interface FormBuilderInterface

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 $form_arg
     *   The value must be one of the following:
     *   - The name of a class that implements \Drupal\Core\Form\FormInterface.
     *   - An instance of a class that implements \Drupal\Core\Form\FormInterface.
     * phpcs:disable Drupal.Commenting
     * @todo Uncomment new method parameters before drupal:11.0.0.
     * @see https://www.drupal.org/project/drupal/issues/3354672
     *
     * @param mixed ...$args
     *   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
     *   $form_state->getBuildInfo()['args'].
     * phpcs:enable
     *
     * @return array
     *   The form array.
     *
     * @see \Drupal\Core\Form\FormBuilderInterface::buildForm()
     */
    public function getForm($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_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 \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_arg, 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().
     *
     * For example:
     * @code
     * // Set the administrator role to 'content_editor'.
     * $values['user_admin_role'] = 'content_editor';
     * $form_state = new FormState();
     * $form_state->setValues($values);
     * \Drupal::formBuilder()->submitForm(RoleSettingsForm::class, $form_state);
     * @endcode
     *
     * @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 \Drupal\Core\Form\FormStateInterface $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 $form_state->getValues(), then the
     *   default value of the respective element is used. To submit an unchecked
     *   checkbox or other control that browsers submit by not having a
     *   \Drupal::request()->request entry, include the key, but set the value to
     *   NULL.
     * phpcs:disable Drupal.Commenting
     * @todo Uncomment new method parameters before drupal:11.0.0.
     * @see https://www.drupal.org/project/drupal/issues/3354672
     *
     * @param mixed ...$args
     *   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
     *   $form_state build info array so that the reference can be preserved. For
     *   example, a form builder function with the following signature:
     *   @code
     *   function my_module_form($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('my_module_form', $form_state);
     *   @endcode
     * phpcs:enable
     */
    public function submitForm($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
     * RendererInterface::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.
     * - $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 RendererInterface::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 (non validated) #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);

}