3 namespace Drupal\Core\Render;
6 * Defines an interface for turning a render array into a string.
8 interface RendererInterface {
11 * Renders final HTML given a structured array tree.
13 * Calls ::render() in such a way that placeholders are replaced.
15 * Should therefore only be used in occasions where the final rendering is
16 * happening, just before sending a Response:
17 * - system internals that are responsible for rendering the final HTML
18 * - render arrays for non-HTML responses, such as feeds
20 * (Cannot be executed within another render context.)
22 * @param array $elements
23 * The structured array describing the data to be rendered.
25 * @return \Drupal\Component\Render\MarkupInterface
28 * @throws \LogicException
29 * When called from inside another renderRoot() call.
31 * @see \Drupal\Core\Render\RendererInterface::render()
33 public function renderRoot(&$elements);
36 * Renders final HTML in situations where no assets are needed.
38 * Calls ::render() in such a way that placeholders are replaced.
40 * Useful for instance when rendering the values of tokens or emails, which
41 * need a render array being turned into a string, but do not need any of the
42 * bubbleable metadata (the attached assets and cache tags).
44 * Some of these are a relatively common use case and happen *within* a
45 * ::renderRoot() call, but that is generally highly problematic (and hence an
46 * exception is thrown when a ::renderRoot() call happens within another
47 * ::renderRoot() call). However, in this case, we only care about the output,
48 * not about the bubbling. Hence this uses a separate render context, to not
49 * affect the parent ::renderRoot() call.
51 * (Can be executed within another render context: it runs in isolation.)
53 * @param array $elements
54 * The structured array describing the data to be rendered.
56 * @return \Drupal\Component\Render\MarkupInterface
59 * @see \Drupal\Core\Render\RendererInterface::renderRoot()
60 * @see \Drupal\Core\Render\RendererInterface::render()
62 public function renderPlain(&$elements);
65 * Renders final HTML for a placeholder.
67 * Renders the placeholder in isolation.
69 * @param string $placeholder
70 * An attached placeholder to render. (This must be a key of one of the
71 * values of $elements['#attached']['placeholders'].)
72 * @param array $elements
73 * The structured array describing the data to be rendered.
76 * The updated $elements.
78 * @see \Drupal\Core\Render\RendererInterface::render()
80 public function renderPlaceholder($placeholder, array $elements);
83 * Renders HTML given a structured array tree.
85 * Renderable arrays have two kinds of key/value pairs: properties and
86 * children. Properties have keys starting with '#' and their values influence
87 * how the array will be rendered. Children are all elements whose keys do not
88 * start with a '#'. Their values should be renderable arrays themselves,
89 * which will be rendered during the rendering of the parent array. The markup
90 * provided by the children is typically inserted into the markup generated by
93 * An important aspect of rendering is caching the result, when appropriate.
94 * Because the HTML of a rendered item includes all of the HTML of the
95 * rendered children, caching it requires certain information to bubble up
96 * from child elements to their parents:
97 * - Cache contexts, so that the render cache is varied by every context that
98 * affects the rendered HTML. Because cache contexts affect the cache ID,
99 * and therefore must be resolved for cache hits as well as misses, it is
100 * up to the implementation of this interface to decide how to implement
101 * the caching of items whose children specify cache contexts not directly
102 * specified by the parent. \Drupal\Core\Render\Renderer implements this
103 * with a lazy two-tier caching strategy. Alternate strategies could be to
104 * not cache such parents at all or to cache them with the child elements
105 * replaced by placeholder tokens that are dynamically rendered after cache
107 * - Cache tags, so that cached renderings are invalidated when site content
108 * or configuration that can affect that rendering changes.
109 * - Placeholders, with associated self-contained placeholder render arrays,
110 * for executing code to handle dynamic requirements that cannot be cached.
111 * A render context (\Drupal\Core\Render\RenderContext) can be used to perform
112 * bubbling; it is a stack of \Drupal\Core\Render\BubbleableMetadata objects.
114 * Additionally, whether retrieving from cache or not, it is important to
115 * know all of the assets (CSS and JavaScript) required by the rendered HTML,
116 * and this must also bubble from child to parent. Therefore,
117 * \Drupal\Core\Render\BubbleableMetadata includes that data as well.
119 * The process of rendering an element is recursive unless the element defines
120 * an implemented theme hook in #theme. During each call to
121 * Renderer::render(), the outermost renderable array (also known as an
122 * "element") is processed using the following steps:
123 * - If this element has already been printed (#printed = TRUE) or the user
124 * does not have access to it (#access = FALSE), then an empty string is
126 * - If no render context is set yet, an exception is thrown. Otherwise,
127 * an empty \Drupal\Core\Render\BubbleableMetadata is pushed onto the
129 * - If this element has #cache defined then the cached markup for this
130 * element will be returned if it exists in Renderer::render()'s cache. To
131 * use Renderer::render() caching, set the element's #cache property to an
132 * associative array with one or several of the following keys:
133 * - 'keys': An array of one or more keys that identify the element. If
134 * 'keys' is set, the cache ID is created automatically from these keys.
135 * - 'contexts': An array of one or more cache context IDs. These are
136 * converted to a final value depending on the request. (For instance,
137 * 'user' is mapped to the current user's ID.)
138 * - 'max-age': A time in seconds. Zero seconds means it is not cacheable.
139 * \Drupal\Core\Cache\Cache::PERMANENT means it is cacheable forever.
140 * - 'bin': Specify a cache bin to cache the element in. Default is
142 * When there is a render cache hit, there is no rendering work left to be
143 * done, so the stack must be updated. The empty (and topmost) frame that
144 * was just pushed onto the stack is updated with all bubbleable rendering
145 * metadata from the element retrieved from render cache. Then, this stack
146 * frame is bubbled: the two topmost frames are popped from the stack,
147 * they are merged, and the result is pushed back onto the stack.
148 * However, also in case of a cache miss we have to do something. Note
149 * that a Renderer renders top-down, which means that we try to render a
150 * parent first, and we try to avoid the work of rendering the children by
151 * using the render cache. Though in this case, we are dealing with a
152 * cache miss. So a Renderer traverses down the tree, rendering all
153 * children. In doing so, the render stack is updated with the bubbleable
154 * metadata of the children. That means that once the children are
155 * rendered, we can render cache this element. But the cache ID may have
156 * *changed* at that point, because the children's cache contexts have
158 * It is for that case that we must store the current (pre-bubbling) cache
159 * ID, so that we can compare it with the new (post-bubbling) cache ID
160 * when writing to the cache. We store the current cache ID in
162 * - If this element has #type defined and the default attributes for this
163 * element have not already been merged in (#defaults_loaded = TRUE) then
164 * the defaults for this type of element, defined by an element plugin,
165 * are merged into the array. #defaults_loaded is set by functions that
166 * process render arrays and call the element info service before passing
167 * the array to Renderer::render(), such as form_builder() in the Form
169 * - If this element has #create_placeholder set to TRUE, and it has a
170 * #lazy_builder callback, then the element is replaced with another
171 * element that has only two properties: #markup and #attached. #markup
172 * will contain placeholder markup, and #attached contains the placeholder
173 * metadata, that will be used for replacing this placeholder. That
174 * metadata contains a very compact render array (containing only
175 * #lazy_builder and #cache) that will be rendered to replace the
176 * placeholder with its final markup. This means that when the
177 * #lazy_builder callback is called, it received a render array to add to
178 * that only contains #cache.
179 * - If this element has a #lazy_builder or an array of #pre_render
180 * functions defined, they are called sequentially to modify the element
181 * before rendering. #lazy_builder is preferred, since it allows for
182 * placeholdering (see previous step), but #pre_render is still supported.
183 * Both have their use case: #lazy_builder is for building a render array,
184 * #pre_render is for decorating an existing render array.
185 * After the #lazy_builder function is called, #lazy_builder is removed,
186 * and #built is set to TRUE.
187 * After the #lazy_builder and all #pre_render functions have been called,
188 * #printed is checked a second time in case a #lazy_builder or
189 * #pre_render function flags the element as printed. If #printed is set,
190 * we return early and hence no rendering work is left to be done,
191 * similarly to a render cache hit. Once again, the empty (and topmost)
192 * frame that was just pushed onto the stack is updated with all
193 * bubbleable rendering metadata from the element whose #printed = TRUE.
194 * Then, this stack frame is bubbled: the two topmost frames are popped
195 * from the stack, they are merged, and the result is pushed back onto the
197 * - The child elements of this element are sorted by weight using uasort()
198 * in \Drupal\Core\Render\Element::children(). Since this is expensive,
199 * when passing already sorted elements to Renderer::render(), for example
200 * from a database query, set $elements['#sorted'] = TRUE to avoid sorting
201 * them a second time.
202 * - The main render phase to produce #children for this element takes
204 * - If this element has #theme defined and #theme is an implemented theme
205 * hook/suggestion then ThemeManagerInterface::render() is called and
206 * must render both the element and its children. If #render_children is
207 * set, ThemeManagerInterface::render() will not be called.
208 * #render_children is usually only set internally by
209 * ThemeManagerInterface::render() so that we can avoid the situation
210 * where Renderer::render() called from within a theme preprocess
211 * function creates an infinite loop.
212 * - If this element does not have a defined #theme, or the defined #theme
213 * hook is not implemented, or #render_children is set, then
214 * Renderer::render() is called recursively on each of the child
215 * elements of this element, and the result of each is concatenated onto
216 * #children. This is skipped if #children is not empty at this point.
217 * - Once #children has been rendered for this element, if #theme is not
218 * implemented and #markup is set for this element, #markup will be
219 * prepended to #children.
220 * - If this element has #states defined then JavaScript state information
221 * is added to this element's #attached attribute by
222 * drupal_process_states().
223 * - If this element has #attached defined then any required libraries,
224 * JavaScript, CSS, or other custom data are added to the current page by
225 * \Drupal\Core\Render\AttachmentsResponseProcessorInterface::processAttachments().
226 * - If this element has an array of #theme_wrappers defined and
227 * #render_children is not set, #children is then re-rendered by passing
228 * the element in its current state to ThemeManagerInterface::render()
229 * successively for each item in #theme_wrappers. Since #theme and
230 * #theme_wrappers hooks often define variables with the same names it is
231 * possible to explicitly override each attribute passed to each
232 * #theme_wrappers hook by setting the hook name as the key and an array
233 * of overrides as the value in #theme_wrappers array.
234 * For example, if we have a render element as follows:
237 * '#theme' => 'image',
238 * '#attributes' => array('class' => array('foo')),
239 * '#theme_wrappers' => array('container'),
242 * and we need to pass the class 'bar' as an attribute for 'container', we
243 * can rewrite our element thus:
246 * '#theme' => 'image',
247 * '#attributes' => array('class' => array('foo')),
248 * '#theme_wrappers' => array(
249 * 'container' => array(
250 * '#attributes' => array('class' => array('bar')),
255 * - If this element has an array of #post_render functions defined, they
256 * are called sequentially to modify the rendered #children. Unlike
257 * #pre_render functions, #post_render functions are passed both the
258 * rendered #children attribute as a string and the element itself.
259 * - If this element has #prefix and/or #suffix defined, they are
260 * concatenated to #children.
261 * - The rendering of this element is now complete. The next step will be
262 * render caching. So this is the perfect time to update the stack. At
263 * this point, children of this element (if any), have been rendered also,
264 * and if there were any, their bubbleable rendering metadata will have
265 * been bubbled up into the stack frame for the element that is currently
266 * being rendered. The render cache item for this element must contain the
267 * bubbleable rendering metadata for this element and all of its children.
268 * However, right now, the topmost stack frame (the one for this element)
269 * currently only contains the metadata for the children. Therefore, the
270 * topmost stack frame is updated with this element's metadata, and then
271 * the element's metadata is replaced with the metadata in the topmost
272 * stack frame. This element now contains all bubbleable rendering
273 * metadata for this element and all its children, so it's now ready for
275 * - If this element has #cache defined, the rendered output of this element
276 * is saved to Renderer::render()'s internal cache. This includes the
277 * changes made by #post_render.
278 * At the same time, if $pre_bubbling_cid is set, it is compared to the
279 * calculated cache ID. If they are different, then a redirecting cache
280 * item is created, containing the #cache metadata of the current element,
281 * and written to cache using the value of $pre_bubbling_cid as the cache
282 * ID. This ensures the pre-bubbling ("wrong") cache ID redirects to the
283 * post-bubbling ("right") cache ID.
284 * - If this element also has #cache_properties defined, all the array items
285 * matching the specified property names will be cached along with the
286 * element markup. If properties include children names, the system
287 * assumes only children's individual markup is relevant and ignores the
288 * parent markup. This approach is normally not needed and should be
289 * adopted only when dealing with very advanced use cases.
290 * - If this element has attached placeholders ([#attached][placeholders]),
291 * or any of its children has (which we would know thanks to the stack
292 * having been updated just before the render caching step), its
293 * placeholder element containing a #lazy_builder function is rendered in
294 * isolation. The resulting markup is used to replace the placeholder, and
295 * any bubbleable metadata is merged.
296 * Placeholders must be unique, to guarantee that for instance, samples of
297 * placeholders are not replaced as well.
298 * - Just before finishing the rendering of this element, this element's
299 * stack frame (the topmost one) is bubbled: the two topmost frames are
300 * popped from the stack, they are merged and the result is pushed back
302 * So if for instance this element was a child element, then a new frame
303 * was pushed onto the stack element at the beginning of rendering this
304 * element, it was updated when the rendering was completed, and now we
305 * merge it with the frame for the parent, so that the parent now has the
306 * bubbleable rendering metadata for its child.
307 * - #printed is set to TRUE for this element to ensure that it is only
309 * - The final value of #children for this element is returned as the
312 * @param array $elements
313 * The structured array describing the data to be rendered.
314 * @param bool $is_root_call
315 * (Internal use only.) Whether this is a recursive call or not. See
318 * @return \Drupal\Component\Render\MarkupInterface
321 * @throws \LogicException
322 * When called outside of a render context (i.e. outside of a renderRoot(),
323 * renderPlain() or executeInRenderContext() call).
325 * If a #pre_render callback throws an exception, it is caught to mark the
326 * renderer as no longer being in a root render call, if any. Then the
327 * exception is rethrown.
329 * @see \Drupal\Core\Render\ElementInfoManagerInterface::getInfo()
330 * @see \Drupal\Core\Theme\ThemeManagerInterface::render()
331 * @see drupal_process_states()
332 * @see \Drupal\Core\Render\AttachmentsResponseProcessorInterface::processAttachments()
333 * @see \Drupal\Core\Render\RendererInterface::renderRoot()
335 public function render(&$elements, $is_root_call = FALSE);
338 * Checks whether a render context is active.
340 * This is useful only in very specific situations to determine whether the
341 * system is already capable of collecting bubbleable metadata. Normally it
342 * should not be necessary to be concerned about this.
345 * TRUE if the renderer has a render context active, FALSE otherwise.
347 public function hasRenderContext();
350 * Executes a callable within a render context.
352 * Only for very advanced use cases. Prefer using ::renderRoot() and
353 * ::renderPlain() instead.
355 * All rendering must happen within a render context. Within a render context,
356 * all bubbleable metadata is bubbled and hence tracked. Outside of a render
357 * context, it would be lost. This could lead to missing assets, incorrect
358 * cache variations (and thus security issues), insufficient cache
359 * invalidations, and so on.
361 * Any and all rendering must therefore happen within a render context, and it
362 * is this method that provides that.
364 * @param \Drupal\Core\Render\RenderContext $context
365 * The render context to execute the callable within.
366 * @param callable $callable
367 * The callable to execute.
370 * The callable's return value.
372 * @throws \LogicException
373 * In case bubbling has failed, can only happen in case of broken code.
375 * @see \Drupal\Core\Render\RenderContext
376 * @see \Drupal\Core\Render\BubbleableMetadata
378 public function executeInRenderContext(RenderContext $context, callable $callable);
381 * Merges the bubbleable rendering metadata o/t 2nd render array with the 1st.
389 * The first render array, modified to also contain the bubbleable rendering
390 * metadata of the second render array.
392 * @see \Drupal\Core\Render\BubbleableMetadata
394 public function mergeBubbleableMetadata(array $a, array $b);
397 * Adds a dependency on an object: merges its cacheability metadata.
399 * For instance, when a render array depends on some configuration, an entity,
400 * or an access result, we must make sure their cacheability metadata is
401 * present on the render array. This method makes doing that simple.
403 * @param array &$elements
404 * The render array to update.
405 * @param \Drupal\Core\Cache\CacheableDependencyInterface|mixed $dependency
406 * The dependency. If the object implements CacheableDependencyInterface,
407 * then its cacheability metadata will be used. Otherwise, the passed in
408 * object must be assumed to be uncacheable, so max-age 0 is set.
410 * @see \Drupal\Core\Cache\CacheableMetadata::createFromObject()
412 public function addCacheableDependency(array &$elements, $dependency);