--- /dev/null
+<?php
+
+namespace Drupal\Core\Form;
+
+/**
+ * Provides an interface for form building and processing.
+ */
+interface FormBuilderInterface {
+
+ /**
+ * Request key for AJAX forms that submit to the form's original route.
+ *
+ * This constant is distinct from a "drupal_ajax" value for
+ * \Drupal\Core\EventSubscriber\MainContentViewSubscriber::WRAPPER_FORMAT,
+ * because that one is set for all AJAX submissions, including ones with
+ * dedicated routes for which self::buildForm() should not exit early via a
+ * \Drupal\Core\Form\FormAjaxException.
+ *
+ * @todo Re-evaluate the need for this constant after
+ * https://www.drupal.org/node/2502785 and
+ * https://www.drupal.org/node/2503429.
+ */
+ const AJAX_FORM_REQUEST = 'ajax_form';
+
+ /**
+ * Determines the ID of a form.
+ *
+ * @param \Drupal\Core\Form\FormInterface|string $form_arg
+ * The value is identical to that of self::getForm()'s $form_arg argument.
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ * The current state of the form.
+ *
+ * @return string
+ * The unique string identifying the desired form.
+ */
+ public function getFormId($form_arg, FormStateInterface &$form_state);
+
+ /**
+ * Gets a renderable form array.
+ *
+ * This function should be used instead of self::buildForm() when $form_state
+ * is not needed (i.e., when initially rendering the form) and is often
+ * used as a menu callback.
+ *
+ * @param \Drupal\Core\Form\FormInterface|string $form_arg
+ * The value must be one of the following:
+ * - The name of a class that implements \Drupal\Core\Form\FormInterface.
+ * - An instance of a class that implements \Drupal\Core\Form\FormInterface.
+ * @param ...
+ * Any additional arguments are passed on to the functions called by
+ * \Drupal::formBuilder()->getForm(), including the unique form constructor
+ * function. For example, the node_edit form requires that a node object is
+ * passed in here when it is called. These are available to implementations
+ * of hook_form_alter() and hook_form_FORM_ID_alter() as the array
+ * $form_state->getBuildInfo()['args'].
+ *
+ * @return array
+ * The form array.
+ *
+ * @see \Drupal\Core\Form\FormBuilderInterface::buildForm()
+ */
+ public function getForm($form_arg);
+
+ /**
+ * Builds and processes a form for a given form ID.
+ *
+ * The form may also be retrieved from the cache if the form was built in a
+ * previous page load. The form is then passed on for processing, validation,
+ * and submission if there is proper input.
+ *
+ * @param \Drupal\Core\Form\FormInterface|string $form_id
+ * The value must be one of the following:
+ * - The name of a class that implements \Drupal\Core\Form\FormInterface.
+ * - An instance of a class that implements \Drupal\Core\Form\FormInterface.
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ * The current state of the form.
+ *
+ * @return array
+ * The rendered form. This function may also perform a redirect and hence
+ * may not return at all depending upon the $form_state flags that were set.
+ *
+ * @throws \Drupal\Core\Form\FormAjaxException
+ * Thrown when a form is triggered via an AJAX submission. It will be
+ * handled by \Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber.
+ * @throws \Drupal\Core\Form\EnforcedResponseException
+ * Thrown when a form builder returns a response directly, usually a
+ * \Symfony\Component\HttpFoundation\RedirectResponse. It will be handled by
+ * \Drupal\Core\EventSubscriber\EnforcedFormResponseSubscriber.
+ *
+ * @see self::redirectForm()
+ */
+ public function buildForm($form_id, FormStateInterface &$form_state);
+
+ /**
+ * Constructs a new $form from the information in $form_state.
+ *
+ * This is the key function for making multi-step forms advance from step to
+ * step. It is called by self::processForm() when all user input processing,
+ * including calling validation and submission handlers, for the request is
+ * finished. If a validate or submit handler set $form_state->isRebuilding()
+ * to TRUE, and if other conditions don't preempt a rebuild from happening,
+ * then this function is called to generate a new $form, the next step in the
+ * form workflow, to be returned for rendering.
+ *
+ * Ajax form submissions are almost always multi-step workflows, so that is
+ * one common use-case during which form rebuilding occurs.
+ *
+ * @param string $form_id
+ * The unique string identifying the desired form. If a function with that
+ * name exists, it is called to build the form array.
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ * The current state of the form.
+ * @param array|null $old_form
+ * (optional) A previously built $form. Used to retain the #build_id and
+ * #action properties in Ajax callbacks and similar partial form rebuilds.
+ * The only properties copied from $old_form are the ones which both exist
+ * in $old_form and for which $form_state->getRebuildInfo()['copy'][PROPERTY]
+ * is TRUE. If $old_form is not passed, the entire $form is rebuilt freshly.
+ * 'rebuild_info' needs to be a separate top-level property next to
+ * 'build_info', since the contained data must not be cached.
+ *
+ * @return array
+ * The newly built form.
+ *
+ * @see self::processForm()
+ */
+ public function rebuildForm($form_id, FormStateInterface &$form_state, $old_form = NULL);
+
+ /**
+ * Retrieves, populates, and processes a form.
+ *
+ * This function allows you to supply values for form elements and submit a
+ * form for processing. Compare to self::getForm(), which also builds and
+ * processes a form, but does not allow you to supply values.
+ *
+ * There is no return value, but you can check to see if there are errors
+ * by calling $form_state->getErrors().
+ *
+ * @param \Drupal\Core\Form\FormInterface|string $form_arg
+ * The value must be one of the following:
+ * - The name of a class that implements \Drupal\Core\Form\FormInterface.
+ * - An instance of a class that implements \Drupal\Core\Form\FormInterface.
+ * @param $form_state
+ * The current state of the form. Most important is the
+ * $form_state->getValues() collection, a tree of data used to simulate the
+ * incoming \Drupal::request()->request information from a user's form
+ * submission. If a key is not filled in $form_state->getValues(), then the
+ * default value of the respective element is used. To submit an unchecked
+ * checkbox or other control that browsers submit by not having a
+ * \Drupal::request()->request entry, include the key, but set the value to
+ * NULL.
+ * @param ...
+ * Any additional arguments are passed on to the functions called by
+ * self::submitForm(), including the unique form constructor function.
+ * For example, the node_edit form requires that a node object be passed
+ * in here when it is called. Arguments that need to be passed by reference
+ * should not be included here, but rather placed directly in the
+ * $form_state build info array so that the reference can be preserved. For
+ * example, a form builder function with the following signature:
+ * @code
+ * function mymodule_form($form, FormStateInterface &$form_state, &$object) {
+ * }
+ * @endcode
+ * would be called via self::submitForm() as follows:
+ * @code
+ * $form_state->setValues($my_form_values);
+ * $form_state->addBuildInfo('args', [&$object]);
+ * \Drupal::formBuilder()->submitForm('mymodule_form', $form_state);
+ * @endcode
+ * For example:
+ * @code
+ * // register a new user
+ * $form_state = new FormState();
+ * $values['name'] = 'robo-user';
+ * $values['mail'] = 'robouser@example.com';
+ * $values['pass']['pass1'] = 'password';
+ * $values['pass']['pass2'] = 'password';
+ * $values['op'] = t('Create new account');
+ * $form_state->setValues($values);
+ * \Drupal::formBuilder()->submitForm('user_register_form', $form_state);
+ * @endcode
+ */
+ public function submitForm($form_arg, FormStateInterface &$form_state);
+
+ /**
+ * Retrieves the structured array that defines a given form.
+ *
+ * @param string $form_id
+ * The unique string identifying the desired form. If a function
+ * with that name exists, it is called to build the form array.
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ * The current state of the form, including the additional arguments to
+ * self::getForm() or self::submitForm() in the 'args' component of the
+ * array.
+ *
+ * @return mixed|\Symfony\Component\HttpFoundation\Response
+ */
+ public function retrieveForm($form_id, FormStateInterface &$form_state);
+
+ /**
+ * Processes a form submission.
+ *
+ * This function is the heart of form API. The form gets built, validated and
+ * in appropriate cases, submitted and rebuilt.
+ *
+ * @param string $form_id
+ * The unique string identifying the current form.
+ * @param array $form
+ * An associative array containing the structure of the form.
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ * The current state of the form. This includes the current persistent
+ * storage data for the form, and any data passed along by earlier steps
+ * when displaying a multi-step form. Additional information, like the
+ * sanitized \Drupal::request()->request data, is also accumulated here.
+ *
+ * @return \Symfony\Component\HttpFoundation\RedirectResponse|null
+ */
+ public function processForm($form_id, &$form, FormStateInterface &$form_state);
+
+ /**
+ * Prepares a structured form array.
+ *
+ * Adds required elements, executes any hook_form_alter functions, and
+ * optionally inserts a validation token to prevent tampering.
+ *
+ * @param string $form_id
+ * A unique string identifying the form for validation, submission,
+ * theming, and hook_form_alter functions.
+ * @param array $form
+ * An associative array containing the structure of the form.
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ * The current state of the form. Passed in here so that hook_form_alter()
+ * calls can use it, as well.
+ */
+ public function prepareForm($form_id, &$form, FormStateInterface &$form_state);
+
+ /**
+ * Builds and processes all elements in the structured form array.
+ *
+ * Adds any required properties to each element, maps the incoming input data
+ * to the proper elements, and executes any #process handlers attached to a
+ * specific element.
+ *
+ * This is one of the three primary functions that recursively iterates a form
+ * array. This one does it for completing the form building process. The other
+ * two are self::doValidateForm() (invoked via self::validateForm() and used
+ * to invoke validation logic for each element) and drupal_render() (for
+ * rendering each element). Each of these three pipelines provides ample
+ * opportunity for modules to customize what happens. For example, during this
+ * function's life cycle, the following functions get called for each element:
+ * - $element['#value_callback']: A callable that implements how user input is
+ * mapped to an element's #value property. This defaults to a function named
+ * 'form_type_TYPE_value' where TYPE is $element['#type'].
+ * - $element['#process']: An array of functions called after user input has
+ * been mapped to the element's #value property. These functions can be used
+ * to dynamically add child elements: for example, for the 'date' element
+ * type, one of the functions in this array is form_process_datetime(),
+ * which adds the individual 'date', and 'time'. child elements. These
+ * functions can also be used to set additional properties or implement
+ * special logic other than adding child elements: for example, for the
+ * 'details' element type, one of the functions in this array is
+ * form_process_details(), which adds the attributes and JavaScript needed
+ * to make the details work in older browsers. The #process functions are
+ * called in preorder traversal, meaning they are called for the parent
+ * element first, then for the child elements.
+ * - $element['#after_build']: An array of callables called after
+ * self::doBuildForm() is done with its processing of the element. These are
+ * called in postorder traversal, meaning they are called for the child
+ * elements first, then for the parent element.
+ * There are similar properties containing callback functions invoked by
+ * self::doValidateForm() and drupal_render(), appropriate for those
+ * operations.
+ *
+ * Developers are strongly encouraged to integrate the functionality needed by
+ * their form or module within one of these three pipelines, using the
+ * appropriate callback property, rather than implementing their own recursive
+ * traversal of a form array. This facilitates proper integration between
+ * multiple modules. For example, module developers are familiar with the
+ * relative order in which hook_form_alter() implementations and #process
+ * functions run. A custom traversal function that affects the building of a
+ * form is likely to not integrate with hook_form_alter() and #process in the
+ * expected way. Also, deep recursion within PHP is both slow and memory
+ * intensive, so it is best to minimize how often it's done.
+ *
+ * As stated above, each element's #process functions are executed after its
+ * #value has been set. This enables those functions to execute conditional
+ * logic based on the current value. However, all of self::doBuildForm() runs
+ * before self::validateForm() is called, so during #process function
+ * execution, the element's #value has not yet been validated, so any code
+ * that requires validated values must reside within a submit handler.
+ *
+ * As a security measure, user input is used for an element's #value only if
+ * the element exists within $form, is not disabled (as per the #disabled
+ * property), and can be accessed (as per the #access property, except that
+ * forms submitted using self::submitForm() bypass #access restrictions). When
+ * user input is ignored due to #disabled and #access restrictions, the
+ * element's default value is used.
+ *
+ * Because of the preorder traversal, where #process functions of an element
+ * run before user input for its child elements is processed, and because of
+ * the Form API security of user input processing with respect to #access and
+ * #disabled described above, this generally means that #process functions
+ * should not use an element's (unvalidated) #value to affect the #disabled or
+ * #access of child elements. Use-cases where a developer may be tempted to
+ * implement such conditional logic usually fall into one of two categories:
+ * - Where user input from the current submission must affect the structure of
+ * a form, including properties like #access and #disabled that affect how
+ * the next submission needs to be processed, a multi-step workflow is
+ * needed. This is most commonly implemented with a submit handler setting
+ * persistent data within $form_state based on *validated* values in
+ * $form_state->getValues() and checking $form_state->isRebuilding(). The
+ * form building functions must then be implemented to use the $form_state
+ * to rebuild the form with the structure appropriate for the new state.
+ * - Where user input must affect the rendering of the form without affecting
+ * its structure, the necessary conditional rendering logic should reside
+ * within functions that run during the rendering phase (#pre_render,
+ * #theme, #theme_wrappers, and #post_render).
+ *
+ * @param string $form_id
+ * A unique string identifying the form for validation, submission,
+ * theming, and hook_form_alter functions.
+ * @param array $element
+ * An associative array containing the structure of the current element.
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ * The current state of the form. In this context, it is used to accumulate
+ * information about which button was clicked when the form was submitted,
+ * as well as the sanitized \Drupal::request()->request data.
+ *
+ * @return array
+ */
+ public function doBuildForm($form_id, &$element, FormStateInterface &$form_state);
+
+}
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