FieldStorageAddForm::buildForm

public FieldStorageAddForm::buildForm(array $form, FormStateInterface $form_state, $entity_type_id = NULL, $bundle = NULL)

public function buildForm(array $form, FormStateInterface $form_state, $entity_type_id = NULL, $bundle = NULL) {
  if (!$form_state->get('entity_type_id')) {
    $form_state->set('entity_type_id', $entity_type_id);
  }
  if (!$form_state->get('bundle')) {
    $form_state->set('bundle', $bundle);
  }

  $this->entityTypeId = $form_state->get('entity_type_id');
  $this->bundle = $form_state->get('bundle');

  // Gather valid field types.
  $field_type_options = array();
  foreach ($this->fieldTypePluginManager->getGroupedDefinitions($this->fieldTypePluginManager->getUiDefinitions()) as $category => $field_types) {
    foreach ($field_types as $name => $field_type) {
      $field_type_options[$category][$name] = $field_type['label'];
    }
  }

  $form['add'] = array(
    '#type' => 'container',
    '#attributes' => array('class' => array('form--inline', 'clearfix')),
  );

  $form['add']['new_storage_type'] = array(
    '#type' => 'select',
    '#title' => $this->t('Add a new field'),
    '#options' => $field_type_options,
    '#empty_option' => $this->t('- Select a field type -'),
  );

  // Re-use existing field.
  if ($existing_field_storage_options = $this->getExistingFieldStorageOptions()) {
    $form['add']['separator'] = array(
      '#type' => 'item',
      '#markup' => $this->t('or'),
    );
    $form['add']['existing_storage_name'] = array(
      '#type' => 'select',
      '#title' => $this->t('Re-use an existing field'),
      '#options' => $existing_field_storage_options,
      '#empty_option' => $this->t('- Select an existing field -'),
    );

    $form['#attached']['drupalSettings']['existingFieldLabels'] = $this->getExistingFieldLabels(array_keys($existing_field_storage_options));
  }
  else {
    // Provide a placeholder form element to simplify the validation code.
    $form['add']['existing_storage_name'] = array(
      '#type' => 'value',
      '#value' => FALSE,
    );
  }

  // Field label and field_name.
  $form['new_storage_wrapper'] = array(
    '#type' => 'container',
    '#states' => array(
      '!visible' => array(
        ':input[name="new_storage_type"]' => array('value' => ''),
      ),
    ),
  );
  $form['new_storage_wrapper']['label'] = array(
    '#type' => 'textfield',
    '#title' => $this->t('Label'),
    '#size' => 15,
  );

  $field_prefix = $this->config('field_ui.settings')->get('field_prefix');
  $form['new_storage_wrapper']['field_name'] = array(
    '#type' => 'machine_name',
    // This field should stay LTR even for RTL languages.
    '#field_prefix' => '<span dir="ltr">' . $field_prefix,
    '#field_suffix' => '</span>&lrm;',
    '#size' => 15,
    '#description' => $this->t('A unique machine-readable name containing letters, numbers, and underscores.'),
    // Calculate characters depending on the length of the field prefix
    // setting. Maximum length is 32.
    '#maxlength' => FieldStorageConfig::NAME_MAX_LENGTH - strlen($field_prefix),
    '#machine_name' => array(
      'source' => array('new_storage_wrapper', 'label'),
      'exists' => array($this, 'fieldNameExists'),
    ),
    '#required' => FALSE,
  );

  // Provide a separate label element for the "Re-use existing field" case
  // and place it outside the $form['add'] wrapper because those elements
  // are displayed inline.
  if ($existing_field_storage_options) {
    $form['existing_storage_label'] = array(
      '#type' => 'textfield',
      '#title' => $this->t('Label'),
      '#size' => 15,
      '#states' => array(
        '!visible' => array(
          ':input[name="existing_storage_name"]' => array('value' => ''),
        ),
      ),
    );
  }

  // Place the 'translatable' property as an explicit value so that contrib
  // modules can form_alter() the value for newly created fields. By default
  // we create field storage as translatable so it will be possible to enable
  // translation at field level.
  $form['translatable'] = array(
    '#type' => 'value',
    '#value' => TRUE,
  );

  $form['actions'] = array('#type' => 'actions');
  $form['actions']['submit'] = array(
    '#type' => 'submit',
    '#value' => $this->t('Save and continue'),
    '#button_type' => 'primary',
  );

  $form['#attached']['library'][] = 'field_ui/drupal.field_ui';

  return $form;
}