Programmatically change block content on page render only

Question

I have a situation where I want to change the content of a block on certain conditions, but not permanently.

I'm using a theme_preprocess_block() hook to change the content of the block, but it's not carrying through to the page render:

function MYTHEME_preprocess_block(&$variables) {

  if ( $variables['base_plugin_id'] == 'block_content'
    && $variables['attributes']['id'] == 'block-mycustomfooterblock' ) {

    $variables['content']['#block_content']->set('body', [[
      'value' => 'New Content',
      'format' => 'full_html',
    ]]);

Even after clearing caches, the block content doesn't get replaced with "New Content".

I added this line

$variables['content']['#block_content']->save();

However, that permanently saved the change to the block, which I don't want.

How can I alter the block content just for the page render?

Answer 1

You should probably try

$variables['content']['body'][0]['#text']= "<strong>html content</strong>";

complete example

function THEMENAME_preprocess_block(&$variables) {
  if ( $variables['attributes']['id'] == "block-newblock" ) {
    $variables['content']['body'][0]['#text']= "<strong>html content</strong>";
    $variables['content']['body'][0]['#format']= "full_html";
  }  
}

Answer 2

You could write a pre_render function that gets called by hook_block_alter. So in your .module file, you might try this:

use Drupal\Core\Render\Markup;

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

function _MYMODULE_block_customfooter_prerender(array $build) {
    $build['content']['#markup'] = Markup::create('New Content');
    return $build;
}