In answer the first checkbox is always checked

Problem/Motivation

Thank you for your work on developing the module!

In the lms_answer_plugins submodule, using the "Multiple answer select", "Multiple answer select with feedback", "Single answer select with feedback", and "Single answer select" types, I noticed that when I check any of the checkboxes, the first option is always selected.

Proposed resolution

The error is caused by the current use of checkboxes. The description of Checkboxes.php states:

/**
 * Provides a form element for a set of checkboxes.
 *
 * Properties:
 * - #options: An associative array whose keys are the values returned for each
 *   checkbox, and whose values are the labels next to each checkbox. The
 *   #options array cannot have a 0 key, as it would not be possible to discern
 *   checked and unchecked states.

However, the following is included in the lms_answer_plugin/src/Plugin/SelectBase.php class (the $delta is a number, started 0) :

  /**
   * {@inheritdoc}
   */
  public function answeringForm(array &$form, FormStateInterface $form_state, Answer $answer): void {
    $activity = $answer->getActivity();
    $data = $answer->getData();
    $options = [];
    foreach ($activity->get('answers') as $delta => $answer_item) {
      $options[$delta] = $answer_item->get('answer')->getValue();
    }

I suggest something like this:

...
  /**
   * {@inheritdoc}
   */
  public function getScore(Answer $answer): float {
    $answers = $answer->getActivity()->get('answers');
    $data = $answer->getData()['answer'];
    if (!\is_array($data)) {
      $data = [$data];
    }

    $data = \array_filter($data, static fn($item) => $item !== 0);
    $score = 0;
    $max_score = 0;
    foreach ($answers as $delta => $answer_item) {
//      if (\in_array((string) $delta, $data, TRUE)) {
      if (\in_array('o-' . $delta, $data, TRUE)) {

        $answer = TRUE;
      }
      else {
        $answer = FALSE;
      }

...

  /**
   * {@inheritdoc}
   */
  public function answeringForm(array &$form, FormStateInterface $form_state, Answer $answer): void {
    $activity = $answer->getActivity();
    $data = $answer->getData();
    $options = [];
    foreach ($activity->get('answers') as $delta => $answer_item) {
//      $options[$delta] = $answer_item->get('answer')->getValue();
      $options['o-' . $delta] = $answer_item->get('answer')->getValue();
    }

...

  /**
   * {@inheritdoc}
   */
  protected function getAnswerRenderable(Answer $answer): array {
    $data = $answer->getData();
    if (!\array_key_exists('answer', $data)) {
      return [];
    }
...
    foreach ($answer->getActivity()->get('answers') as $delta => $answer_item) {
//      $renderable['#items'][] = $answer_item->get('answer')->getValue() . ' - ' . (\array_key_exists($delta, $data['answer']) ? $this->t('Selected') : $this->t('Not selected'));
      $renderable['#items'][] = $answer_item->get('answer')->getValue() . ' - ' . (\array_key_exists('o-' . $delta, $data['answer']) ? $this->t('Selected') : $this->t('Not selected'));
    }

...