WidgetBase::form

public WidgetBase::form(FieldItemListInterface $items, array &$form, FormStateInterface $form_state, $get_delta = NULL)

public function form(FieldItemListInterface $items, array &$form, FormStateInterface $form_state, $get_delta = NULL) {
  $field_name = $this->fieldDefinition->getName();
  $parents = $form['#parents'];

  // Store field information in $form_state.
  if (!static::getWidgetState($parents, $field_name, $form_state)) {
    $field_state = array(
      'items_count' => count($items),
      'array_parents' => array(),
    );
    static::setWidgetState($parents, $field_name, $form_state, $field_state);
  }

  // Collect widget elements.
  $elements = array();

  // If the widget is handling multiple values (e.g Options), or if we are
  // displaying an individual element, just get a single form element and make
  // it the $delta value.
  if ($this->handlesMultipleValues() || isset($get_delta)) {
    $delta = isset($get_delta) ? $get_delta : 0;
    $element = array(
      '#title' => $this->fieldDefinition->getLabel(),
      '#description' => FieldFilteredMarkup::create(\Drupal::token()->replace($this->fieldDefinition->getDescription())),
    );
    $element = $this->formSingleElement($items, $delta, $element, $form, $form_state);

    if ($element) {
      if (isset($get_delta)) {
        // If we are processing a specific delta value for a field where the
        // field module handles multiples, set the delta in the result.
        $elements[$delta] = $element;
      }
      else {
        // For fields that handle their own processing, we cannot make
        // assumptions about how the field is structured, just merge in the
        // returned element.
        $elements = $element;
      }
    }
  }
  // If the widget does not handle multiple values itself, (and we are not
  // displaying an individual element), process the multiple value form.
  else {
    $elements = $this->formMultipleElements($items, $form, $form_state);
  }

  // Populate the 'array_parents' information in $form_state->get('field')
  // after the form is built, so that we catch changes in the form structure
  // performed in alter() hooks.
  $elements['#after_build'][] = array(get_class($this), 'afterBuild');
  $elements['#field_name'] = $field_name;
  $elements['#field_parents'] = $parents;
  // Enforce the structure of submitted values.
  $elements['#parents'] = array_merge($parents, array($field_name));
  // Most widgets need their internal structure preserved in submitted values.
  $elements += array('#tree' => TRUE);

  return array(
    // Aid in theming of widgets by rendering a classified container.
    '#type' => 'container',
    // Assign a different parent, to keep the main id for the widget itself.
    '#parents' => array_merge($parents, array($field_name . '_wrapper')),
    '#attributes' => array(
      'class' => array(
        'field--type-' . Html::getClass($this->fieldDefinition->getType()),
        'field--name-' . Html::getClass($field_name),
        'field--widget-' . Html::getClass($this->getPluginId()),
      ),
    ),
    'widget' => $elements,
  );
}