Add a custom variable to an existing block

Question

Is there a way to alter the render array of an existing block? I want to do it for a commerce block (cart block) but the question remains for any core or contrib module block.

When a block uses a custom #theme for it's block, is there a way to alter the definition?

Let's say the module defines the following theme :

/**
 * Implements hook_theme().
 */
function commerce_cart_theme($existing, $type, $theme, $path) {
  return [
    'commerce_cart_block' => [
      'variables' => [
        'icon' => NULL,
        'count' => NULL,
        'count_text' => '',
        'content' => NULL,
        'url' => NULL,
        'links' => [],
      ],
    ],
  ];
}

and the block uses '#theme' => 'commerce_cart_block' as it's build render array.

I'd like to know if there's a way to add a new custom variable to commerce_cart_block in a hook somewhere.

I tried to rewrite the commerce-cart-block.html.twig in my theme and add a new variable in it and I'd set my variable in THEME_preprocess_block__commerce_cart(), but it doesnt work.

I also tried to alter the block's content with HOOK_block_view_alter in a module, but it also didn't work.

Answer 1

You can do it with hook_preprocess_HOOK and the all caps HOOK part refers to the array key in hook_theme().

/**
 * Implements hook_preprocess_HOOK().
 */
function mytheme_preprocess_commerce_cart_block(&$variables) {
  // Adds a custom variable
  $variables['hello_world'] = 'Hello World';
}

BTW, this hook can be used either in a custom theme or custom module.

and in commerce-cart-block.html.twig

{{ hello_world }}

should print "Hello World".