How do I add a variable for a Twig template in my preprocess function?

Question

I'm using the following code to preprocess the variables for my template:

function template_preprocess_imagegallery_format(&$vars) {
  template_preprocess_image_formatter($vars);
  $vars['image']['#theme'] = 'igimage';
  $vars['image']['#thumbnail'] = ImageStyle::load('thumbnail')->buildUrl($vars ['image']['#uri']);
  $vars['image']['#fullimage'] = file_create_url($vars ['image']['#uri']);
}

The #uri variable is already present, and if I just modify it everything works fine. But I need to add additional variables here as I need two different styles of the same image. But simply adding those variables to the array doesn't work.

If I dump the available variables in my Twig template with {{ dump(_context|keys) }}, I don't see the variables I added there. If I try to use them in my template, nothing gets inserted.

I can see that the values are added to the array, when I dump the image variable in the theme that renders the entire field, I see my variables in the array. But they're still not available in the template that renders the image variable itself.

What else do I need to do to add a variable in my preprocess function that I can use in my Twig template?

Answer 1

I finally solved the problem, it turns out the problem was that I tried to add the variables in the wrong preprocessor. Editing the #theme works there, but to add variables I needed to preprocess the theme I set there:

function template_preprocess_imagegallery_format(&$vars) {
  template_preprocess_image_formatter($vars);
  $vars['image']['#theme'] = 'igimage';
}

function template_preprocess_igimage(&$vars) {
    $vars['thumbnail'] = ImageStyle::load('thumbnail')->buildUrl($vars['uri']);
    $vars['fullimage'] = file_create_url($vars['uri']);
}

Answer 2

For a theme whose machine name is atvdirect, create the atvdirect.theme file in the directory containing the .info.yml file for the theme, and add the following code.

function atvdirect_preprocess_page(&$variables) {
  $variables['logopath'] = '/'.drupal_get_path('theme','atvdirect') .'/logo.png';
}

Now you can use {{ logopath }} in the page.html.twig template file.