block_form_alter-8.x-1.x-dev/tests/modules/block_form_alter_test/block_form_alter_test.module

<?php

/**
 * @file
 * This module provides block form alter test functions.
 */

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_block_type_form_alter().
 */
function block_form_alter_test_block_type_form_alter(array &$form, FormStateInterface &$form_state, string $block_type) {
  if ($block_type == 'test_block') {
    $form['body']['widget'][0]['#default_value'] = 'test body string';
  }
}

/**
 * Implements hook_block_plugin_form_alter().
 */
function block_form_alter_test_block_plugin_form_alter(array &$form, FormStateInterface &$form_state, string $plugin) {
  if ($plugin == 'system_branding_block') {
    $form['settings']['block_branding']['use_site_logo']['#default_value'] = FALSE;
  }
}

/**
 * Implements hook_theme_registry_alter().
 */
function block_form_alter_test_theme_registry_alter(&$theme_registry) {
  $theme_registry['block']['path'] = \Drupal::service('extension.list.module')->getPath('block_form_alter_test') . '/templates/block';
}