<?php

/**
 * @file
 * Module for adding classes to blocks.
 */

use Drupal\block\Entity\Block;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function block_class_form_block_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if (\Drupal::currentUser()->hasPermission('administer block classes')) {

    /** @var \Drupal\block\BlockInterface $block */
    $block = $form_state->getFormObject()->getEntity();

    // This will automatically be saved in the third party settings.
    $form['third_party_settings']['#tree'] = TRUE;
    $form['third_party_settings']['block_class']['classes'] = array(
      '#type' => 'textfield',
      '#title' => t('CSS class(es)'),
      '#description' => t('Customize the styling of this block by adding CSS classes. Separate multiple classes by spaces.'),
      '#default_value' => $block->getThirdPartySetting('block_class', 'classes'),
    );

  }
}

/**
 * Implements hook_preprocess_HOOK().
 */
function block_class_preprocess_block(&$variables) {
  // Blocks coming from page manager widget does not have id.
  if (!empty($variables['elements']['#id'])) {
    $block = Block::load($variables['elements']['#id']);
    if ($classes = $block->getThirdPartySetting('block_class', 'classes')) {
      $variables['attributes']['class'][] = $classes;
    }
  }
}