interface FormInterface

interface FormInterface {
    
    /**
     * Returns a unique string identifying the form.
     *
     * The returned ID should be a unique string that can be a valid PHP function
     * name, since it's used in hook implementation names such as
     * hook_form_FORM_ID_alter().
     *
     * @return string
     *   The unique string identifying the form.
     */
    public function getFormId();
    
    /**
     * Form constructor.
     *
     * @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.
     *
     * @return array
     *   The form structure.
     */
    public function buildForm(array $form, FormStateInterface $form_state);
    
    /**
     * Form validation handler.
     *
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public function validateForm(array &$form, FormStateInterface $form_state);
    
    /**
     * Form submission handler.
     *
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public function submitForm(array &$form, FormStateInterface $form_state);

}