<?php

namespace Drupal\Core\Theme;

use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Provides a class which determines the active theme of the page.
 *
 * It therefore uses ThemeNegotiatorInterface objects which are passed in
 * using the 'theme_negotiator' tag.
 */
class ThemeNegotiator implements ThemeNegotiatorInterface {

  /**
   * Holds arrays of theme negotiators, keyed by priority.
   *
   * @var array
   */
  protected $negotiators = [];

  /**
   * Holds the array of theme negotiators sorted by priority.
   *
   * Set to NULL if the array needs to be re-calculated.
   *
   * @var array|null
   */
  protected $sortedNegotiators;

  /**
   * The access checker for themes.
   *
   * @var \Drupal\Core\Theme\ThemeAccessCheck
   */
  protected $themeAccess;

  /**
   * Constructs a new ThemeNegotiator.
   *
   * @param \Drupal\Core\Theme\ThemeAccessCheck $theme_access
   *   The access checker for themes.
   */
  public function __construct(ThemeAccessCheck $theme_access) {
    $this->themeAccess = $theme_access;
  }

  /**
   * Adds a active theme negotiation service.
   *
   * @param \Drupal\Core\Theme\ThemeNegotiatorInterface $negotiator
   *   The theme negotiator to add.
   * @param int $priority
   *   Priority of the theme negotiator.
   */
  public function addNegotiator(ThemeNegotiatorInterface $negotiator, $priority) {
    $this->negotiators[$priority][] = $negotiator;
    // Force the negotiators to be re-sorted.
    $this->sortedNegotiators = NULL;
  }

  /**
   * Returns the sorted array of theme negotiators.
   *
   * @return array|\Drupal\Core\Theme\ThemeNegotiatorInterface[]
   *   An array of theme negotiator objects.
   */
  protected function getSortedNegotiators() {
    if (!isset($this->sortedNegotiators)) {
      // Sort the negotiators according to priority.
      krsort($this->negotiators);
      // Merge nested negotiators from $this->negotiators into
      // $this->sortedNegotiators.
      $this->sortedNegotiators = [];
      foreach ($this->negotiators as $builders) {
        $this->sortedNegotiators = array_merge($this->sortedNegotiators, $builders);
      }
    }
    return $this->sortedNegotiators;
  }

  /**
   * {@inheritdoc}
   */
  public function applies(RouteMatchInterface $route_match) {
    return TRUE;
  }

  /**
   * {@inheritdoc}
   */
  public function determineActiveTheme(RouteMatchInterface $route_match) {
    foreach ($this->getSortedNegotiators() as $negotiator) {
      if ($negotiator->applies($route_match)) {
        $theme = $negotiator->determineActiveTheme($route_match);
        if ($theme !== NULL && $this->themeAccess->checkAccess($theme)) {
          return $theme;
        }
      }
    }
  }

}