theme.inc

<?php

/**
 * @file
 * The theme system, which controls the output of Drupal.
 *
 * The theme system allows for nearly all output of the Drupal system to be
 * customized by user themes.
 */

use Drupal\Component\Utility\String;
use Drupal\Component\Utility\Url;
use Drupal\Core\Config\Config;
use Drupal\Core\Language\Language;
use Drupal\Core\Extension\ExtensionNameLengthException;
use Drupal\Core\Template\Attribute;
use Drupal\Core\Template\RenderWrapper;
use Drupal\Core\Theme\ThemeSettings;
use Drupal\Component\Utility\NestedArray;
use Drupal\Component\Utility\MapArray;

/**
 * @defgroup content_flags Content markers
 * @{
 * Markers used by theme_mark() and node_mark() to designate content.
 * @see theme_mark(), node_mark()
 */

/**
 * Mark content as read.
 */
const MARK_READ = 0;

/**
 * Mark content as being new.
 */
const MARK_NEW = 1;

/**
 * Mark content as being updated.
 */
const MARK_UPDATED = 2;

/**
 * A responsive table class; hide table cell on narrow devices.
 *
 * Indicates that a column has medium priority and thus can be hidden on narrow
 * width devices and shown on medium+ width devices (i.e. tablets and desktops).
 */
const RESPONSIVE_PRIORITY_MEDIUM = 'priority-medium';

/**
 * A responsive table class; only show table cell on wide devices.
 *
 * Indicates that a column has low priority and thus can be hidden on narrow
 * and medium viewports and shown on wide devices (i.e. desktops).
 */
const RESPONSIVE_PRIORITY_LOW = 'priority-low';

/**
 * @} End of "defgroup content_flags".
 */

/**
 * Determines if a theme is available to use.
 *
 * @param $theme
 *   Either the name of a theme or a full theme object.
 *
 * @return
 *   Boolean TRUE if the theme is enabled or is the site administration theme;
 *   FALSE otherwise.
 *
 * @deprecated Use \Drupal::service('access_check.theme')->checkAccess().
 */
function drupal_theme_access($theme) {
  if (is_object($theme)) {
    $theme = $theme->name;
  }
  return \Drupal::service('access_check.theme')->checkAccess($theme);
}

/**
 * Initializes the theme system by loading the theme.
 */
function drupal_theme_initialize() {
  global $theme, $user, $theme_key;

  // If $theme is already set, assume the others are set, too, and do nothing
  if (isset($theme)) {
    return;
  }

  drupal_bootstrap(DRUPAL_BOOTSTRAP_DATABASE);
  $themes = list_themes();

  // Only select the user selected theme if it is available in the
  // list of themes that can be accessed.
  $theme = !empty($user->theme) && drupal_theme_access($user->theme) ? $user->theme : \Drupal::config('system.theme')->get('default');

  // Allow modules to override the theme. Validation has already been performed
  // inside menu_get_custom_theme(), so we do not need to check it again here.
  $custom_theme = menu_get_custom_theme();
  $theme = !empty($custom_theme) ? $custom_theme : $theme;

  // Store the identifier for retrieving theme settings with.
  $theme_key = $theme;

  // Find all our ancestor themes and put them in an array.
  $base_theme = array();
  $ancestor = $theme;
  while ($ancestor && isset($themes[$ancestor]->base_theme)) {
    $ancestor = $themes[$ancestor]->base_theme;
    $base_theme[] = $themes[$ancestor];
  }
  _drupal_theme_initialize($themes[$theme], array_reverse($base_theme));

  // Themes can have alter functions, so reset the drupal_alter() cache.
  drupal_static_reset('drupal_alter');
}

/**
 * Initializes the theme system given already loaded information.
 *
 * This function is useful to initialize a theme when no database is present.
 *
 * @param $theme
 *   An object with the following information:
 *     filename
 *       The .info.yml file for this theme. The 'path' to
 *       the theme will be in this file's directory. (Required)
 *     owner
 *       The path to the .theme file or the .engine file to load for
 *       the theme. (Required)
 *     stylesheet
 *       The primary stylesheet for the theme. (Optional)
 *     engine
 *       The name of theme engine to use. (Optional)
 * @param $base_theme
 *    An optional array of objects that represent the 'base theme' if the
 *    theme is meant to be derivative of another theme. It requires
 *    the same information as the $theme object. It should be in
 *    'oldest first' order, meaning the top level of the chain will
 *    be first.
 */
function _drupal_theme_initialize($theme, $base_theme = array()) {
  global $theme_info, $base_theme_info, $theme_engine, $theme_path;
  $theme_info = $theme;
  $base_theme_info = $base_theme;

  $theme_path = dirname($theme->filename);

  // Prepare stylesheets from this theme as well as all ancestor themes.
  // We work it this way so that we can have child themes override parent
  // theme stylesheets easily.
  $final_stylesheets = array();
  // CSS file basenames to override, pointing to the final, overridden filepath.
  $theme->stylesheets_override = array();
  // CSS file basenames to remove.
  $theme->stylesheets_remove = array();

  // Grab stylesheets from base theme
  foreach ($base_theme as $base) {
    if (!empty($base->stylesheets)) {
      foreach ($base->stylesheets as $media => $stylesheets) {
        foreach ($stylesheets as $name => $stylesheet) {
          $final_stylesheets[$media][$name] = $stylesheet;
        }
      }
    }
    $base_theme_path = dirname($base->filename);
    if (!empty($base->info['stylesheets-remove'])) {
      foreach ($base->info['stylesheets-remove'] as $basename) {
        $theme->stylesheets_remove[$basename] = $base_theme_path . '/' . $basename;
      }
    }
    if (!empty($base->info['stylesheets-override'])) {
      foreach ($base->info['stylesheets-override'] as $name) {
        $basename = drupal_basename($name);
        $theme->stylesheets_override[$basename] = $base_theme_path . '/' . $name;
      }
    }
  }

  // Add stylesheets used by this theme.
  if (!empty($theme->stylesheets)) {
    foreach ($theme->stylesheets as $media => $stylesheets) {
      foreach ($stylesheets as $name => $stylesheet) {
        $final_stylesheets[$media][$name] = $stylesheet;
      }
    }
  }
  if (!empty($theme->info['stylesheets-remove'])) {
    foreach ($theme->info['stylesheets-remove'] as $basename) {
      $theme->stylesheets_remove[$basename] = $theme_path . '/' . $basename;

      if (isset($theme->stylesheets_override[$basename])) {
        unset($theme->stylesheets_override[$basename]);
      }
    }
  }
  if (!empty($theme->info['stylesheets-override'])) {
    foreach ($theme->info['stylesheets-override'] as $name) {
      $basename = drupal_basename($name);
      $theme->stylesheets_override[$basename] = $theme_path . '/' . $name;

      if (isset($theme->stylesheets_remove[$basename])) {
        unset($theme->stylesheets_remove[$basename]);
      }
    }
  }

  // And now add the stylesheets properly
  foreach ($final_stylesheets as $media => $stylesheets) {
    foreach ($stylesheets as $stylesheet) {
      drupal_add_css($stylesheet, array('group' => CSS_AGGREGATE_THEME, 'every_page' => TRUE, 'media' => $media));
    }
  }

  // Do basically the same as the above for scripts
  $final_scripts = array();

  // Grab scripts from base theme
  foreach ($base_theme as $base) {
    if (!empty($base->scripts)) {
      foreach ($base->scripts as $name => $script) {
        $final_scripts[$name] = $script;
      }
    }
  }

  // Add scripts used by this theme.
  if (!empty($theme->scripts)) {
    foreach ($theme->scripts as $name => $script) {
      $final_scripts[$name] = $script;
    }
  }

  // Add scripts used by this theme.
  foreach ($final_scripts as $script) {
    drupal_add_js($script, array('group' => JS_THEME, 'every_page' => TRUE));
  }

  $theme_engine = NULL;

  // Initialize the theme.
  if (isset($theme->engine)) {
    // Include the engine.
    include_once DRUPAL_ROOT . '/' . $theme->owner;

    $theme_engine = $theme->engine;
    if (function_exists($theme_engine . '_init')) {
      foreach ($base_theme as $base) {
        call_user_func($theme_engine . '_init', $base);
      }
      call_user_func($theme_engine . '_init', $theme);
    }
  }
  else {
    // include non-engine theme files
    foreach ($base_theme as $base) {
      // Include the theme file or the engine.
      if (!empty($base->owner)) {
        include_once DRUPAL_ROOT . '/' . $base->owner;
      }
    }
    // and our theme gets one too.
    if (!empty($theme->owner)) {
      include_once DRUPAL_ROOT . '/' . $theme->owner;
    }
  }

  // Always include Twig as the default theme engine.
  include_once DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine';
}

/**
 * Gets the theme registry.
 *
 * @param bool $complete
 *   Optional boolean to indicate whether to return the complete theme registry
 *   array or an instance of the Drupal\Core\Utility\ThemeRegistry class.
 *   If TRUE, the complete theme registry array will be returned. This is useful
 *   if you want to foreach over the whole registry, use array_* functions or
 *   inspect it in a debugger. If FALSE, an instance of the
 *   Drupal\Core\Utility\ThemeRegistry class will be returned, this provides an
 *   ArrayObject which allows it to be accessed with array syntax and isset(),
 *   and should be more lightweight than the full registry. Defaults to TRUE.
 *
 * @return
 *   The complete theme registry array, or an instance of the
 *   Drupal\Core\Utility\ThemeRegistry class.
 */
function theme_get_registry($complete = TRUE) {
  $theme_registry = \Drupal::service('theme.registry');
  if ($complete) {
    return $theme_registry->get();
  }
  else {
    return $theme_registry->getRuntime();
  }
}

/**
 * Forces the system to rebuild the theme registry.
 *
 * This function should be called when modules are added to the system, or when
 * a dynamic system needs to add more theme hooks.
 */
function drupal_theme_rebuild() {
  \Drupal::service('theme.registry')->reset();
}

/**
 * Returns a list of all currently available themes.
 *
 * Retrieved from the database, if available and the site is not in maintenance
 * mode; otherwise compiled freshly from the filesystem.
 *
 * @param $refresh
 *   Whether to reload the list of themes from the database. Defaults to FALSE.
 *
 * @return
 *   An associative array of the currently available themes. The keys are the
 *   themes' machine names and the values are objects having the following
 *   properties:
 *   - filename: The filepath and name of the .info.yml file.
 *   - name: The machine name of the theme.
 *   - status: 1 for enabled, 0 for disabled themes.
 *   - info: The contents of the .info.yml file.
 *   - stylesheets: A two dimensional array, using the first key for the
 *     media attribute (e.g. 'all'), the second for the name of the file
 *     (e.g. style.css). The value is a complete filepath (e.g.
 *     themes/bartik/style.css). Not set if no stylesheets are defined in the
 *     .info.yml file.
 *   - scripts: An associative array of JavaScripts, using the filename as key
 *     and the complete filepath as value. Not set if no scripts are defined in
 *     the .info.yml file.
 *   - prefix: The base theme engine prefix.
 *   - engine: The machine name of the theme engine.
 *   - base_theme: If this is a sub-theme, the machine name of the base theme
 *     defined in the .info.yml file. Otherwise, the element is not set.
 *   - base_themes: If this is a sub-theme, an associative array of the
 *     base-theme ancestors of this theme, starting with this theme's base
 *     theme, then the base theme's own base theme, etc. Each entry has an
 *     array key equal to the theme's machine name, and a value equal to the
 *     human-readable theme name; if a theme with matching machine name does
 *     not exist in the system, the value will instead be NULL (and since the
 *     system would not know whether that theme itself has a base theme, that
 *     will end the array of base themes). This is not set if the theme is not
 *     a sub-theme.
 *   - sub_themes: An associative array of themes on the system that are
 *     either direct sub-themes (that is, they declare this theme to be
 *     their base theme), direct sub-themes of sub-themes, etc. The keys are
 *     the themes' machine names, and the values are the themes' human-readable
 *     names. This element is not set if there are no themes on the system that
 *     declare this theme as their base theme.
 */
function list_themes($refresh = FALSE) {
  $list = &drupal_static(__FUNCTION__, array());

  if ($refresh) {
    $list = array();
    system_list_reset();
  }

  if (empty($list)) {
    $list = array();
    // Extract from the database only when it is available.
    // Also check that the site is not in the middle of an install or update.
    if (!defined('MAINTENANCE_MODE')) {
      try {
        $themes = system_list('theme');
      }
      catch (Exception $e) {
        // If the database is not available, rebuild the theme data.
        $themes = _system_rebuild_theme_data();
      }
    }
    else {
      // Scan the installation when the database should not be read.
      $themes = _system_rebuild_theme_data();
    }

    foreach ($themes as $theme) {
      foreach ($theme->info['stylesheets'] as $media => $stylesheets) {
        foreach ($stylesheets as $stylesheet => $path) {
          $theme->stylesheets[$media][$stylesheet] = $path;
        }
      }
      foreach ($theme->info['scripts'] as $script => $path) {
        $theme->scripts[$script] = $path;
      }
      if (isset($theme->info['engine'])) {
        $theme->engine = $theme->info['engine'];
      }
      if (isset($theme->info['base theme'])) {
        $theme->base_theme = $theme->info['base theme'];
      }
      // Status is normally retrieved from the database. Add zero values when
      // read from the installation directory to prevent notices.
      if (!isset($theme->status)) {
        $theme->status = 0;
      }
      $list[$theme->name] = $theme;
    }
  }

  return $list;
}

/**
 * Finds all the base themes for the specified theme.
 *
 * Themes can inherit templates and function implementations from earlier
 * themes.
 *
 * @param $themes
 *   An array of available themes.
 * @param $key
 *   The name of the theme whose base we are looking for.
 * @param $used_keys
 *   (optional) A recursion parameter preventing endless loops. Defaults to
 *   NULL.
 *
 * @return
 *   Returns an array of all of the theme's ancestors; the first element's value
 *   will be NULL if an error occurred.
 */
function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  $base_key = $themes[$key]->info['base theme'];
  // Does the base theme exist?
  if (!isset($themes[$base_key])) {
    return array($base_key => NULL);
  }

  $current_base_theme = array($base_key => $themes[$base_key]->info['name']);

  // Is the base theme itself a child of another theme?
  if (isset($themes[$base_key]->info['base theme'])) {
    // Do we already know the base themes of this theme?
    if (isset($themes[$base_key]->base_themes)) {
      return $themes[$base_key]->base_themes + $current_base_theme;
    }
    // Prevent loops.
    if (!empty($used_keys[$base_key])) {
      return array($base_key => NULL);
    }
    $used_keys[$base_key] = TRUE;
    return drupal_find_base_themes($themes, $base_key, $used_keys) + $current_base_theme;
  }
  // If we get here, then this is our parent theme.
  return $current_base_theme;
}

/**
 * Generates themed output.
 *
 * All requests for themed output must go through this function (however,
 * calling the theme() function directly is strongly discouraged - see next
 * paragraph). It examines the request and routes it to the appropriate
 * @link themeable theme function or template @endlink, by checking the theme
 * registry.
 *
 * Avoid calling this function directly. It is preferable to replace direct
 * calls to the theme() function with calls to drupal_render() by passing a
 * render array with a #theme key to drupal_render(), which in turn calls
 * theme().
 *
 * @section sec_theme_hooks Theme Hooks
 * Most commonly, the first argument to this function is the name of the theme
 * hook. For instance, to theme a taxonomy term, the theme hook name is
 * 'taxonomy_term'. Modules register theme hooks within a hook_theme()
 * implementation and provide a default implementation via a function named
 * theme_HOOK() (e.g., theme_taxonomy_term()) or via a template file named
 * according to the value of the 'template' key registered with the theme hook
 * (see hook_theme() for details). Default templates are implemented with the
 * Twig rendering engine and are named the same as the theme hook, with
 * underscores changed to hyphens, so for the 'taxonomy_term' theme hook, the
 * default template is 'taxonomy-term.html.twig'.
 *
 * @subsection sub_overriding_theme_hooks Overriding Theme Hooks
 * Themes may also register new theme hooks within a hook_theme()
 * implementation, but it is more common for themes to override default
 * implementations provided by modules than to register entirely new theme
 * hooks. Themes can override a default implementation by implementing a
 * function named THEME_HOOK() (for example, the 'bartik' theme overrides the
 * default implementation of the 'menu_tree' theme hook by implementing a
 * bartik_menu_tree() function), or by adding a template file within its folder
 * structure that follows the template naming structure used by the theme's
 * rendering engine (for example, since the Bartik theme uses the Twig rendering
 * engine, it overrides the default implementation of the 'page' theme hook by
 * containing a 'page.html.twig' file within its folder structure).
 *
 * @subsection sub_preprocess_templates Preprocessing for Template Files
 * If the implementation is a template file, several functions are called before
 * the template file is invoked to modify the $variables array. These make up
 * the "preprocessing" phase, and are executed (if they exist), in the following
 * order (note that in the following list, HOOK indicates the theme hook name,
 * MODULE indicates a module name, THEME indicates a theme name, and ENGINE
 * indicates a theme engine name):
 * - template_preprocess(&$variables, $hook): Creates a default set of variables
 *   for all theme hooks with template implementations.
 * - template_preprocess_HOOK(&$variables): Should be implemented by the module
 *   that registers the theme hook, to set up default variables.
 * - MODULE_preprocess(&$variables, $hook): hook_preprocess() is invoked on all
 *   implementing modules.
 * - MODULE_preprocess_HOOK(&$variables): hook_preprocess_HOOK() is invoked on
 *   all implementing modules, so that modules that didn't define the theme hook
 *   can alter the variables.
 * - ENGINE_engine_preprocess(&$variables, $hook): Allows the theme engine to
 *   set necessary variables for all theme hooks with template implementations.
 * - ENGINE_engine_preprocess_HOOK(&$variables): Allows the theme engine to set
 *   necessary variables for the particular theme hook.
 * - THEME_preprocess(&$variables, $hook): Allows the theme to set necessary
 *   variables for all theme hooks with template implementations.
 * - THEME_preprocess_HOOK(&$variables): Allows the theme to set necessary
 *   variables specific to the particular theme hook.
 *
 * @subsection sub_preprocess_theme_funcs Preprocessing for Theme Functions
 * If the implementation is a function, only the theme-hook-specific preprocess
 * functions (the ones ending in _HOOK) are called from the list above. This is
 * because theme hooks with function implementations need to be fast, and
 * calling the non-theme-hook-specific preprocess functions for them would incur
 * a noticeable performance penalty.
 *
 * @subsection sub_alternate_suggestions Suggesting Alternate Hooks
 * Alternate hooks can be suggested by implementing the hook-specific
 * hook_theme_suggestions_HOOK_alter() or the generic
 * hook_theme_suggestions_alter(). These alter hooks are used to manipulate an
 * array of suggested alternate theme hooks to use, in reverse order of
 * priority. theme() will use the highest priority implementation that exists.
 * If none exists, theme() will use the implementation for the theme hook it was
 * called with. These suggestions are similar to and are used for similar
 * reasons as calling theme() with an array as the $hook parameter (see below).
 * The difference is whether the suggestions are determined by the code that
 * calls theme() or by altering the suggestions via the suggestion alter hooks.
 *
 * @param $hook
 *   The name of the theme hook to call. If the name contains a
 *   double-underscore ('__') and there isn't an implementation for the full
 *   name, the part before the '__' is checked. This allows a fallback to a
 *   more generic implementation. For example, if theme('links__node', ...) is
 *   called, but there is no implementation of that theme hook, then the
 *   'links' implementation is used. This process is iterative, so if
 *   theme('links__contextual__node', ...) is called, theme() checks for the
 *   following implementations, and uses the first one that exists:
 *   - links__contextual__node
 *   - links__contextual
 *   - links
 *   This allows themes to create specific theme implementations for named
 *   objects and contexts of otherwise generic theme hooks. The $hook parameter
 *   may also be an array, in which case the first theme hook that has an
 *   implementation is used. This allows for the code that calls theme() to
 *   explicitly specify the fallback order in a situation where using the '__'
 *   convention is not desired or is insufficient.
 * @param $variables
 *   An associative array of variables to merge with defaults from the theme
 *   registry, pass to preprocess functions for modification, and finally, pass
 *   to the function or template implementing the theme hook. Alternatively,
 *   this can be a renderable array, in which case, its properties are mapped to
 *   variables expected by the theme hook implementations.
 *
 * @return string|false
 *   An HTML string representing the themed output or FALSE if the passed $hook
 *   is not implemented.
 *
 * @see drupal_render()
 * @see themeable
 * @see hook_theme()
 * @see template_preprocess()
 */
function theme($hook, $variables = array()) {
  static $default_attributes;
  // If called before all modules are loaded, we do not necessarily have a full
  // theme registry to work with, and therefore cannot process the theme
  // request properly. See also \Drupal\Core\Theme\Registry::get().
  if (!\Drupal::moduleHandler()->isLoaded() && !defined('MAINTENANCE_MODE')) {
    throw new Exception(t('theme() may not be called until all modules are loaded.'));
  }

  /** @var \Drupal\Core\Utility\ThemeRegistry $theme_registry */
  $theme_registry = \Drupal::service('theme.registry')->getRuntime();

  // If an array of hook candidates were passed, use the first one that has an
  // implementation.
  if (is_array($hook)) {
    foreach ($hook as $candidate) {
      if ($theme_registry->has($candidate)) {
        break;
      }
    }
    $hook = $candidate;
  }
  // Save the original theme hook, so it can be supplied to theme variable
  // preprocess callbacks.
  $original_hook = $hook;

  // If there's no implementation, check for more generic fallbacks. If there's
  // still no implementation, log an error and return an empty string.
  if (!$theme_registry->has($hook)) {
    // Iteratively strip everything after the last '__' delimiter, until an
    // implementation is found.
    while ($pos = strrpos($hook, '__')) {
      $hook = substr($hook, 0, $pos);
      if ($theme_registry->has($hook)) {
        break;
      }
    }
    if (!$theme_registry->has($hook)) {
      // Only log a message when not trying theme suggestions ($hook being an
      // array).
      if (!isset($candidate)) {
        watchdog('theme', 'Theme hook %hook not found.', array('%hook' => $hook), WATCHDOG_WARNING);
      }
      // There is no theme implementation for the hook passed. Return FALSE so
      // the function calling theme() can differentiate between a hook that
      // exists and renders an empty string and a hook that is not implemented.
      return FALSE;
    }
  }

  $info = $theme_registry->get($hook);
  global $theme_path;
  $temp = $theme_path;
  // point path_to_theme() to the currently used theme path:
  $theme_path = $info['theme path'];

  // Include a file if the theme function or variable preprocessor is held
  // elsewhere.
  if (!empty($info['includes'])) {
    foreach ($info['includes'] as $include_file) {
      include_once DRUPAL_ROOT . '/' . $include_file;
    }
  }

  // If a renderable array is passed as $variables, then set $variables to
  // the arguments expected by the theme function.
  if (isset($variables['#theme']) || isset($variables['#theme_wrappers'])) {
    $element = $variables;
    $variables = array();
    if (isset($info['variables'])) {
      foreach (array_keys($info['variables']) as $name) {
        if (isset($element["#$name"]) || array_key_exists("#$name", $element)) {
          $variables[$name] = $element["#$name"];
        }
      }
    }
    else {
      $variables[$info['render element']] = $element;
      // Give a hint to render engines to prevent infinite recursion.
      $variables[$info['render element']]['#render_children'] = TRUE;
    }
  }

  // Merge in argument defaults.
  if (!empty($info['variables'])) {
    $variables += $info['variables'];
  }
  elseif (!empty($info['render element'])) {
    $variables += array($info['render element'] => array());
  }
  // Supply original caller info.
  $variables += array(
    'theme_hook_original' => $original_hook,
  );

  // Set base hook for later use. For example if '#theme' => 'node__article'
  // is called, we run hook_theme_suggestions_node_alter() rather than
  // hook_theme_suggestions_node__article_alter(), and also pass in the base
  // hook as the last parameter to the suggestions alter hooks.
  if (isset($info['base hook'])) {
    $base_theme_hook = $info['base hook'];
  }
  else {
    $base_theme_hook = $hook;
  }

  // Invoke hook_theme_suggestions_HOOK().
  $suggestions = Drupal::moduleHandler()->invokeAll('theme_suggestions_' . $base_theme_hook, array($variables));
  // If theme() was invoked with a direct theme suggestion like
  // '#theme' => 'node__article', add it to the suggestions array before
  // invoking suggestion alter hooks.
  if (isset($info['base hook'])) {
    $suggestions[] = $hook;
  }
  // Allow suggestions to be altered via hook_theme_suggestions_HOOK_alter().
  Drupal::moduleHandler()->alter('theme_suggestions_' . $base_theme_hook, $suggestions, $variables);

  // Check if each suggestion exists in the theme registry, and if so,
  // use it instead of the hook that theme() was called with. For example, a
  // function may call theme('node', ...), but a module can add
  // 'node__article' as a suggestion via hook_theme_suggestions_HOOK_alter(),
  // enabling a theme to have an alternate template file for article nodes.
  foreach (array_reverse($suggestions) as $suggestion) {
    if ($theme_registry->has($suggestion)) {
      $info = $theme_registry->get($suggestion);
      break;
    }
  }

  // Invoke the variable preprocessors, if any.
  if (isset($info['base hook'])) {
    $base_hook = $info['base hook'];
    $base_hook_info = $theme_registry->get($base_hook);
    // Include files required by the base hook, since its variable preprocessors
    // might reside there.
    if (!empty($base_hook_info['includes'])) {
      foreach ($base_hook_info['includes'] as $include_file) {
        include_once DRUPAL_ROOT . '/' . $include_file;
      }
    }
    // Replace the preprocess functions with those from the base hook.
    if (isset($base_hook_info['preprocess functions'])) {
      // Set a variable for the 'theme_hook_suggestion'. This is used to
      // maintain backwards compatibility with template engines.
      $theme_hook_suggestion = $hook;
      $info['preprocess functions'] = $base_hook_info['preprocess functions'];
    }
  }
  if (isset($info['preprocess functions'])) {
    foreach ($info['preprocess functions'] as $preprocessor_function) {
      if (function_exists($preprocessor_function)) {
        $preprocessor_function($variables, $hook, $info);
      }
    }
  }

  // Generate the output using either a function or a template.
  $output = '';
  if (isset($info['function'])) {
    if (function_exists($info['function'])) {
      $output = $info['function']($variables);
    }
  }
  else {
    $render_function = 'twig_render_template';
    $extension = '.html.twig';

    // The theme engine may use a different extension and a different renderer.
    global $theme_engine;
    if (isset($theme_engine)) {
      if ($info['type'] != 'module') {
        if (function_exists($theme_engine . '_render_template')) {
          $render_function = $theme_engine . '_render_template';
        }
        $extension_function = $theme_engine . '_extension';
        if (function_exists($extension_function)) {
          $extension = $extension_function();
        }
      }
    }

    // In some cases, a template implementation may not have had
    // template_preprocess() run (for example, if the default implementation is
    // a function, but a template overrides that default implementation). In
    // these cases, a template should still be able to expect to have access to
    // the variables provided by template_preprocess(), so we add them here if
    // they don't already exist. We don't want the overhead of running
    // template_preprocess() twice, so we use the 'directory' variable to
    // determine if it has already run, which while not completely intuitive,
    // is reasonably safe, and allows us to save on the overhead of adding some
    // new variable to track that.
    if (!isset($variables['directory'])) {
      $default_template_variables = array();
      template_preprocess($default_template_variables, $hook, $info);
      $variables += $default_template_variables;
    }
    if (!isset($default_attributes)) {
      $default_attributes = new Attribute();
    }
    foreach (array('attributes', 'title_attributes', 'content_attributes') as $key) {
      if (isset($variables[$key]) && !($variables[$key] instanceof Attribute)) {
        if ($variables[$key]) {
          $variables[$key] = new Attribute($variables[$key]);
        }
        else {
          // Create empty attributes.
          $variables[$key] = clone $default_attributes;
        }
      }
    }

    // Render the output using the template file.
    $template_file = $info['template'] . $extension;
    if (isset($info['path'])) {
      $template_file = $info['path'] . '/' . $template_file;
    }
    // Add the theme suggestions to the variables array just before rendering
    // the template for backwards compatibility with template engines.
    $variables['theme_hook_suggestions'] = $suggestions;
    // For backwards compatibility, pass 'theme_hook_suggestion' on to the
    // template engine. This is only set when calling a direct suggestion like
    // '#theme' => 'menu_tree__shortcut_default' when the template exists in the
    // current theme.
    if (isset($theme_hook_suggestion)) {
      $variables['theme_hook_suggestion'] = $theme_hook_suggestion;
    }
    $output = $render_function($template_file, $variables);
  }

  // restore path_to_theme()
  $theme_path = $temp;
  return (string) $output;
}

/**
 * Returns the path to the current themed element.
 *
 * It can point to the active theme or the module handling a themed
 * implementation. For example, when invoked within the scope of a theming call
 * it will depend on where the theming function is handled. If implemented from
 * a module, it will point to the module. If implemented from the active theme,
 * it will point to the active theme. When called outside the scope of a
 * theming call, it will always point to the active theme.
 */
function path_to_theme() {
  global $theme_path;

  if (!isset($theme_path)) {
    drupal_theme_initialize();
  }

  return $theme_path;
}

/**
 * Allows themes and/or theme engines to discover overridden theme functions.
 *
 * @param $cache
 *   The existing cache of theme hooks to test against.
 * @param $prefixes
 *   An array of prefixes to test, in reverse order of importance.
 *
 * @return $implementations
 *   The functions found, suitable for returning from hook_theme;
 */
function drupal_find_theme_functions($cache, $prefixes) {
  $implementations = array();
  $functions = get_defined_functions();

  foreach ($cache as $hook => $info) {
    foreach ($prefixes as $prefix) {
      // Find theme functions that implement possible "suggestion" variants of
      // registered theme hooks and add those as new registered theme hooks.
      // The 'pattern' key defines a common prefix that all suggestions must
      // start with. The default is the name of the hook followed by '__'. An
      // 'base hook' key is added to each entry made for a found suggestion,
      // so that common functionality can be implemented for all suggestions of
      // the same base hook. To keep things simple, deep hierarchy of
      // suggestions is not supported: each suggestion's 'base hook' key
      // refers to a base hook, not to another suggestion, and all suggestions
      // are found using the base hook's pattern, not a pattern from an
      // intermediary suggestion.
      $pattern = isset($info['pattern']) ? $info['pattern'] : ($hook . '__');
      if (!isset($info['base hook']) && !empty($pattern)) {
        $matches = preg_grep('/^' . $prefix . '_' . $pattern . '/', $functions['user']);
        if ($matches) {
          foreach ($matches as $match) {
            $new_hook = substr($match, strlen($prefix) + 1);
            $arg_name = isset($info['variables']) ? 'variables' : 'render element';
            $implementations[$new_hook] = array(
              'function' => $match,
              $arg_name => $info[$arg_name],
              'base hook' => $hook,
            );
          }
        }
      }
      // Find theme functions that implement registered theme hooks and include
      // that in what is returned so that the registry knows that the theme has
      // this implementation.
      if (function_exists($prefix . '_' . $hook)) {
        $implementations[$hook] = array(
          'function' => $prefix . '_' . $hook,
        );
      }
    }
  }

  return $implementations;
}

/**
 * Allows themes and/or theme engines to easily discover overridden templates.
 *
 * @param $cache
 *   The existing cache of theme hooks to test against.
 * @param $extension
 *   The extension that these templates will have.
 * @param $path
 *   The path to search.
 */
function drupal_find_theme_templates($cache, $extension, $path) {
  $implementations = array();

  // Collect paths to all sub-themes grouped by base themes. These will be
  // used for filtering. This allows base themes to have sub-themes in its
  // folder hierarchy without affecting the base themes template discovery.
  $theme_paths = array();
  foreach (list_themes() as $theme_info) {