RenderContext.php

<?php

namespace Drupal\Core\Render;


/**
 * The render context: a stack containing bubbleable rendering metadata.
 *
 * A stack of \Drupal\Core\Render\BubbleableMetadata objects.
 *
 * @see \Drupal\Core\Render\RendererInterface
 * @see \Drupal\Core\Render\Renderer
 * @see \Drupal\Core\Render\BubbleableMetadata
 */
class RenderContext extends \SplStack {
    
    /**
     * Updates the current frame of the stack.
     *
     * @param array &$element
     *   The element of the render array that has just been rendered. The stack
     *   frame for this element will be updated with the bubbleable rendering
     *   metadata of this element.
     */
    public function update(&$element) {
        // The latest frame represents the bubbleable metadata for the subtree.
        $frame = $this->pop();
        // Update the frame, but also update the current element, to ensure it
        // contains up-to-date information in case it gets render cached.
        $updated_frame = BubbleableMetadata::createFromRenderArray($element)->merge($frame);
        $updated_frame->applyTo($element);
        $this->push($updated_frame);
    }
    
    /**
     * Bubbles the stack.
     *
     * Whenever another level in the render array has been rendered, the stack
     * must be bubbled, to merge its rendering metadata with that of the parent
     * element.
     */
    public function bubble() {
        // If there's only one frame on the stack, then this is the root call, and
        // we can't bubble up further. ::renderRoot() will reset the stack, but we
        // must not reset it here to allow users of ::executeInRenderContext() to
        // access the stack directly.
        if ($this->count() === 1) {
            return;
        }
        // Merge the current and the parent stack frame.
        $current = $this->pop();
        $parent = $this->pop();
        $this->push($current->merge($parent));
    }

}