Remove support for function-based forms

It is RTBC. Thank you @tim.plunkett for revamping the form component.

+++ b/core/lib/Drupal/Core/Controller/FormController.php
@@ -77,8 +77,7 @@ public function getContentResult(Request $request) {
-    return $this->formBuilder->buildForm($form_id, $form_state);
+    return $this->formBuilder->buildForm($form_object, $form_state);

Yay!!!!

+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -685,40 +675,8 @@ public function prepareForm($form_id, &$form, &$form_state) {
-    }
+    $form['#validate'][] = array($form_state['build_info']['callback_object'], 'validateForm');
+    $form['#submit'][] = array($form_state['build_info']['callback_object'], 'submitForm');
 

Are you sure that we should do this? Doesn't this break forms that don't want to call the default methods by default? I guess it's only on the default submit and usually you have non-standard submits on specific buttons, but still?

Before, the form object versions were still inside the !isset().

I keep forgetting/avoiding it, but I'd still like to see us find a way to avoid serializing the form object and specifying it like this, because what happens is that during submit, we have two different instances, the unserialized one in those form submits and the one that was re-created. To avoid nasty stuff like https://github.com/karanpoddar/monitoring/blob/gsoc-plugin/src/SensorFor... and the corresponding one inside EntityForm.

I think supporting a way to only specify the method name for submit/validate should bring us pretty far, possibly serialize the callback_object object as a string and re-initalize it.

1. +++ b/core/lib/Drupal/Core/Controller/FormController.php
   @@ -77,8 +77,7 @@ public function getContentResult(Request $request) {
   -    $form_id = $this->formBuilder->getFormId($form_object, $form_state);
   -    return $this->formBuilder->buildForm($form_id, $form_state);
   +    return $this->formBuilder->buildForm($form_object, $form_state);
   
   +++ b/core/lib/Drupal/Core/Entity/EntityFormBuilder.php
   @@ -52,7 +52,7 @@ public function getForm(EntityInterface $entity, $operation = 'default', array $
    
   -    return $this->formBuilder->buildForm($form_object->getFormId(), $form_state);
   +    return $this->formBuilder->buildForm($form_object, $form_state);
   

   <3 This is much better to use

2. +++ b/core/lib/Drupal/Core/Form/FormBuilder.php
   @@ -139,18 +140,16 @@ public function getFormId($form_arg, &$form_state) {
   +    if (!is_object($form_arg) || !($form_arg instanceof FormInterface)) {
   +      throw new \Exception(String::format('The form argument @form_arg is not a valid form.', array('@form_arg' => $form_arg)));
        }
   

   I wonder whether we can use a more specific exception, at least InvalidArgument or a custom one for forms?

3. +++ b/core/lib/Drupal/Core/Form/FormBuilder.php
   @@ -685,40 +675,8 @@ public function prepareForm($form_id, &$form, &$form_state) {
   -    if (!isset($form['#validate'])) {
   ...
   -    if (!isset($form['#submit'])) {
   ...
   +    $form['#validate'][] = array($form_state['build_info']['callback_object'], 'validateForm');
   +    $form['#submit'][] = array($form_state['build_info']['callback_object'], 'submitForm');
   

   This is a small behavior change. Previously you seem to be able to skip the default validate/submit handling? some clarification would be cool

4. +++ b/core/lib/Drupal/Core/Form/FormBuilder.php
   --- a/core/modules/field/src/Tests/FieldAttachOtherTest.php
   +++ b/core/modules/field/src/Tests/FieldAttachOtherTest.php
   
   +++ b/core/modules/field/src/Tests/FieldAttachOtherTest.php
   @@ -342,6 +342,7 @@ function testEntityFormDisplayExtractFormValues() {
        // Pretend the form has been built.
   +    $form_state['build_info']['callback_object'] = \Drupal::entityManager()->getFormObject($entity_type, 'default');
   

   This is odd ... why was that not needed previously?

5. +++ b/core/modules/views/includes/ajax.inc
   @@ -14,7 +14,7 @@
   -function views_ajax_form_wrapper($form_id, &$form_state) {
   +function views_ajax_form_wrapper($form_class, &$form_state) {
   

   We need a follow up to add proper documentation for the parameters: #2303761: Move views_ajax_form_wrapper() to ViewsFormBase

Thank you, this is just perfect.

1. +++ b/core/includes/batch.inc
   @@ -379,7 +379,7 @@ function _batch_next_set() {
   -      call_user_func_array($callback, array($batch['form_state']['complete_form'], &$batch['form_state']));
   +      call_user_func_array($callback, array(&$batch['form_state']['complete_form'], &$batch['form_state']));
   

   How is this change related?

2. +++ b/core/lib/Drupal/Core/Controller/FormController.php
   @@ -77,8 +77,7 @@ public function getContentResult(Request $request) {
   -    return $this->formBuilder->buildForm($form_id, $form_state);
   +    return $this->formBuilder->buildForm($form_object, $form_state);
   

   Does the docblock of FormBuilder::buildForm need to change?

Committed fc939d4 and pushed to 8.x. Thanks!

diff --git a/core/modules/system/src/Tests/Form/FormDefaultHandlersTest.php b/core/modules/system/src/Tests/Form/FormDefaultHandlersTest.php
index 812f368..9c980a6 100644
--- a/core/modules/system/src/Tests/Form/FormDefaultHandlersTest.php
+++ b/core/modules/system/src/Tests/Form/FormDefaultHandlersTest.php
@@ -72,7 +72,7 @@ public function submitForm(array &$form, array &$form_state) {
   /**
    * Tests that default handlers are added even if custom are specified.
    */
-  function testLimitValidationErrors() {
+  function testDefaultAndCustomHandlers() {
     $form_state['values'] = array();
     $form_builder = $this->container->get('form_builder');
     $form_builder->submitForm($this, $form_state);

The method name is wrong - fixed on commit.