How do I pass a theme variable from a template to another?

Question

Several fields in a node need to be displayed in a particular way depending upon which newsletter body they are a part of. How do I pass a variable from the enclosing newsletter body to enclosed fields so that I can inform the field of its context and control the way the field is displayed?

I have a theme that has preprocess functions both for the newsletter body and for fields contained within the body.

I want to set a variable in the body preprocess function, and its related twig template, that can be inspected by the field preprocess function, and/or its own template.

In other words, I want each field to be aware of the body context in which it is being rendered.

Specifically, I have mytheme_preprocess_simplenews_newsletter_body() that sets a variable ($variables['summons'] = 'placeholder';) which can be inspected (and also set) by the theme's simplenews-newsletter-body.html.twig template, but (because it's in a different render array) is subsequently neither visible to mytheme_preprocess_field() nor field--node--title.html.twig.

How do I communicate between the two render arrays?

Note that

1. There are apparently global variables like $db_is_active and $is_admin.

2. Sometimes, it is possible to achieve this kind of thing by cascading CSS class attributes from a containing element to a contained element. This is not generally satisfactory and in this particular case, it won't wash on different target mail clients unpredictable treatment of CSS.

Answer 1

The solution I am working with is to use drupal_static(); simplified essentials in code below - take care with the &ampersands. It is giving me control where I need it.

So, using this approach, I can - taking their global context into account - per field, set variables in the usual way e.g. in mytheme_preprocess_field() subsequently to be used in a related template like field--node--title--agenda.html.twig

function mytheme_preprocess_simplenews_newsletter_body(&$variables) {
  // prepare to alter static var ...
  // get a reference binding to static var
  $var = &myvariable_function();
  // alter static var
  $var = 'simplenews_newsletter';
}

function mytheme_preprocess_field(&$variables, $hook) {
  // prepare to read static var
  // assign static var
  $var = myvariable_function();
  // $element is the render array for the field
  $element = $variables['element'];
  // filter those cases where it matters that we're theming a newsletter
  if ($element['#view_mode'] == 'email_html' and $var == 'simplenews_newsletter') {
    // Do stuff here like setting a variable for twig ...
    $variables['summons'] = 'some text';
  }
}
function &myvariable_function() {
  $var = &drupal_static(__FUNCTION__);
  if (!isset($var)) {
    // generate contents of static variable
    $var =  'initial_value';
  }
  return $var;
}

I am sure that there are other ways of communicating common context between different levels of a page's render hierarchy. Let me know how you'd do it.

Answer 2

I use this code:

/**
 * Helper function to pass slider variables from preprocess_page to preprocess_page_title_custom
 * @param  string $new_image Image URL
 * @return array             Images URL Array
 */
function _core_slider_images($new_image = NULL) {
  $vars = &drupal_static(__FUNCTION__, array());
  // If a new value has been passed add to the array.
  if ($new_image) {
    $vars[] = $new_image;
  }
  return $vars;
}

From this template:

In page.html.twig we have access to the $node var, and we need to pass the values to another template that don't have access to this var. We will search the values that we need to pass and call the _core_slider_images(), this function will be the bridge between the two templates (preprocess functions)

/**
 * Implements hook_preprocess_page().
 */
function MODULE_preprocess_page(&$variables) {
  // ...
  // Sending Images URL and titles from page to page_title.
    for ($i = 0; $i < $cant; $i++) {
      // Adding the images to the static function.
      _core_slider_images($variables['node']->field_image[$i]->entity->url());
    }
  }

To this template:

page-title-custom.html.twig is a custom template and don't have access to $node. As hook_preprocess_page() is called first we have stored the $node values in a static var, and we only need to call the _core_slider_images() function.

/**
 * Implements hook_preprocess_HOOK().
 */
function MODULE_preprocess_page_title_custom(&$variables) {
  // Searching the variables for sending to the page-title-custom template.
   $images = _core_slider_images();
}

Answer 3

I'm still new to D8, so I could easily be missing a trick. Generally in this situation you want to look for the most specific hook that gives you access to everything you want to alter. So altering at the field level is too specific and at the page level (for example) is too general.

Unfortunately MailEntity isn't a real (Drupal) entity, so you can't use standard hooks to alter its render array. It doesn't invoke any custom hooks I can see to allow you to alter the content either. In D7 in this situation I'd be tempted to use hook_page_alter() which is dirty but would work I think, but that's been removed from D8.

You could always override the MailEntity class and provide your own customisations directly there, but needing to do that for theme changes does seem a bit wrong to me.