FieldConfigEditForm::form

public FieldConfigEditForm::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $form = parent::form($form, $form_state);

  $field_storage = $this->entity->getFieldStorageDefinition();
  $bundles = $this->entityManager->getBundleInfo($this->entity->getTargetEntityTypeId());

  $form_title = $this->t('%field settings for %bundle', array(
    '%field' => $this->entity->getLabel(),
    '%bundle' => $bundles[$this->entity->getTargetBundle()]['label'],
  ));
  $form['#title'] = $form_title;

  if ($field_storage->isLocked()) {
    $form['locked'] = array(
      '#markup' => $this->t('The field %field is locked and cannot be edited.', array('%field' => $this->entity->getLabel())),
    );
    return $form;
  }

  // Build the configurable field values.
  $form['label'] = array(
    '#type' => 'textfield',
    '#title' => $this->t('Label'),
    '#default_value' => $this->entity->getLabel() ? : $field_storage->getName(),
    '#required' => TRUE,
    '#weight' => -20,
  );

  $form['description'] = array(
    '#type' => 'textarea',
    '#title' => $this->t('Help text'),
    '#default_value' => $this->entity->getDescription(),
    '#rows' => 5,
    '#description' => $this->t('Instructions to present to the user below this field on the editing form.<br />Allowed HTML tags: @tags', array('@tags' => FieldFilteredMarkup::displayAllowedTags())) . '<br />' . $this->t('This field supports tokens.'),
    '#weight' => -10,
  );

  $form['required'] = array(
    '#type' => 'checkbox',
    '#title' => $this->t('Required field'),
    '#default_value' => $this->entity->isRequired(),
    '#weight' => -5,
  );

  // Create an arbitrary entity object (used by the 'default value' widget).
  $ids = (object) array(
    'entity_type' => $this->entity->getTargetEntityTypeId(),
    'bundle' => $this->entity->getTargetBundle(),
    'entity_id' => NULL
  );
  $form['#entity'] = _field_create_entity_from_ids($ids);
  $items = $form['#entity']->get($this->entity->getName());
  $item = $items->first() ? : $items->appendItem();

  // Add field settings for the field type and a container for third party
  // settings that modules can add to via hook_form_FORM_ID_alter().
  $form['settings'] = array(
    '#tree' => TRUE,
    '#weight' => 10,
  );
  $form['settings'] += $item->fieldSettingsForm($form, $form_state);
  $form['third_party_settings'] = array(
    '#tree' => TRUE,
    '#weight' => 11,
  );

  // Add handling for default value.
  if ($element = $items->defaultValuesForm($form, $form_state)) {
    $element = array_merge($element, array(
      '#type' => 'details',
      '#title' => $this->t('Default value'),
      '#open' => TRUE,
      '#tree' => TRUE,
      '#description' => $this->t('The default value for this field, used when creating new content.'),
    ));

    $form['default_value'] = $element;
  }

  return $form;
}