drupal_set_title() and drupal_get_title() were removed

drupal_set_title()

Titles on routes now can be set on various ways, depending on your usecase.
Previously just drupal_set_title() was called in whatever place. The following usecases exist:

1. Static title:
   For static titles you set a '_title' on the routing definition:

   block.admin_add:
     path: '/admin/structure/block/add/{plugin_id}/{theme}'
     defaults:
       _controller: '\Drupal\block\Controller\BlockAddController::blockAddConfigureForm'
       _title: 'Configure block'
     requirements:
       _permission: 'administer blocks'
   

2. Dynamic title:
   If you write a controller and you need a dynamic title, for example depending on the site configuration, use _title_callback in the route defaults.

   mymodule.test:
     path: '/mymodule/test'
     defaults:
       _controller: '\Drupal\mymodule\Controller\Test::getContent'
       _title_callback: '\Drupal\mymodule\Controller\Test::getTitle'
   

   class Test {
   
     /**
      * Returns a page title.
      */
     public function getTitle() {
       return  'Foo: ' . \Drupal::config('system.site')->get('name');
     }
   
     /**
      * Returns a page render array.
      */
     public function getContent() {
       $build = array();
       $build['#markup'] = 'Hello Drupal';
       return $build;
     }
   }
   

3. Final title override:

   If you write a controller and you need to override the title from the route, you can return #title in the render array. This should generally be avoided, since the title for the page when fully rendered could be different from the title in other contexts (like in the breadcrumb).

   class Test {
   
     /**
      * Renders a page with a title.
      *
      * @return array
      *   A render array as expected by drupal_render()
      */
     public function getContentWithTitle() {
       $build = array();
       $build['#markup'] = 'Hello Drupal';
       $build['#title'] = 'Foo: ' . \Drupal::config('system.site')->get('name');
   
       return $build;
     }
   
   }
   

Output flag of drupal_set_title #

The output validation in Drupal 8 is opposite to Drupal 7. We have to explicitly specify PASS_THROUGH and CHECK_PLAIN is by default in Drupal 7, whereas in Drupal 8 the situation is different. Output is autoescaped unless it is marked safe. Both t() and new FormattableMarkup return objects that will not be autoescaped.

Drupal 7:

  drupal_set_title(t('Add new shortcut'));

Drupal 8:

  $form['#title'] = $this->t('Add new shortcut');

Drupal 7:

  drupal_set_title(t("%name block", array('%name' => $info[$block->delta]['info'])), PASS_THROUGH);

Drupal 8

  $form['#title'] = $this->t("'%name' block", array('%name' => $info[$block->delta]['info']));

drupal_get_title()

As titles on routes now can be set on various ways (see above), drupal_get_title() has been removed. In its place you should call the title_resolver service.

Drupal 7:

  $title = drupal_get_title();

Drupal 8:

  $request = \Drupal::request();
  $route_match = \Drupal::routeMatch();
  $title = \Drupal::service('title_resolver')->getTitle($request, $route_match->getRouteObject());