Get list of preprocess block variables

Question

How can I display a list of preprocess block variables so I can conditionally include a library? Using the 'plugin_id' doesn't seem to work and when I echo all of the variables all I see is a blank page. Is there a Drupal page that will show me what all the variables are for blocks?

My custom block module is called, 'leaflet_block'.

function leaflet_block_preprocess_block(&$variables) {
  echo "<pre>"; print_r($variables); echo "</pre>"; // shows a blank page

  if ($variables['plugin_id'] == 'leaflet_block') {
    $variables['#attached']['library'][] = 'leaflet_block/leaflet_block';
  }
}

Answer 1

echo "<pre>"; print_r($variables); echo "</pre>";

That's not how it should be done, instead, install the Devel module.

This will allow you to do:

dump($variables);

If you haven't turned caching off, for developing purposes, then you will have to flush all caches to see the dump.

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In addition, if you also install the kint package/library with:

composer require kint-php/kint

now you could also do:

kint($variables);

_{Related: Welcome to Kint 101}