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author | Chris Cannam |
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date | Wed, 28 Aug 2019 13:14:47 +0100 |
parents | 4c8ae668cc8c |
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<?php namespace Drupal\Core\Render; /** * Defines an interface for turning a render array into a string. */ 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_process_states(). * - 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 $is_root_call * (Internal use only.) Whether this is a recursive call or not. See * ::renderRoot(). * * @return \Drupal\Component\Render\MarkupInterface * The rendered HTML. * * @throws \LogicException * When called outside of a render context (i.e. outside of a renderRoot(), * renderPlain() or executeInRenderContext() call). * @throws \Exception * If a #pre_render callback throws an exception, it is caught to mark the * renderer as no longer being in a root render call, if any. Then the * exception is rethrown. * * @see \Drupal\Core\Render\ElementInfoManagerInterface::getInfo() * @see \Drupal\Core\Theme\ThemeManagerInterface::render() * @see drupal_process_states() * @see \Drupal\Core\Render\AttachmentsResponseProcessorInterface::processAttachments() * @see \Drupal\Core\Render\RendererInterface::renderRoot() */ public function render(&$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); }