From 6a6e601d4a77bb8269a66b1d0d9438ae7048609a Mon Sep 17 00:00:00 2001
From: webchick <drupal@webchick.net>
Date: Sat, 23 Aug 2014 14:18:18 -0700
Subject: [PATCH] Issue #2309323 by Berdir, tim.plunkett: Allow #submit and
 #validate to be specified as methods of the form object.

---
 core/lib/Drupal/Core/Entity/EntityForm.php    | 16 ++-----
 core/lib/Drupal/Core/Form/FormBuilder.php     |  8 ++--
 core/lib/Drupal/Core/Form/FormState.php       | 11 +++++
 .../Drupal/Core/Form/FormStateInterface.php   | 14 ++++++
 core/lib/Drupal/Core/Form/FormSubmitter.php   |  6 +--
 core/lib/Drupal/Core/Form/FormValidator.php   |  6 +--
 .../Core/Installer/Form/SiteSettingsForm.php  |  2 +-
 core/modules/block/src/BlockForm.php          |  2 +-
 core/modules/comment/src/CommentForm.php      |  9 +---
 .../src/Form/ConfigSingleExportForm.php       |  4 +-
 core/modules/contact/src/MessageForm.php      |  9 +---
 .../dblog/src/Form/DblogFilterForm.php        |  2 +-
 .../editor/src/Form/EditorImageDialog.php     |  2 +-
 .../editor/src/Form/EditorLinkDialog.php      |  2 +-
 .../src/Form/FieldInstanceEditForm.php        |  2 +-
 core/modules/forum/src/Form/Overview.php      |  2 +-
 .../language/src/Form/LanguageAddForm.php     |  8 ++--
 .../language/src/Form/LanguageEditForm.php    |  4 +-
 .../locale/src/Form/TranslateFilterForm.php   |  2 +-
 core/modules/node/src/NodeForm.php            | 13 ++---
 core/modules/path/src/Form/EditForm.php       |  2 +-
 core/modules/path/src/Form/PathFilterForm.php |  2 +-
 .../search/src/SearchPageListBuilder.php      |  6 +--
 .../shortcut/src/Form/SetCustomize.php        |  5 +-
 .../src/Controller/FormAjaxController.php     |  2 +
 core/modules/system/src/Form/CronForm.php     |  2 +-
 .../system/src/Form/DateFormatFormBase.php    |  2 +-
 .../system/src/Form/PerformanceForm.php       |  2 +-
 .../Tests/Form/FormDefaultHandlersTest.php    |  4 +-
 .../ajax_test/src/Form/AjaxTestDialogForm.php |  4 +-
 .../FormTestLimitValidationErrorsForm.php     | 14 +++---
 .../src/Form/FormTestProgrammaticForm.php     |  2 +-
 .../FormTestRebuildPreserveValuesForm.php     |  2 +-
 .../src/Form/FormTestStorageForm.php          |  4 +-
 .../src/Form/FormTestValidateRequiredForm.php |  2 +-
 .../taxonomy/src/Form/OverviewTerms.php       |  2 +-
 core/modules/taxonomy/src/VocabularyForm.php  |  2 +-
 core/modules/user/src/Form/UserLoginForm.php  |  6 +--
 core/modules/user/src/ProfileForm.php         |  2 +-
 .../src/Form/AdvancedSettingsForm.php         |  2 +-
 core/modules/views_ui/src/ViewAddForm.php     |  4 +-
 .../views_ui/src/ViewDuplicateForm.php        |  4 +-
 core/modules/views_ui/src/ViewEditForm.php    | 18 ++++---
 core/modules/views_ui/src/ViewPreviewForm.php |  2 +-
 .../Drupal/Tests/Core/Form/FormStateTest.php  | 47 +++++++++++++++++++
 .../Tests/Core/Form/FormSubmitterTest.php     | 11 ++++-
 46 files changed, 164 insertions(+), 115 deletions(-)

diff --git a/core/lib/Drupal/Core/Entity/EntityForm.php b/core/lib/Drupal/Core/Entity/EntityForm.php
index 3ca1d3838a6..3cf5558de3b 100644
--- a/core/lib/Drupal/Core/Entity/EntityForm.php
+++ b/core/lib/Drupal/Core/Entity/EntityForm.php
@@ -138,7 +138,7 @@ class EntityForm extends FormBase implements EntityFormInterface {
     $entity = $this->entity;
 
     // Add a process callback.
-    $form['#process'][] = array($this, 'processForm');
+    $form['#process'][] = '::processForm';
 
     if (!isset($form['langcode'])) {
       // If the form did not specify otherwise, default to keeping the existing
@@ -210,13 +210,8 @@ class EntityForm extends FormBase implements EntityFormInterface {
     $actions['submit'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Save'),
-      '#validate' => array(
-        array($this, 'validate'),
-      ),
-      '#submit' => array(
-        array($this, 'submit'),
-        array($this, 'save'),
-      ),
+      '#validate' => array('::validate'),
+      '#submit' => array('::submit', '::save'),
     );
 
     if (!$this->entity->isNew() && $this->entity->hasLinkTemplate('delete-form')) {
@@ -317,11 +312,6 @@ class EntityForm extends FormBase implements EntityFormInterface {
    */
   public function buildEntity(array $form, FormStateInterface $form_state) {
     $entity = clone $this->entity;
-    // If you submit a form, the form state comes from caching, which forces
-    // the form object to be the one before caching. Ensure to have the
-    // form object of the current request.
-    $form_state['controller'] = $this;
-
     $this->copyFormValuesToEntity($entity, $form, $form_state);
 
     // Invoke all specified builders for copying form values to entity
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 7d7ab24fd4f..bf9aa882c44 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -639,8 +639,8 @@ class FormBuilder implements FormBuilderInterface, FormValidatorInterface, FormS
 
     $form += $this->getElementInfo('form');
     $form += array('#tree' => FALSE, '#parents' => array());
-    $form['#validate'][] = array($form_state['build_info']['callback_object'], 'validateForm');
-    $form['#submit'][] = array($form_state['build_info']['callback_object'], 'submitForm');
+    $form['#validate'][] = '::validateForm';
+    $form['#submit'][] = '::submitForm';
 
     // If no #theme has been set, automatically apply theme suggestions.
     // theme_form() itself is in #theme_wrappers and not #theme. Therefore, the
@@ -767,9 +767,9 @@ class FormBuilder implements FormBuilderInterface, FormValidatorInterface, FormS
     // Allow for elements to expand to multiple elements, e.g., radios,
     // checkboxes and files.
     if (isset($element['#process']) && !$element['#processed']) {
-      foreach ($element['#process'] as $process) {
+      foreach ($element['#process'] as $callback) {
         $complete_form = &$form_state->getCompleteForm();
-        $element = call_user_func_array($process, array(&$element, &$form_state, &$complete_form));
+        $element = call_user_func_array($form_state->prepareCallback($callback), array(&$element, &$form_state, &$complete_form));
       }
       $element['#processed'] = TRUE;
     }
diff --git a/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
index 038e63f338b..50b75f7c222 100644
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -796,6 +796,17 @@ class FormState implements FormStateInterface, \ArrayAccess {
     return $this;
   }
 
+
+  /**
+   * {@inheritdoc}
+   */
+  public function prepareCallback($callback) {
+    if (is_string($callback) && substr($callback, 0, 2) == '::') {
+      $callback = array($this->get('build_info')['callback_object'], substr($callback, 2));
+    }
+    return $callback;
+  }
+
   /**
    * Wraps drupal_set_message().
    *
diff --git a/core/lib/Drupal/Core/Form/FormStateInterface.php b/core/lib/Drupal/Core/Form/FormStateInterface.php
index f7abf0e1332..164078a20a1 100644
--- a/core/lib/Drupal/Core/Form/FormStateInterface.php
+++ b/core/lib/Drupal/Core/Form/FormStateInterface.php
@@ -456,4 +456,18 @@ interface FormStateInterface {
    */
   public function setRebuild($rebuild = TRUE);
 
+  /**
+   * Converts support notations for a form callback to a valid callable.
+   *
+   * Specifically, supports methods on the form/callback object as strings when
+   * they start with ::, for example "::submitForm()".
+   *
+   * @param string|array $callback
+   *   The callback.
+   *
+   * @return array|string
+   *   A valid callable.
+   */
+  public function prepareCallback($callback);
+
 }
diff --git a/core/lib/Drupal/Core/Form/FormSubmitter.php b/core/lib/Drupal/Core/Form/FormSubmitter.php
index e99727e6040..ab7e76ef424 100644
--- a/core/lib/Drupal/Core/Form/FormSubmitter.php
+++ b/core/lib/Drupal/Core/Form/FormSubmitter.php
@@ -105,7 +105,7 @@ class FormSubmitter implements FormSubmitterInterface {
       $handlers = array();
     }
 
-    foreach ($handlers as $function) {
+    foreach ($handlers as $callback) {
       // Check if a previous _submit handler has set a batch, but make sure we
       // do not react to a batch that is already being processed (for instance
       // if a batch operation performs a
@@ -114,11 +114,11 @@ class FormSubmitter implements FormSubmitterInterface {
         // Some previous submit handler has set a batch. To ensure correct
         // execution order, store the call in a special 'control' batch set.
         // See _batch_next_set().
-        $batch['sets'][] = array('form_submit' => $function);
+        $batch['sets'][] = array('form_submit' => $callback);
         $batch['has_form_submits'] = TRUE;
       }
       else {
-        call_user_func_array($function, array(&$form, &$form_state));
+        call_user_func_array($form_state->prepareCallback($callback), array(&$form, &$form_state));
       }
     }
   }
diff --git a/core/lib/Drupal/Core/Form/FormValidator.php b/core/lib/Drupal/Core/Form/FormValidator.php
index de48d4c0ee6..5efedb089d6 100644
--- a/core/lib/Drupal/Core/Form/FormValidator.php
+++ b/core/lib/Drupal/Core/Form/FormValidator.php
@@ -79,8 +79,8 @@ class FormValidator implements FormValidatorInterface {
       $handlers = array();
     }
 
-    foreach ($handlers as $function) {
-      call_user_func_array($function, array(&$form, &$form_state));
+    foreach ($handlers as $callback) {
+      call_user_func_array($form_state->prepareCallback($callback), array(&$form, &$form_state));
     }
   }
 
@@ -263,7 +263,7 @@ class FormValidator implements FormValidatorInterface {
       elseif (isset($elements['#element_validate'])) {
         foreach ($elements['#element_validate'] as $callback) {
           $complete_form = &$form_state->getCompleteForm();
-          call_user_func_array($callback, array(&$elements, &$form_state, &$complete_form));
+          call_user_func_array($form_state->prepareCallback($callback), array(&$elements, &$form_state, &$complete_form));
         }
       }
 
diff --git a/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php b/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
index 1b08b475e09..48fadf64de9 100644
--- a/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
+++ b/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
@@ -104,7 +104,7 @@ class SiteSettingsForm extends FormBase {
         array('driver'),
         array($default_driver),
       ),
-      '#submit' => array(array($this, 'submitForm')),
+      '#submit' => array('::submitForm'),
     );
 
     $form['errors'] = array();
diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index e289086e4a4..e3d442d4e6f 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -101,7 +101,7 @@ class BlockForm extends EntityForm {
         '#title' => t('Theme'),
         '#default_value' => $theme,
         '#ajax' => array(
-          'callback' => array($this, 'themeSwitch'),
+          'callback' => '::themeSwitch',
           'wrapper' => 'edit-block-region-wrapper',
         ),
       );
diff --git a/core/modules/comment/src/CommentForm.php b/core/modules/comment/src/CommentForm.php
index 3ba804d3d7b..075e463f576 100644
--- a/core/modules/comment/src/CommentForm.php
+++ b/core/modules/comment/src/CommentForm.php
@@ -244,13 +244,8 @@ class CommentForm extends ContentEntityForm {
       '#type' => 'submit',
       '#value' => $this->t('Preview'),
       '#access' => $preview_mode != DRUPAL_DISABLED,
-      '#validate' => array(
-        array($this, 'validate'),
-      ),
-      '#submit' => array(
-        array($this, 'submit'),
-        array($this, 'preview'),
-      ),
+      '#validate' => array('::validate'),
+      '#submit' => array('::submit', '::preview'),
     );
 
     return $element;
diff --git a/core/modules/config/src/Form/ConfigSingleExportForm.php b/core/modules/config/src/Form/ConfigSingleExportForm.php
index 579501a4e09..6a22b168541 100644
--- a/core/modules/config/src/Form/ConfigSingleExportForm.php
+++ b/core/modules/config/src/Form/ConfigSingleExportForm.php
@@ -95,7 +95,7 @@ class ConfigSingleExportForm extends FormBase {
       '#options' => $config_types,
       '#default_value' => $config_type,
       '#ajax' => array(
-        'callback' => array($this, 'updateConfigurationType'),
+        'callback' => '::updateConfigurationType',
         'wrapper' => 'edit-config-type-wrapper',
       ),
     );
@@ -109,7 +109,7 @@ class ConfigSingleExportForm extends FormBase {
       '#prefix' => '<div id="edit-config-type-wrapper">',
       '#suffix' => '</div>',
       '#ajax' => array(
-        'callback' => array($this, 'updateExport'),
+        'callback' => '::updateExport',
         'wrapper' => 'edit-export-wrapper',
       ),
     );
diff --git a/core/modules/contact/src/MessageForm.php b/core/modules/contact/src/MessageForm.php
index 72aaeb33ca0..ca7be751bb2 100644
--- a/core/modules/contact/src/MessageForm.php
+++ b/core/modules/contact/src/MessageForm.php
@@ -156,13 +156,8 @@ class MessageForm extends ContentEntityForm {
     $elements['submit']['#value'] = $this->t('Send message');
     $elements['preview'] = array(
       '#value' => $this->t('Preview'),
-      '#validate' => array(
-        array($this, 'validate'),
-      ),
-      '#submit' => array(
-        array($this, 'submit'),
-        array($this, 'preview'),
-      ),
+      '#validate' => array('::validate'),
+      '#submit' => array('::submit', '::preview'),
     );
     return $elements;
   }
diff --git a/core/modules/dblog/src/Form/DblogFilterForm.php b/core/modules/dblog/src/Form/DblogFilterForm.php
index 4804e7ed8bb..140189396e5 100644
--- a/core/modules/dblog/src/Form/DblogFilterForm.php
+++ b/core/modules/dblog/src/Form/DblogFilterForm.php
@@ -59,7 +59,7 @@ class DblogFilterForm extends FormBase {
         '#type' => 'submit',
         '#value' => $this->t('Reset'),
         '#limit_validation_errors' => array(),
-        '#submit' => array(array($this, 'resetForm')),
+        '#submit' => array('::resetForm'),
       );
     }
     return $form;
diff --git a/core/modules/editor/src/Form/EditorImageDialog.php b/core/modules/editor/src/Form/EditorImageDialog.php
index 66438f9027f..8193cb67e1d 100644
--- a/core/modules/editor/src/Form/EditorImageDialog.php
+++ b/core/modules/editor/src/Form/EditorImageDialog.php
@@ -189,7 +189,7 @@ class EditorImageDialog extends FormBase {
       // No regular submit-handler. This form only works via JavaScript.
       '#submit' => array(),
       '#ajax' => array(
-        'callback' => array($this, 'submitForm'),
+        'callback' => '::submitForm',
         'event' => 'click',
       ),
     );
diff --git a/core/modules/editor/src/Form/EditorLinkDialog.php b/core/modules/editor/src/Form/EditorLinkDialog.php
index 47b78c85d77..758b489d14c 100644
--- a/core/modules/editor/src/Form/EditorLinkDialog.php
+++ b/core/modules/editor/src/Form/EditorLinkDialog.php
@@ -69,7 +69,7 @@ class EditorLinkDialog extends FormBase {
       // No regular submit-handler. This form only works via JavaScript.
       '#submit' => array(),
       '#ajax' => array(
-        'callback' => array($this, 'submitForm'),
+        'callback' => '::submitForm',
         'event' => 'click',
       ),
     );
diff --git a/core/modules/field_ui/src/Form/FieldInstanceEditForm.php b/core/modules/field_ui/src/Form/FieldInstanceEditForm.php
index 6061d9f1148..bc722483296 100644
--- a/core/modules/field_ui/src/Form/FieldInstanceEditForm.php
+++ b/core/modules/field_ui/src/Form/FieldInstanceEditForm.php
@@ -157,7 +157,7 @@ class FieldInstanceEditForm extends FormBase {
     $form['actions']['delete'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Delete field'),
-      '#submit' => array(array($this, 'delete')),
+      '#submit' => array('::delete'),
     );
     return $form;
   }
diff --git a/core/modules/forum/src/Form/Overview.php b/core/modules/forum/src/Form/Overview.php
index 98688ab03a5..126d845cc61 100644
--- a/core/modules/forum/src/Form/Overview.php
+++ b/core/modules/forum/src/Form/Overview.php
@@ -99,7 +99,7 @@ class Overview extends OverviewTerms {
 
     // The form needs to have submit and validate handlers set explicitly.
     // Use the existing taxonomy overview submit handler.
-    $form['#submit'] = array(array($this, 'submitForm'));
+    $form['#submit'] = array('::submitForm');
     $form['terms']['#empty'] = $this->t('No containers or forums available. <a href="@container">Add container</a> or <a href="@forum">Add forum</a>.', array(
       '@container' => $this->url('forum.add_container'),
       '@forum' => $this->url('forum.add_forum')
diff --git a/core/modules/language/src/Form/LanguageAddForm.php b/core/modules/language/src/Form/LanguageAddForm.php
index 9b6445cf553..acef8631f81 100644
--- a/core/modules/language/src/Form/LanguageAddForm.php
+++ b/core/modules/language/src/Form/LanguageAddForm.php
@@ -49,8 +49,8 @@ class LanguageAddForm extends LanguageFormBase {
           'select#edit-predefined-langcode' => array('value' => 'custom'),
         ),
       ),
-      '#validate' => array(array($this, 'validatePredefined')),
-      '#submit' => array(array($this, 'submitForm')),
+      '#validate' => array('::validatePredefined'),
+      '#submit' => array('::submitForm'),
     );
 
     $custom_language_states_conditions = array(
@@ -72,8 +72,8 @@ class LanguageAddForm extends LanguageFormBase {
     $form['custom_language']['submit'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Add custom language'),
-      '#validate' => array(array($this, 'validateCustom')),
-      '#submit' => array(array($this, 'submitForm')),
+      '#validate' => array('::validateCustom'),
+      '#submit' => array('::submitForm'),
     );
 
     return $form;
diff --git a/core/modules/language/src/Form/LanguageEditForm.php b/core/modules/language/src/Form/LanguageEditForm.php
index abac06d2cb7..de2213a3899 100644
--- a/core/modules/language/src/Form/LanguageEditForm.php
+++ b/core/modules/language/src/Form/LanguageEditForm.php
@@ -38,8 +38,8 @@ class LanguageEditForm extends LanguageFormBase {
     $actions['submit'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Save language'),
-      '#validate' => array(array($this, 'validateCommon')),
-      '#submit' => array(array($this, 'submitForm')),
+      '#validate' => array('::validateCommon'),
+      '#submit' => array('::submitForm'),
     );
     return $actions;
   }
diff --git a/core/modules/locale/src/Form/TranslateFilterForm.php b/core/modules/locale/src/Form/TranslateFilterForm.php
index 1a109b53a88..d283724f97c 100644
--- a/core/modules/locale/src/Form/TranslateFilterForm.php
+++ b/core/modules/locale/src/Form/TranslateFilterForm.php
@@ -74,7 +74,7 @@ class TranslateFilterForm extends TranslateFormBase {
       $form['filters']['actions']['reset'] = array(
         '#type' => 'submit',
         '#value' => $this->t('Reset'),
-        '#submit' => array(array($this, 'resetForm')),
+        '#submit' => array('::resetForm'),
       );
     }
 
diff --git a/core/modules/node/src/NodeForm.php b/core/modules/node/src/NodeForm.php
index 327aee5d18a..a100487a5a5 100644
--- a/core/modules/node/src/NodeForm.php
+++ b/core/modules/node/src/NodeForm.php
@@ -288,7 +288,7 @@ class NodeForm extends ContentEntityForm {
         $element['publish']['#value'] = $node->isPublished() ? t('Save and keep published') : t('Save and publish');
       }
       $element['publish']['#weight'] = 0;
-      array_unshift($element['publish']['#submit'], array($this, 'publish'));
+      array_unshift($element['publish']['#submit'], '::publish');
 
       // Add a "Unpublish" button.
       $element['unpublish'] = $element['submit'];
@@ -300,7 +300,7 @@ class NodeForm extends ContentEntityForm {
         $element['unpublish']['#value'] = !$node->isPublished() ? t('Save and keep unpublished') : t('Save and unpublish');
       }
       $element['unpublish']['#weight'] = 10;
-      array_unshift($element['unpublish']['#submit'], array($this, 'unpublish'));
+      array_unshift($element['unpublish']['#submit'], '::unpublish');
 
       // If already published, the 'publish' button is primary.
       if ($node->isPublished()) {
@@ -321,13 +321,8 @@ class NodeForm extends ContentEntityForm {
       '#access' => $preview_mode != DRUPAL_DISABLED && ($node->access('create') || $node->access('update')),
       '#value' => t('Preview'),
       '#weight' => 20,
-      '#validate' => array(
-        array($this, 'validate'),
-      ),
-      '#submit' => array(
-        array($this, 'submit'),
-        array($this, 'preview'),
-      ),
+      '#validate' => array('::validate'),
+      '#submit' => array('::submit', '::preview'),
     );
 
     $element['delete']['#access'] = $node->access('delete');
diff --git a/core/modules/path/src/Form/EditForm.php b/core/modules/path/src/Form/EditForm.php
index e86c3ec4227..95d5e205d7f 100644
--- a/core/modules/path/src/Form/EditForm.php
+++ b/core/modules/path/src/Form/EditForm.php
@@ -44,7 +44,7 @@ class EditForm extends PathFormBase {
     $form['actions']['delete'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Delete'),
-      '#submit' => array(array($this, 'deleteSubmit')),
+      '#submit' => array('::deleteSubmit'),
     );
     return $form;
   }
diff --git a/core/modules/path/src/Form/PathFilterForm.php b/core/modules/path/src/Form/PathFilterForm.php
index 42e50a45610..30374be54f3 100644
--- a/core/modules/path/src/Form/PathFilterForm.php
+++ b/core/modules/path/src/Form/PathFilterForm.php
@@ -50,7 +50,7 @@ class PathFilterForm extends FormBase {
       $form['basic']['reset'] = array(
         '#type' => 'submit',
         '#value' => $this->t('Reset'),
-        '#submit' => array(array($this, 'resetForm')),
+        '#submit' => array('::resetForm'),
       );
     }
     return $form;
diff --git a/core/modules/search/src/SearchPageListBuilder.php b/core/modules/search/src/SearchPageListBuilder.php
index c5b939ac6bb..5e6f1b5d55d 100644
--- a/core/modules/search/src/SearchPageListBuilder.php
+++ b/core/modules/search/src/SearchPageListBuilder.php
@@ -185,7 +185,7 @@ class SearchPageListBuilder extends DraggableListBuilder implements FormInterfac
     $form['status']['wipe'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Re-index site'),
-      '#submit' => array(array($this, 'searchAdminReindexSubmit')),
+      '#submit' => array('::searchAdminReindexSubmit'),
     );
 
     $items = array(10, 20, 50, 100, 200, 500);
@@ -271,8 +271,8 @@ class SearchPageListBuilder extends DraggableListBuilder implements FormInterfac
     $form['search_pages']['add_page']['add_search_submit'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Add new page'),
-      '#validate' => array(array($this, 'validateAddSearchPage')),
-      '#submit' => array(array($this, 'submitAddSearchPage')),
+      '#validate' => array('::validateAddSearchPage'),
+      '#submit' => array('::submitAddSearchPage'),
       '#limit_validation_errors' => array(array('search_type')),
     );
 
diff --git a/core/modules/shortcut/src/Form/SetCustomize.php b/core/modules/shortcut/src/Form/SetCustomize.php
index d19381d6dc7..b0aaef36131 100644
--- a/core/modules/shortcut/src/Form/SetCustomize.php
+++ b/core/modules/shortcut/src/Form/SetCustomize.php
@@ -89,10 +89,7 @@ class SetCustomize extends EntityForm {
       'submit' => array(
         '#value' => t('Save changes'),
         '#access' => (bool) Element::getVisibleChildren($form['shortcuts']['links']),
-        '#submit' => array(
-          array($this, 'submit'),
-          array($this, 'save'),
-        ),
+        '#submit' => array('::submit', '::save'),
       ),
     );
   }
diff --git a/core/modules/system/src/Controller/FormAjaxController.php b/core/modules/system/src/Controller/FormAjaxController.php
index 84d1f86445e..5285cfde156 100644
--- a/core/modules/system/src/Controller/FormAjaxController.php
+++ b/core/modules/system/src/Controller/FormAjaxController.php
@@ -72,9 +72,11 @@ class FormAjaxController implements ContainerInjectionInterface {
     // up to the #ajax['callback'] function of the element (may or may not be a
     // button) that triggered the Ajax request to determine what needs to be
     // rendered.
+    $callback = NULL;
     if (!empty($form_state['triggering_element'])) {
       $callback = $form_state['triggering_element']['#ajax']['callback'];
     }
+    $callback = $form_state->prepareCallback($callback);
     if (empty($callback) || !is_callable($callback)) {
       throw new HttpException(500, t('Internal Server Error'));
     }
diff --git a/core/modules/system/src/Form/CronForm.php b/core/modules/system/src/Form/CronForm.php
index 63a17d1014a..14900ea297f 100644
--- a/core/modules/system/src/Form/CronForm.php
+++ b/core/modules/system/src/Form/CronForm.php
@@ -92,7 +92,7 @@ class CronForm extends ConfigFormBase {
     $form['run'] = array(
       '#type' => 'submit',
       '#value' => t('Run cron'),
-      '#submit' => array(array($this, 'submitCron')),
+      '#submit' => array('::submitCron'),
     );
 
     $status = '<p>' . t('Last run: %cron-last ago.', array('%cron-last' => $this->dateFormatter->formatInterval(REQUEST_TIME - $this->state->get('system.cron_last')))) . '</p>';
diff --git a/core/modules/system/src/Form/DateFormatFormBase.php b/core/modules/system/src/Form/DateFormatFormBase.php
index 311f5db7933..a84e571e12b 100644
--- a/core/modules/system/src/Form/DateFormatFormBase.php
+++ b/core/modules/system/src/Form/DateFormatFormBase.php
@@ -135,7 +135,7 @@ abstract class DateFormatFormBase extends EntityForm {
       '#default_value' => '',
       '#field_suffix' => ' <small id="edit-date-format-suffix"></small>',
       '#ajax' => array(
-        'callback' => array($this, 'dateTimeLookup'),
+        'callback' => '::dateTimeLookup',
         'event' => 'keyup',
         'progress' => array('type' => 'throbber', 'message' => NULL),
       ),
diff --git a/core/modules/system/src/Form/PerformanceForm.php b/core/modules/system/src/Form/PerformanceForm.php
index f5261f846a6..ed631f32b5a 100644
--- a/core/modules/system/src/Form/PerformanceForm.php
+++ b/core/modules/system/src/Form/PerformanceForm.php
@@ -107,7 +107,7 @@ class PerformanceForm extends ConfigFormBase {
     $form['clear_cache']['clear'] = array(
       '#type' => 'submit',
       '#value' => t('Clear all caches'),
-      '#submit' => array(array($this, 'submitCacheClear')),
+      '#submit' => array('::submitCacheClear'),
     );
 
     $form['caching'] = array(
diff --git a/core/modules/system/src/Tests/Form/FormDefaultHandlersTest.php b/core/modules/system/src/Tests/Form/FormDefaultHandlersTest.php
index 7fa6a7c2df9..51c79f85212 100644
--- a/core/modules/system/src/Tests/Form/FormDefaultHandlersTest.php
+++ b/core/modules/system/src/Tests/Form/FormDefaultHandlersTest.php
@@ -37,8 +37,8 @@ class FormDefaultHandlersTest extends KernelTestBase implements FormInterface {
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state) {
-    $form['#validate'][] = array($this, 'customValidateForm');
-    $form['#submit'][] = array($this, 'customSubmitForm');
+    $form['#validate'][] = '::customValidateForm';
+    $form['#submit'][] = '::customSubmitForm';
     $form['submit'] = array('#type' => 'submit', '#value' => 'Save');
     return $form;
   }
diff --git a/core/modules/system/tests/modules/ajax_test/src/Form/AjaxTestDialogForm.php b/core/modules/system/tests/modules/ajax_test/src/Form/AjaxTestDialogForm.php
index 22dc4ee616c..21b3ac6bfee 100644
--- a/core/modules/system/tests/modules/ajax_test/src/Form/AjaxTestDialogForm.php
+++ b/core/modules/system/tests/modules/ajax_test/src/Form/AjaxTestDialogForm.php
@@ -42,7 +42,7 @@ class AjaxTestDialogForm extends FormBase {
       '#name' => 'button1',
       '#value' => 'Button 1 (modal)',
       '#ajax' => array(
-        'callback' => array($this, 'modal'),
+        'callback' => '::modal',
       ),
     );
     $form['button2'] = array(
@@ -50,7 +50,7 @@ class AjaxTestDialogForm extends FormBase {
       '#name' => 'button2',
       '#value' => 'Button 2 (non-modal)',
       '#ajax' => array(
-        'callback' => array($this, 'nonModal'),
+        'callback' => '::nonModal',
       ),
     );
 
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestLimitValidationErrorsForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestLimitValidationErrorsForm.php
index 8f260be9e70..dab4ac6164a 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestLimitValidationErrorsForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestLimitValidationErrorsForm.php
@@ -35,7 +35,7 @@ class FormTestLimitValidationErrorsForm extends FormBase {
     $form['test'] = array(
       '#title' => 'Test',
       '#type' => 'textfield',
-      '#element_validate' => array(array($this, 'elementValidateLimitValidationErrors')),
+      '#element_validate' => array('::elementValidateLimitValidationErrors'),
     );
     $form['test_numeric_index'] = array(
       '#tree' => TRUE,
@@ -43,7 +43,7 @@ class FormTestLimitValidationErrorsForm extends FormBase {
     $form['test_numeric_index'][0] = array(
       '#title' => 'Test (numeric index)',
       '#type' => 'textfield',
-      '#element_validate' => array(array($this, 'elementValidateLimitValidationErrors')),
+      '#element_validate' => array('::elementValidateLimitValidationErrors'),
     );
 
     $form['test_substring'] = array(
@@ -52,30 +52,30 @@ class FormTestLimitValidationErrorsForm extends FormBase {
     $form['test_substring']['foo'] = array(
       '#title' => 'Test (substring) foo',
       '#type' => 'textfield',
-      '#element_validate' => array(array($this, 'elementValidateLimitValidationErrors')),
+      '#element_validate' => array('::elementValidateLimitValidationErrors'),
     );
     $form['test_substring']['foobar'] = array(
       '#title' => 'Test (substring) foobar',
       '#type' => 'textfield',
-      '#element_validate' => array(array($this, 'elementValidateLimitValidationErrors')),
+      '#element_validate' => array('::elementValidateLimitValidationErrors'),
     );
 
     $form['actions']['partial'] = array(
       '#type' => 'submit',
       '#limit_validation_errors' => array(array('test')),
-      '#submit' => array(array($this, 'partialSubmitForm')),
+      '#submit' => array('::partialSubmitForm'),
       '#value' => t('Partial validate'),
     );
     $form['actions']['partial_numeric_index'] = array(
       '#type' => 'submit',
       '#limit_validation_errors' => array(array('test_numeric_index', 0)),
-      '#submit' => array(array($this, 'partialSubmitForm')),
+      '#submit' => array('::partialSubmitForm'),
       '#value' => t('Partial validate (numeric index)'),
     );
     $form['actions']['substring'] = array(
       '#type' => 'submit',
       '#limit_validation_errors' => array(array('test_substring', 'foo')),
-      '#submit' => array(array($this, 'partialSubmitForm')),
+      '#submit' => array('::partialSubmitForm'),
       '#value' => t('Partial validate (substring)'),
     );
     $form['actions']['full'] = array(
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestProgrammaticForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestProgrammaticForm.php
index 86689a00ca6..baa146bc44c 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestProgrammaticForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestProgrammaticForm.php
@@ -74,7 +74,7 @@ class FormTestProgrammaticForm extends FormBase {
       // Use the same submit handler for this button as for the form itself.
       // (This must be set explicitly or otherwise the form API will ignore the
       // #limit_validation_errors property.)
-      '#submit' => array(array($this, 'submitForm')),
+      '#submit' => array('::submitForm'),
     );
     $user_input = $form_state->getUserInput();
     if (!empty($user_input['field_to_validate']) && $user_input['field_to_validate'] != 'all') {
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestRebuildPreserveValuesForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestRebuildPreserveValuesForm.php
index 47f1f392adf..00a95705d30 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestRebuildPreserveValuesForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestRebuildPreserveValuesForm.php
@@ -53,7 +53,7 @@ class FormTestRebuildPreserveValuesForm extends FormBase {
       $form['add_more'] = array(
         '#type' => 'submit',
         '#value' => 'Add more',
-        '#submit' => array(array($this, 'addMoreSubmitForm')),
+        '#submit' => array('::addMoreSubmitForm'),
       );
     }
     else {
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php
index c7ca2f4bea0..fb231607fdd 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php
@@ -63,7 +63,7 @@ class FormTestStorageForm extends FormBase {
       '#type' => 'textfield',
       '#title' => 'Value',
       '#default_value' => $form_state['storage']['thing']['value'],
-      '#element_validate' => array(array($this, 'elementValidateValueCached')),
+      '#element_validate' => array('::elementValidateValueCached'),
     );
     $form['continue_button'] = array(
       '#type' => 'button',
@@ -73,7 +73,7 @@ class FormTestStorageForm extends FormBase {
     $form['continue_submit'] = array(
       '#type' => 'submit',
       '#value' => 'Continue submit',
-      '#submit' => array(array($this, 'continueSubmitForm')),
+      '#submit' => array('::continueSubmitForm'),
     );
     $form['submit'] = array(
       '#type' => 'submit',
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestValidateRequiredForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestValidateRequiredForm.php
index 35908ad5e26..40803de7e0f 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestValidateRequiredForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestValidateRequiredForm.php
@@ -27,7 +27,7 @@ class FormTestValidateRequiredForm extends FormBase {
    */
   public function buildForm(array $form, FormStateInterface $form_state) {
     $options = array('foo' => 'foo', 'bar' => 'bar');
-    $validate = array(array($this, 'elementValidateRequired'));
+    $validate = array('::elementValidateRequired');
 
     $form['textfield'] = array(
       '#type' => 'textfield',
diff --git a/core/modules/taxonomy/src/Form/OverviewTerms.php b/core/modules/taxonomy/src/Form/OverviewTerms.php
index d28b53c5b6d..e2eeef0cfc2 100644
--- a/core/modules/taxonomy/src/Form/OverviewTerms.php
+++ b/core/modules/taxonomy/src/Form/OverviewTerms.php
@@ -339,7 +339,7 @@ class OverviewTerms extends FormBase {
       );
       $form['actions']['reset_alphabetical'] = array(
         '#type' => 'submit',
-        '#submit' => array(array($this, 'submitReset')),
+        '#submit' => array('::submitReset'),
         '#value' => $this->t('Reset to alphabetical'),
       );
     }
diff --git a/core/modules/taxonomy/src/VocabularyForm.php b/core/modules/taxonomy/src/VocabularyForm.php
index d34d3e2464d..5a70f8d8b1a 100644
--- a/core/modules/taxonomy/src/VocabularyForm.php
+++ b/core/modules/taxonomy/src/VocabularyForm.php
@@ -97,7 +97,7 @@ class VocabularyForm extends EntityForm {
       // the submit button has custom submit handlers.
       if ($this->moduleHandler->moduleExists('language')) {
         array_unshift($actions['submit']['#submit'], 'language_configuration_element_submit');
-        array_unshift($actions['submit']['#submit'], array($this, 'languageConfigurationSubmit'));
+        array_unshift($actions['submit']['#submit'], '::languageConfigurationSubmit');
       }
       // We cannot leverage the regular submit handler definition because we
       // have button-specific ones here. Hence we need to explicitly set it for
diff --git a/core/modules/user/src/Form/UserLoginForm.php b/core/modules/user/src/Form/UserLoginForm.php
index 1a8736cab20..25511a82ccc 100644
--- a/core/modules/user/src/Form/UserLoginForm.php
+++ b/core/modules/user/src/Form/UserLoginForm.php
@@ -105,9 +105,9 @@ class UserLoginForm extends FormBase {
     $form['actions'] = array('#type' => 'actions');
     $form['actions']['submit'] = array('#type' => 'submit', '#value' => $this->t('Log in'));
 
-    $form['#validate'][] = array($this, 'validateName');
-    $form['#validate'][] = array($this, 'validateAuthentication');
-    $form['#validate'][] = array($this, 'validateFinal');
+    $form['#validate'][] = '::validateName';
+    $form['#validate'][] = '::validateAuthentication';
+    $form['#validate'][] = '::validateFinal';
 
     return $form;
   }
diff --git a/core/modules/user/src/ProfileForm.php b/core/modules/user/src/ProfileForm.php
index 998d29b2a88..e1196931c51 100644
--- a/core/modules/user/src/ProfileForm.php
+++ b/core/modules/user/src/ProfileForm.php
@@ -37,7 +37,7 @@ class ProfileForm extends AccountForm {
     $user = $this->currentUser();
     $element['delete']['#type'] = 'submit';
     $element['delete']['#value'] = $this->t('Cancel account');
-    $element['delete']['#submit'] = array(array($this, 'editCancelSubmit'));
+    $element['delete']['#submit'] = array('::editCancelSubmit');
     $element['delete']['#access'] = $account->id() > 1 && (($account->id() == $user->id() && $user->hasPermission('cancel account')) || $user->hasPermission('administer users'));
 
     return $element;
diff --git a/core/modules/views_ui/src/Form/AdvancedSettingsForm.php b/core/modules/views_ui/src/Form/AdvancedSettingsForm.php
index 56254a25071..44a2f36e2a8 100644
--- a/core/modules/views_ui/src/Form/AdvancedSettingsForm.php
+++ b/core/modules/views_ui/src/Form/AdvancedSettingsForm.php
@@ -46,7 +46,7 @@ class AdvancedSettingsForm extends ConfigFormBase {
     $form['cache']['clear_cache'] = array(
       '#type' => 'submit',
       '#value' => $this->t("Clear Views' cache"),
-      '#submit' => array(array($this, 'cacheSubmit')),
+      '#submit' => array('::cacheSubmit'),
     );
 
     $form['debug'] = array(
diff --git a/core/modules/views_ui/src/ViewAddForm.php b/core/modules/views_ui/src/ViewAddForm.php
index 8717ff93a10..78fb0490825 100644
--- a/core/modules/views_ui/src/ViewAddForm.php
+++ b/core/modules/views_ui/src/ViewAddForm.php
@@ -153,9 +153,7 @@ class ViewAddForm extends ViewFormBase {
     $actions['cancel'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Cancel'),
-      '#submit' => array(
-        array($this, 'cancel'),
-      ),
+      '#submit' => array('::cancel'),
       '#limit_validation_errors' => array(),
     );
     return $actions;
diff --git a/core/modules/views_ui/src/ViewDuplicateForm.php b/core/modules/views_ui/src/ViewDuplicateForm.php
index e817e36263b..c081b3066c6 100644
--- a/core/modules/views_ui/src/ViewDuplicateForm.php
+++ b/core/modules/views_ui/src/ViewDuplicateForm.php
@@ -58,9 +58,7 @@ class ViewDuplicateForm extends ViewFormBase {
     $actions['submit'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Duplicate'),
-      '#submit' => array(
-        array($this, 'submit'),
-      ),
+      '#submit' => array('::submit'),
     );
     return $actions;
   }
diff --git a/core/modules/views_ui/src/ViewEditForm.php b/core/modules/views_ui/src/ViewEditForm.php
index 6d42b52ba59..1c9522eaf1c 100644
--- a/core/modules/views_ui/src/ViewEditForm.php
+++ b/core/modules/views_ui/src/ViewEditForm.php
@@ -240,9 +240,7 @@ class ViewEditForm extends ViewFormBase {
     $actions['cancel'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Cancel'),
-      '#submit' => array(
-        array($this, 'cancel'),
-      ),
+      '#submit' => array('::cancel'),
     );
     if ($this->entity->isLocked()) {
       $actions['submit']['#access'] = FALSE;
@@ -418,7 +416,7 @@ class ViewEditForm extends ViewFormBase {
             '#type' => 'submit',
             '#value' => $this->t('Enable !display_title', array('!display_title' => $display_title)),
             '#limit_validation_errors' => array(),
-            '#submit' => array(array($this, 'submitDisplayEnable'), array($this, 'submitDelayDestination')),
+            '#submit' => array('::submitDisplayEnable', '::submitDelayDestination'),
             '#prefix' => '<li class="enable">',
             "#suffix" => '</li>',
           );
@@ -443,7 +441,7 @@ class ViewEditForm extends ViewFormBase {
             '#type' => 'submit',
             '#value' => $this->t('Duplicate !display_title', array('!display_title' => $display_title)),
             '#limit_validation_errors' => array(),
-            '#submit' => array(array($this, 'submitDisplayDuplicate'), array($this, 'submitDelayDestination')),
+            '#submit' => array('::submitDisplayDuplicate', '::submitDelayDestination'),
             '#prefix' => '<li class="duplicate">',
             "#suffix" => '</li>',
           );
@@ -453,7 +451,7 @@ class ViewEditForm extends ViewFormBase {
           '#type' => 'submit',
           '#value' => $this->t('Delete !display_title', array('!display_title' => $display_title)),
           '#limit_validation_errors' => array(),
-          '#submit' => array(array($this, 'submitDisplayDelete'), array($this, 'submitDelayDestination')),
+          '#submit' => array('::submitDisplayDelete', '::submitDelayDestination'),
           '#prefix' => '<li class="delete">',
           "#suffix" => '</li>',
         );
@@ -467,7 +465,7 @@ class ViewEditForm extends ViewFormBase {
             '#type' => 'submit',
             '#value' => $this->t('Duplicate as !type', array('!type' => $label)),
             '#limit_validation_errors' => array(),
-            '#submit' => array(array($this, 'submitDuplicateDisplayAsType'), array($this, 'submitDelayDestination')),
+            '#submit' => array('::submitDuplicateDisplayAsType', '::submitDelayDestination'),
             '#prefix' => '<li class="duplicate">',
             '#suffix' => '</li>',
           );
@@ -478,7 +476,7 @@ class ViewEditForm extends ViewFormBase {
           '#type' => 'submit',
           '#value' => $this->t('Undo delete of !display_title', array('!display_title' => $display_title)),
           '#limit_validation_errors' => array(),
-          '#submit' => array(array($this, 'submitDisplayUndoDelete'), array($this, 'submitDelayDestination')),
+          '#submit' => array('::submitDisplayUndoDelete', '::submitDelayDestination'),
           '#prefix' => '<li class="undo-delete">',
           "#suffix" => '</li>',
         );
@@ -488,7 +486,7 @@ class ViewEditForm extends ViewFormBase {
           '#type' => 'submit',
           '#value' => $this->t('Disable !display_title', array('!display_title' => $display_title)),
           '#limit_validation_errors' => array(),
-          '#submit' => array(array($this, 'submitDisplayDisable'), array($this, 'submitDelayDestination')),
+          '#submit' => array('::submitDisplayDisable', '::submitDelayDestination'),
           '#prefix' => '<li class="disable">',
           "#suffix" => '</li>',
         );
@@ -760,7 +758,7 @@ class ViewEditForm extends ViewFormBase {
         '#type' => 'submit',
         '#value' => $this->t('Add !display', array('!display' => $label)),
         '#limit_validation_errors' => array(),
-        '#submit' => array(array($this, 'submitDisplayAdd'), array($this, 'submitDelayDestination')),
+        '#submit' => array('::submitDisplayAdd', '::submitDelayDestination'),
         '#attributes' => array('class' => array('add-display')),
         // Allow JavaScript to remove the 'Add ' prefix from the button label when
         // placing the button in a "Add" dropdown menu.
diff --git a/core/modules/views_ui/src/ViewPreviewForm.php b/core/modules/views_ui/src/ViewPreviewForm.php
index b5ecd9c402c..9ba3b991688 100644
--- a/core/modules/views_ui/src/ViewPreviewForm.php
+++ b/core/modules/views_ui/src/ViewPreviewForm.php
@@ -112,7 +112,7 @@ class ViewPreviewForm extends ViewFormBase {
         '#type' => 'submit',
         '#value' => $this->t('Update preview'),
         '#attributes' => array('class' => array('arguments-preview')),
-        '#submit' => array(array($this, 'submitPreview')),
+        '#submit' => array('::submitPreview'),
         '#id' => 'preview-submit',
         '#ajax' => array(
           'path' => 'admin/structure/views/view/' . $view->id() . '/preview/' . $this->displayID,
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
index 4cd45e770f7..3f391dd65a5 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
@@ -7,7 +7,9 @@
 
 namespace Drupal\Tests\Core\Form;
 
+use Drupal\Core\Form\FormInterface;
 use Drupal\Core\Form\FormState;
+use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Url;
 use Drupal\Tests\UnitTestCase;
 use Symfony\Component\HttpFoundation\RedirectResponse;
@@ -218,6 +220,38 @@ class FormStateTest extends UnitTestCase {
     $this->assertSame($expected, $form_state->getValues());
   }
 
+  /**
+   * @covers ::prepareCallback()
+   */
+  public function testPrepareCallbackValidMethod() {
+    $form_state = new FormState();
+    $form_state['build_info']['callback_object'] = new PrepareCallbackTestForm();
+    $processed_callback = $form_state->prepareCallback('::buildForm');
+    $this->assertEquals(array($form_state['build_info']['callback_object'], 'buildForm'), $processed_callback);
+  }
+
+  /**
+   * @covers ::prepareCallback()
+   */
+  public function testPrepareCallbackInValidMethod() {
+    $form_state = new FormState();
+    $form_state['build_info']['callback_object'] = new PrepareCallbackTestForm();
+    $processed_callback = $form_state->prepareCallback('not_a_method');
+    // The callback was not changed as no such method exists.
+    $this->assertEquals('not_a_method', $processed_callback);
+  }
+
+  /**
+   * @covers ::prepareCallback()
+   */
+  public function testPrepareCallbackArray() {
+    $form_state = new FormState();
+    $form_state['build_info']['callback_object'] = new PrepareCallbackTestForm();
+    $callback = array($form_state['build_info']['callback_object'], 'buildForm');
+    $processed_callback = $form_state->prepareCallback($callback);
+    $this->assertEquals($callback, $processed_callback);
+  }
+
   public function providerTestSetValue() {
     $data = array();
     $data[] = array(
@@ -315,3 +349,16 @@ class FormStateTest extends UnitTestCase {
   }
 
 }
+
+/**
+ * A test form used for the prepareCallback() tests.
+ */
+class PrepareCallbackTestForm implements FormInterface {
+  public function getFormId() {
+    return 'test_form';
+  }
+
+  public function buildForm(array $form, FormStateInterface $form_state) {}
+  public function validateForm(array &$form, FormStateInterface $form_state) { }
+  public function submitForm(array &$form, FormStateInterface $form_state) { }
+}
diff --git a/core/tests/Drupal/Tests/Core/Form/FormSubmitterTest.php b/core/tests/Drupal/Tests/Core/Form/FormSubmitterTest.php
index 9a7833d92d1..e1c9462e3ed 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormSubmitterTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormSubmitterTest.php
@@ -202,13 +202,16 @@ class FormSubmitterTest extends UnitTestCase {
    */
   public function testExecuteSubmitHandlers() {
     $form_submitter = $this->getFormSubmitter();
-    $mock = $this->getMock('stdClass', array('submit_handler', 'hash_submit'));
+    $mock = $this->getMock('stdClass', array('submit_handler', 'hash_submit', 'simple_string_submit'));
     $mock->expects($this->once())
       ->method('submit_handler')
       ->with($this->isType('array'), $this->isInstanceOf('Drupal\Core\Form\FormStateInterface'));
     $mock->expects($this->once())
       ->method('hash_submit')
       ->with($this->isType('array'), $this->isInstanceOf('Drupal\Core\Form\FormStateInterface'));
+    $mock->expects($this->once())
+      ->method('simple_string_submit')
+      ->with($this->isType('array'), $this->isInstanceOf('Drupal\Core\Form\FormStateInterface'));
 
     $form = array();
     $form_state = new FormState();
@@ -220,6 +223,12 @@ class FormSubmitterTest extends UnitTestCase {
     // $form_state submit handlers will supersede $form handlers.
     $form_state['submit_handlers'][] = array($mock, 'submit_handler');
     $form_submitter->executeSubmitHandlers($form, $form_state);
+
+    // Methods directly on the form object can be specified as a string.
+    $form_state = new FormState();
+    $form_state['build_info']['callback_object'] = $mock;
+    $form_state['submit_handlers'][] = '::simple_string_submit';
+    $form_submitter->executeSubmitHandlers($form, $form_state);
   }
 
   /**