Alter block content

Question

I am trying to alter a custom module block, without success. My block class is the following.

class BreadcrumbBlock extends BlockBase {

public function build() {
    return [
        '#theme' => 'breadcrumb_block',
        '#content' => array('value' => 1),
        '#cache' => array(
                'contexts' => array('url'),
        ),
    ];
 }
 .....

In hook_block_view_alter() where can I alter the block? Where are $data['subject'] and $data['content'] in a block?

Answer 1

I think you must use a 'pre_render'. Some thing like : http://www.drupal8.ovh/en/tutoriels/150/change-drupal-8-powered-by-block

use Drupal\Core\Render\Markup;

function yourmodule_block_view_alter(array &$build, \Drupal\Core\Block\BlockPluginInterface $block) {
  if ($block->getBaseId() === 'system_powered_by_block') {
    $build['#pre_render'][] = '_yourmodule_block_poweredby_prerender';
  }
}

function _yourmodule_block_poweredby_prerender(array $build) {
    $build['content']['#markup'] = Markup::create('Your text');
    return $build;
}

Answer 2

Another method which can be used to alter any block without additional #pre_render callback. Note that custom callback function is called after the system Drupal\block\BlockViewBuilder::preRender() and may be not rendered properly. For example, if you add content to an empty block using the method from the above comment then the title of the block will be lost.

Go to the src folder in your module and create OverrideAnyBlockContent.php file with the following content:

<?php

namespace Drupal\YOUR_MODULE;

/**
 * Helper class to override any block content in a HOOK_block_view_alter().
 */
class OverrideAnyBlockContent {

  /**
   * The render array.
   *
   * @var array
   * @see https://www.drupal.org/docs/8/api/render-api/render-arrays
   */
  protected $renderArray;

  public function __construct(array $render_array) {
    $this->renderArray = $render_array;
  }

  public function getPlugin() {
    return $this;
  }

  public function build() {
    return $this->renderArray;
  }

}

Then in YOUR_MODULE.module file:

function YOUR_MODULE_block_view_alter(array &$build, Drupal\Core\Block\BlockPluginInterface $block) {
if ($block->getBaseId() === 'system_powered_by_block') {
    // @see https://www.drupal.org/docs/8/api/render-api/render-arrays
    $render_array = [
      '#type' => 'markup',
      '#markup' => "Any text, HTML, {$php_variable}, whatever, ....",
    ];

    // Uncomment the line below if you need to merge new render array with the
    // original one.
    // $render_array += $build['#block']->getPlugin()->build();

    // Now block will be pre-rendered using new render array. No additional
    // '#pre_render' callback is required.
    // @see Drupal\block\BlockViewBuilder::preRender()
    $build['#block'] = new Drupal\YOUR_MODULE\OverrideAnyBlockContent($render_array);
  }
}

Taken from examples module issue: https://www.drupal.org/node/2893964

Answer 3

The simpliest way would be to use a preprocess hook like so:

function MODULE_OR_THEME_NAME_preprocess_block__BLOCK_ID(&$variables, $hook) {
   // block render array is here to be altered => $variables['content']
}