which will be rendered during the rendering of the parent array. The markup
     * provided by the children is typically inserted into the markup generated by
     * the parent array.
     *
     * An important aspect of rendering is caching the result, when appropriate.
     * Because the HTML of a rendered item includes all of the HTML of the
     * rendered children, caching it requires certain information to bubble up
     * from child elements to their parents:
     * - Cache contexts, so that the render cache is varied by every context that
     *   affects the rendered HTML. Because cache contexts affect the cache ID,
     *   and therefore must be resolved for cache hits as well as misses, it is
     *   up to the implementation of this interface to decide how to implement
     *   the caching of items whose children specify cache contexts not directly
     *   specified by the parent. \Drupal\Core\Render\Renderer implements this
     *   with a lazy two-tier caching strategy. Alternate strategies could be to
     *   not cache such parents at all or to cache them with the child elements
     *   replaced by placeholder tokens that are dynamically rendered after cache
     *   retrieval.
     * - Cache tags, so that cached renderings are invalidated when site content
     *   or configuration that can affect that rendering changes.
     * - Placeholders, with associated self-contained placeholder render arrays,
     *   for executing code to handle dynamic requirements that cannot be cached.
     * A render context (\Drupal\Core\Render\RenderContext) can be used to perform
     * bubbling; it is a stack of \Drupal\Core\Render\BubbleableMetadata objects.
     *
     * Additionally, whether retrieving from cache or not, it is important to
     * know all of the assets (CSS and JavaScript) required by the rendered HTML,
     * and this must also bubble from child to parent. Therefore,
     * \Drupal\Core\Render\BubbleableMetadata includes that data as well.
     *
     * The process of rendering an element is recursive unless the element defines
     * an implemented theme hook in #theme. During each call to
     * Renderer::render(), the outermost renderable array (also known as an
     * "element") is processed using the following steps:
     * - If this element has already been printed (#printed = TRUE) or the user
     *   does not have access to it (#access = FALSE), then an empty string is
     *   returned.
     * - If no render context is set yet, an exception is thrown. Otherwise,
     *   an empty \Drupal\Core\Render\BubbleableMetadata is pushed onto the
     *   render context.
     * - If this element has #cache defined then the cached markup for this
     *   element will be returned if it exists in Renderer::render()'s cache. To
     *   use Renderer::render() caching, set the element's #cache property to an
     *   associative array with one or several of the following keys:
     *   - 'keys': An array of one or more keys that identify the element. If
     *     'keys' is set, the cache ID is created automatically from these keys.
     *   - 'contexts': An array of one or more cache context IDs. These are
     *     converted to a final value depending on the request. (For instance,
     *     'user' is mapped to the current user's ID.)
     *   - 'max-age': A time in seconds. Zero seconds means it is not cacheable.
     *     \Drupal\Core\Cache\Cache::PERMANENT means it is cacheable forever.
     *   - 'bin': Specify a cache bin to cache the element in. Default is
     *     'default'.
     *   When there is a render cache hit, there is no rendering work left to be
     *   done, so the stack must be updated. The empty (and topmost) frame that
     *   was just pushed onto the stack is updated with all bubbleable rendering
     *   metadata from the element retrieved from render cache. Then, this stack
     *   frame is bubbled: the two topmost frames are popped from the stack,
     *   they are merged, and the result is pushed back onto the stack.
     *   However, also in case of a cache miss we have to do something. Note
     *   that a Renderer renders top-down, which means that we try to render a
     *   parent first, and we try to avoid the work of rendering the children by
     *   using the render cache. Though in this case, we are dealing with a
     *   cache miss. So a Renderer traverses down the tree, rendering all
     *   children. In doing so, the render stack is updated with the bubbleable
     *   metadata of the children. That means that once the children are
     *   rendered, we can render cache this element. But the cache ID may have
     *   *changed* at that point, because the children's cache contexts have
     *   been bubbled!
     *   It is for that case that we must store the current (pre-bubbling) cache
     *   ID, so that we can compare it with the new (post-bubbling) cache ID
     *   when writing to the cache. We store the current cache ID in
     *   $pre_bubbling_cid.
     * - If this element has #type defined and the default attributes for this
     *   element have not already been merged in (#defaults_loaded = TRUE) then
     *   the defaults for this type of element, defined by an element plugin,
     *   are merged into the array. #defaults_loaded is set by functions that
     *   process render arrays and call the element info service before passing
     *   the array to Renderer::render(), such as form_builder() in the Form
     *   API.
     * - If this element has #create_placeholder set to TRUE, and it has a
     *   #lazy_builder callback, then the element is replaced with another
     *   element that has only two properties: #markup and #attached. #markup
     *   will contain placeholder markup, and #attached contains the placeholder
     *   metadata, that will be used for replacing this placeholder. That
     *   metadata contains a very compact render array (containing only
     *   #lazy_builder and #cache) that will be rendered to replace the
     *   placeholder with its final markup. This means that when the
     *   #lazy_builder callback is called, it received a render array to add to
     *   that only contains #cache.
     * - If this element has a #lazy_builder or an array of #pre_render
     *   functions defined, they are called sequentially to modify the element
     *   before rendering. #lazy_builder is preferred, since it allows for
     *   placeholdering (see previous step), but #pre_render is still supported.
     *   Both have their use case: #lazy_builder is for building a render array,
     *   #pre_render is for decorating an existing render array.
     *   After the #lazy_builder function is called, #lazy_builder is removed,
     *   and #built is set to TRUE.
     *   After the #lazy_builder and all #pre_render functions have been called,
     *   #printed is checked a second time in case a #lazy_builder or
     *   #pre_render function flags the element as printed. If #printed is set,
     *   we return early and hence no rendering work is left to be done,
     *   similarly to a render cache hit. Once again, the empty (and topmost)
     *   frame that was just pushed onto the stack is updated with all
     *   bubbleable rendering metadata from the element whose #printed = TRUE.
     *   Then, this stack frame is bubbled: the two topmost frames are popped
     *   from the stack, they are merged, and the result is pushed back onto the
     *   stack.
     * - The child elements of this element are sorted by weight using uasort()
     *   in \Drupal\Core\Render\Element::children(). Since this is expensive,
     *   when passing already sorted elements to Renderer::render(), for example
     *   from a database query, set $elements['#sorted'] = TRUE to avoid sorting
     *   them a second time.
     * - The main render phase to produce #children for this element takes
     *   place:
     *   - If this element has #theme defined and #theme is an implemented theme
     *     hook/suggestion then ThemeManagerInterface::render() is called and
     *     must render both the element and its children. If #render_children is
     *     set, ThemeManagerInterface::render() will not be called.
     *     #render_children is usually only set internally by
     *     ThemeManagerInterface::render() so that we can avoid the situation
     *     where Renderer::render() called from within a theme preprocess
     *     function creates an infinite loop.
     *   - If this element does not have a defined #theme, or the defined #theme
     *     hook is not implemented, or #render_children is set, then
     *     Renderer::render() is called recursively on each of the child
     *     elements of this element, and the result of each is concatenated onto
     *     #children. This is skipped if #children is not empty at this point.
     *   - Once #children has been rendered for this element, if #theme is not
     *     implemented and #markup is set for this element, #markup will be
     *     prepended to #children.
     * - If this element has #states defined then JavaScript state information
     *   is added to this element's #attached attribute by
     *   \Drupal\Core\Form\FormHelper::processStates().
     * - If this element has #attached defined then any required libraries,
     *   JavaScript, CSS, or other custom data are added to the current page by
     *   \Drupal\Core\Render\AttachmentsResponseProcessorInterface::processAttachments().
     * - If this element has an array of #theme_wrappers defined and
     *   #render_children is not set, #children is then re-rendered by passing
     *   the element in its current state to ThemeManagerInterface::render()
     *   successively for each item in #theme_wrappers. Since #theme and
     *   #theme_wrappers hooks often define variables with the same names it is
     *   possible to explicitly override each attribute passed to each
     *   #theme_wrappers hook by setting the hook name as the key and an array
     *   of overrides as the value in #theme_wrappers array.
     *   For example, if we have a render element as follows:
     *   @code
     *   array(
     *     '#theme' => 'image',
     *     '#attributes' => array('class' => array('foo')),
     *     '#theme_wrappers' => array('container'),
     *   );
     *   @endcode
     *   and we need to pass the class 'bar' as an attribute for 'container', we
     *   can rewrite our element thus:
     *   @code
     *   array(
     *     '#theme' => 'image',
     *     '#attributes' => array('class' => array('foo')),
     *     '#theme_wrappers' => array(
     *       'container' => array(
     *         '#attributes' => array('class' => array('bar')),
     *       ),
     *     ),
     *   );
     *    @endcode
     * - If this element has an array of #post_render functions defined, they
     *   are called sequentially to modify the rendered #children. Unlike
     *   #pre_render functions, #post_render functions are passed both the
     *   rendered #children attribute as a string and the element itself.
     * - If this element has #prefix and/or #suffix defined, they are
     *   concatenated to #children.
     * - The rendering of this element is now complete. The next step will be
     *   render caching. So this is the perfect time to update the stack. At
     *   this point, children of this element (if any), have been rendered also,
     *   and if there were any, their bubbleable rendering metadata will have
     *   been bubbled up into the stack frame for the element that is currently
     *   being rendered. The render cache item for this element must contain the
     *   bubbleable rendering metadata for this element and all of its children.
     *   However, right now, the topmost stack frame (the one for this element)
     *   currently only contains the metadata for the children. Therefore, the
     *   topmost stack frame is updated with this element's metadata, and then
     *   the element's metadata is replaced with the metadata in the topmost
     *   stack frame. This element now contains all bubbleable rendering
     *   metadata for this element and all its children, so it's now ready for
     *   render caching.
     * - If this element has #cache defined, the rendered output of this element
     *   is saved to Renderer::render()'s internal cache. This includes the
     *   changes made by #post_render.
     *   At the same time, if $pre_bubbling_cid is set, it is compared to the
     *   calculated cache ID. If they are different, then a redirecting cache
     *   item is created, containing the #cache metadata of the current element,
     *   and written to cache using the value of $pre_bubbling_cid as the cache
     *   ID. This ensures the pre-bubbling ("wrong") cache ID redirects to the
     *   post-bubbling ("right") cache ID.
     * - If this element also has #cache_properties defined, all the array items
     *   matching the specified property names will be cached along with the
     *   element markup. If properties include children names, the system
     *   assumes only children's individual markup is relevant and ignores the
     *   parent markup. This approach is normally not needed and should be
     *   adopted only when dealing with very advanced use cases.
     * - If this element has attached placeholders ([#attached][placeholders]),
     *   or any of its children has (which we would know thanks to the stack
     *   having been updated just before the render caching step), its
     *   placeholder element containing a #lazy_builder function is rendered in
     *   isolation. The resulting markup is used to replace the placeholder, and
     *   any bubbleable metadata is merged.
     *   Placeholders must be unique, to guarantee that for instance, samples of
     *   placeholders are not replaced as well.
     * - Just before finishing the rendering of this element, this element's
     *   stack frame (the topmost one) is bubbled: the two topmost frames are
     *   popped from the stack, they are merged and the result is pushed back
     *   onto the stack.
     *   So if for instance this element was a child element, then a new frame
     *   was pushed onto the stack element at the beginning of rendering this
     *   element, it was updated when the rendering was completed, and now we
     *   merge it with the frame for the parent, so that the parent now has the
     *   bubbleable rendering metadata for its child.
     * - #printed is set to TRUE for this element to ensure that it is only
     *   rendered once.
     * - The final value of #children for this element is returned as the
     *   rendered output.
     *
     * @param array $elements
     *   The structured array describing the data to be rendered.
     * @param bool $is_root_call
     *   (Internal use only.) Whether this is a recursive call or not. See
     *   ::renderRoot().
     *
     * @return \Drupal\Component\Render\MarkupInterface
     *   The rendered HTML.
     *
     * @throws \LogicException
     *   When called outside of a render context (i.e. outside of a renderRoot(),
     *   renderPlain() or executeInRenderContext() call).
     * @throws \Exception
     *   If a #pre_render callback throws an exception, it is caught to mark the
     *   renderer as no longer being in a root render call, if any. Then the
     *   exception is rethrown.
     *
     * @see \Drupal\Core\Render\ElementInfoManagerInterface::getInfo()
     * @see \Drupal\Core\Theme\ThemeManagerInterface::render()
     * @see \Drupal\Core\Form\FormHelper::processStates()
     * @see \Drupal\Core\Render\AttachmentsResponseProcessorInterface::processAttachments()
     * @see \Drupal\Core\Render\RendererInterface::renderRoot()
     */
    public function render(&$elements, $is_root_call = FALSE);
    
    /**
     * Checks whether a render context is active.
     *
     * This is useful only in very specific situations to determine whether the
     * system is already capable of collecting bubbleable metadata. Normally it
     * should not be necessary to be concerned about this.
     *
     * @return bool
     *   TRUE if the renderer has a render context active, FALSE otherwise.
     */
    public function hasRenderContext();
    
    /**
     * Executes a callable within a render context.
     *
     * Only for very advanced use cases. Prefer using ::renderRoot() and
     * ::renderPlain() instead.
     *
     * All rendering must happen within a render context. Within a render context,
     * all bubbleable metadata is bubbled and hence tracked. Outside of a render
     * context, it would be lost. This could lead to missing assets, incorrect
     * cache variations (and thus security issues), insufficient cache
     * invalidations, and so on.
     *
     * Any and all rendering must therefore happen within a render context, and it
     * is this method that provides that.
     *
     * @param \Drupal\Core\Render\RenderContext $context
     *   The render context to execute the callable within.
     * @param callable $callable
     *   The callable to execute.
     *
     * @return mixed
     *   The callable's return value.
     *
     * @throws \LogicException
     *   In case bubbling has failed, can only happen in case of broken code.
     *
     * @see \Drupal\Core\Render\RenderContext
     * @see \Drupal\Core\Render\BubbleableMetadata
     */
    public function executeInRenderContext(RenderContext $context, callable $callable);
    
    /**
     * Merges the bubbleable rendering metadata o/t 2nd render array with the 1st.
     *
     * @param array $a
     *   A render array.
     * @param array $b
     *   A render array.
     *
     * @return array
     *   The first render array, modified to also contain the bubbleable rendering
     *   metadata of the second render array.
     *
     * @see \Drupal\Core\Render\BubbleableMetadata
     */
    public function mergeBubbleableMetadata(array $a, array $b);
    
    /**
     * Adds a dependency on an object: merges its cacheability metadata.
     *
     * For instance, when a render array depends on some configuration, an entity,
     * or an access result, we must make sure their cacheability metadata is
     * present on the render array. This method makes doing that simple.
     *
     * @param array &$elements
     *   The render array to update.
     * @param \Drupal\Core\Cache\CacheableDependencyInterface|mixed $dependency
     *   The dependency. If the object implements CacheableDependencyInterface,
     *   then its cacheability metadata will be used. Otherwise, the passed in
     *   object must be assumed to be uncacheable, so max-age 0 is set.
     *
     * @see \Drupal\Core\Cache\CacheableMetadata::createFromObject()
     */
    public function addCacheableDependency(array &$elements, $dependency);

}