Issue #2760609 by Fabianx, stefan.r, chx: Allow the use of callbacks instead...

 *   generate the same form (or very similar forms) using different $form_ids

 *   can implement hook_forms(), which maps different $form_id values to the

 *   proper form constructor function. Examples may be found in node_forms(),

 *   and search_forms().

 *   and search_forms(). hook_forms() can also be used to define forms in

 *   classes.

 * @param ...

 *   Any additional arguments are passed on to the functions called by

 *   drupal_get_form(), including the unique form constructor function. For

    }

    if (isset($form_definition['callback'])) {

      $callback = $form_definition['callback'];

      $form_state['build_info']['base_form_id'] = $callback;

      $form_state['build_info']['base_form_id'] = isset($form_definition['base_form_id']) ? $form_definition['base_form_id'] : $callback;

    }

    // In case $form_state['wrapper_callback'] is not defined already, we also

    // allow hook_forms() to define one.

  // the actual form builder function ($callback) expects. This allows for

  // pre-populating a form with common elements for certain forms, such as

  // back/next/save buttons in multi-step form wizards. See drupal_build_form().

  if (isset($form_state['wrapper_callback']) && function_exists($form_state['wrapper_callback'])) {

  if (isset($form_state['wrapper_callback']) && is_callable($form_state['wrapper_callback'])) {

    $form = call_user_func_array($form_state['wrapper_callback'], $args);

    // Put the prepopulated $form into $args.

    $args[0] = $form;

 * the $form_id input matched your module's format for dynamically-generated

 * form IDs, and if so, act appropriately.

 *

 * Third, forms defined in classes can be defined this way.

 *

 * @param $form_id

 *   The unique string identifying the desired form.

 * @param $args

 * @return

 *   An associative array whose keys define form_ids and whose values are an

 *   associative array defining the following keys:

 *   - callback: The name of the form builder function to invoke. This will be

 *     used for the base form ID, for example, to target a base form using

 *     hook_form_BASE_FORM_ID_alter().

 *   - callback: The callable returning the form array. If it is the name of

 *     the form builder function then this will be used for the base

 *     form ID, for example, to target a base form using

 *     hook_form_BASE_FORM_ID_alter(). Otherwise use the base_form_id key to

 *     define the base form ID.

 *   - callback arguments: (optional) Additional arguments to pass to the

 *     function defined in 'callback', which are prepended to $args.

 *   - wrapper_callback: (optional) The name of a form builder function to

 *     invoke before the form builder defined in 'callback' is invoked. This

 *     wrapper callback may prepopulate the $form array with form elements,

 *     which will then be already contained in the $form that is passed on to

 *     the form builder defined in 'callback'. For example, a wrapper callback

 *     could setup wizard-alike form buttons that are the same for a variety of

 *     forms that belong to the wizard, which all share the same wrapper

 *     callback.

 *   - base_form_id: The base form ID can be specified explicitly. This is

 *     required when callback is not the name of a function.

 *   - wrapper_callback: (optional) Any callable to invoke before the form

 *     builder defined in 'callback' is invoked. This wrapper callback may

 *     prepopulate the $form array with form elements, which will then be

 *     already contained in the $form that is passed on to the form builder

 *     defined in 'callback'. For example, a wrapper callback could setup

 *     wizard-like form buttons that are the same for a variety of forms that

 *     belong to the wizard, which all share the same wrapper callback.

 */

function hook_forms($form_id, $args) {

  // Simply reroute the (non-existing) $form_id 'mymodule_first_form' to

    'wrapper_callback' => 'mymodule_main_form_wrapper',

  );

  // Build a form with a static class callback.

  $forms['mymodule_class_generated_form'] = array(

    // This will call: MyClass::generateMainForm().

    'callback' => array('MyClass', 'generateMainForm'),

    // The base_form_id is required when the callback is a static function in

    // a class. This can also be used to keep newer code backwards compatible.

    'base_form_id' => 'mymodule_main_form',

  );

  return $forms;

}