How to validate a field value programatically in Drupal 8?

Question

I've created a module for Drupal8 that checks against values in the user register and set an error if the values do no match with something you have entered in a custom form.

My module has got a form where you add a list of username you don't want the user to take and a custom message to display as an error.

Forms works perfectly, I can store data, edit it etc... It is stored under src/Form/ and settings are under config/install

Now when I'm trying to hook the user register form validate I got an error regarding form_set_error():

PHP Fatal error: Call to undefined function form_set_error()

Also I think I'm not using $form_state in the right way (but it does the job)

Here's the code in my_module.module:

function my_module_form_user_register_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
 $form['#validate'][] = 'my_module_user_validate';
}

/**
 * Custom validation function.
 */
function my_module_user_validate(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
    // get module settings
    $config = \Drupal::config('my_module.settings');

    // Convert $form_state to an array as it has been passed as object, it looks wrong but I could not find any documentation at the moment
    $form_state = get_object_vars($form_state);
    $name = $form_state['values']['name']);
    $usernames = $config->get('my_modules_usernames');
    // Username are inserted in a text area separated by an enter key
    $usernames = $usernames != '' ? explode("\r\n", $usernames) : array();
    foreach ($usernames as $username) {
      if ($username == $name[1]) {
        //apparently in Drupal 8 form_set_error does not work anymmore
        form_set_error('mail', $config->get('my_module_message'));
      }
    }
  } 

Answer 1

The $form_state now become a classed object as documented here

You can use getValue() to get field value instead of converting it into an array

$form_state->getValue('name');

instead of:

$form_state = get_object_vars($form_state);
$name = $form_state['values']['name']);

Same thing for form_set_value.

And form_set_error() became $form_state->setErrorByName().

Answer 2

The form_set_error() has been dropped in favor of the following $form_state class methods:

$form_state->setError($element, $message);
$form_state->setErrorByName($element, $message);

So the code would look like:

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // Use this to find out about the current form id.
  drupal_set_message('Form ID: ' . $form_id);
}

/**
 * Implements hook_form_FORM_ID_alter() for the FORM_ID() form.
 */
function mymodule_form_FORM_ID_alter(&$form, FormStateInterface $form_state, $form_id) {
  $form['#validate'][] = '_mymodule_form_validate';
}

/**
 * Validates submission values in the FORM_ID() form.
 */
function _mymodule_form_validate(array &$form, FormStateInterface $form_state) {
  $values = $form_state->getValue('field_myfield');
  foreach ($values as $value) {
    $plain_value = is_array($value) ? current($value) : NULL;
    if (empty($plain_value)) {
      $form_state->setErrorByName('field_myfield', t('Please enter a value!'));
    }
  }
}