How to create a custom Twig Template for Menu Block

Question

I'm using a menu block to output a section of my main menu into a region so I can have a "In This Section" type of sub menu on every page.

Unfortunately, I can't find a way to pinpoint in menu.html.twig which rendition of "main_menu" I am working with. (they both have the same "menu_name") I want to adjust the markup for the menu block, but not for my main menu.

A similar question was asked here: Additional twig suggestions for menus in Drupal 8

I don't quite understand the suggested answer. Is there some way I can extend some class to call a different Twig Template? Is there some hook I'm missing?

Answer 1

Ok, I think I found the answer. Kristian Kaa answers it on his blog here: http://kristiankaa.dk/article/drupal8-region-specific-menu-theme-hook-suggestion

Basically you use 2 preprocess functions like so:

/**
 * Implements hook_preprocess_block().
 */
function mymodule_preprocess_block(&$variables) {
  $variables['content']['#attributes']['block'] =    $variables['attributes']['id'];
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function mymodule_theme_suggestions_menu_alter(array &$suggestions, array $variables) {
  // Remove the block and replace dashes with underscores in the block ID to
  // use for the hook name.
  if (isset($variables['attributes']['block'])) {
    $hook = str_replace(array('block-', '-'), array('', '_'),    $variables['attributes']['block']);
    $suggestions[] = $variables['theme_hook_original'] . '__' . $hook;
  }
}

This will add the block ID as a new theme hook suggestion. Many thanks to Kristian.

Answer 2

FWIW, I ran into an issue while using Greg's answer where I was getting errors about $variables['attributes']['id'] not having the "id" key. To fix this I added a check to ensure that this key is set.

/**
 * Implements hook_preprocess_block().
 * http://kristiankaa.dk/article/drupal8-region-specific-menu-theme-hook-suggestion
 */
function THEMENAME_preprocess_block(&$variables) {
  if(isset($variables['attributes']['id'])) {
    $variables['content']['#attributes']['block'] = $variables['attributes']['id'];
  }
}