Block Caching “Gotcha” in Drupal 8

I recently had a fight with the Block system in Drupal 8. To be brief, if you’re trying to disable caching on a block, make sure to set the #cache element regardless of whether the block has output or not.

This does not work (empty result gets cached):

  /**
   * {@inheritdoc}
   */
  public function build() {
    $output = [];

    $user_id = \Drupal::routeMatch()->getRawParameter('user');
    $current_user_id = \Drupal::currentUser()->id();

    if ($user_id == $current_user_id) {
      $output = [
        '#theme' => 'item_list',
        '#list_type' => 'ul',
        '#items' => [
          Link::fromTextAndUrl(t('Edit profile'), Url::fromRoute('entity.user.edit_form', ['user' => $user_id])),
          Link::fromTextAndUrl(t('Logout'), Url::fromUserInput('/user/logout')),
        ],
        '#wrapper-attributes' => ['class' => 'member-utilities'],
        '#cache' => [
          'max-age' => 0,
        ],
      ];
    }

    return $output;
  }

/**

* {@inheritdoc}

public function build() {

$output = [];

$user_id = \Drupal::routeMatch()->getRawParameter('user');

$current_user_id = \Drupal::currentUser()->id();

if ($user_id == $current_user_id) {

$output = [

'#theme' => 'item_list',

'#list_type' => 'ul',

'#items' => [

Link::fromTextAndUrl(t('Edit profile'), Url::fromRoute('entity.user.edit_form', ['user' => $user_id])),

Link::fromTextAndUrl(t('Logout'), Url::fromUserInput('/user/logout')),

'#wrapper-attributes' => ['class' => 'member-utilities'],

'#cache' => [

'max-age' => 0,

];

}

return $output;

}

This does work (nothing gets cached, as desired):

  /**
   * {@inheritdoc}
   */
  public function build() {
    $output = [
      '#cache' => [
        'max-age' => 0,
      ],
    ];

    $user_id = \Drupal::routeMatch()->getRawParameter('user');
    $current_user_id = \Drupal::currentUser()->id();

    if ($user_id == $current_user_id) {
      $output['#theme'] = 'item_list';
      $output['#list_type'] = 'ul';
      $output['#items'] = [
        Link::fromTextAndUrl(t('Edit profile'), Url::fromRoute('entity.user.edit_form', ['user' => $user_id])),
        Link::fromTextAndUrl(t('Logout'), Url::fromUserInput('/user/logout')),
      ];
    }

    return $output;
  }

/**