How to clear the a block cache programmically

Question

I have a custom block that display some information from a custom configuration form. I want the block to be cached, however, I want the block cache to be cleared every time I save the form.

I would like to do this in the submitForm function. How do I go about and do this?

Answer 1

When displaying information from a config object add it also as cache dependency to the render array.

For example in a preprocess hook:

  $config = \Drupal::config('mymodule.settings');
  $config_value = $config->get('some_field');
  $variables['some_field'] = [
    '#markup' => $config_value,
  ];
  // add the cache tag, so that the output gets invalidated when the config is saved
  \Drupal::service('renderer')->addCacheableDependency($variables, $config);

See How to clear cache for config entity after making changes?

You don't need to change form submit because saving a config object automatically invalidates cache entries having this cache dependency (cache tag).

---

Another example based on the block code from your answer and using a different method to add the cache tag:

  /**
   * Config settings.
   *
   * @var string
   */
  const SETTINGS = 'uw_cbl_special_alert.settings';
  /**
   * {@inheritdoc}
   */
  public function build() {
    $config = \Drupal::config(static::SETTINGS);
    return [
      '#markup' => $config->get('message'),
      '#cache' => [
        'tags' => $config->getCacheTags(),
      ],
    ];
  }

---

Access caching

As asked in the comments, you can add cache dependencies to access results as well:

AccessResult::allowed()->addCacheableDependency($config)
AccessResult::forbidden()->addCacheableDependency($config)

Answer 2

Ok I believe I figured it out. This works for me.

use Drupal\Core\Cache\Cache;
class ConfigurationForm extends ConfigFormBase {
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Invalidate block cache.
    Cache::invalidateTags(['config:block.block.{block_id_tag}']);
  }
}

I had to search the cachetags table in the database to find the block id tag.
In my case my custom block id in the annotation was uw_cbl_special_alert but the block id tag was specialalert

In case you are interested, code snippet from custom block

  /**
   * Config settings.
   *
   * @var string
   */
  const SETTINGS = 'uw_cbl_special_alert.settings';
  /**
   * {@inheritdoc}
   */
  public function build() {
    $config = \Drupal::config(static::SETTINGS);
    // Turn caching off as this is a special message and show never be cached.
    return [
      '#markup' => $config->get('message'),
    ];
  }