Create a custom field widget

Field widgets are used to render the field inside forms. Field widgets are defined as plugins, so it's a good idea to familiarize yourself with the Plugin API before diving into writing a new field type.

To create a field widget in Drupal 8 you need a class with the FieldWidget annotation.

The location of the field widget class should be /[MODULE_NAME]/src/Plugin/Field/FieldWidget/. For example, /foo/src/Plugin/Field/FieldWidget/BarWidget.php.

The namespace of that class should be \Drupal\[MODULE_NAME]\Plugin\Field\FieldWidget. For example, \Drupal\foo\Plugin\Field\FieldWidget.

The annotation above the class should include a unique id, a label and an array of ids of field types that this widget can handle.

/**
 * A widget bar.
 *
 * @FieldWidget(
 *   id = "bar",
 *   label = @Translation("Bar widget"),
 *   field_types = {
 *     "baz",
 *     "string"
 *   }
 * )
 */

The class needs to implement the WidgetInterface interface. And can extend the WidgetBase class for common implementation of the interface. The only required method to implement is ::formElement() which should return the actual form elements that represent your widget.

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;

//...
class BarWidget extends WidgetBase {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element += [
      // Add to the element render array passed in.
      // See WidgetInterface::formElement().
    ];
    
    return ['value' => $element];
  }

}

Widget Settings

If your widget needs additional settings, there are three steps required to achieve this:

1. Override PluginSettingsBase::defaultSettings() in order to set the defaults
2. Create the configuration schema for the settings you've created
3. Create a form to allow users to change the settings

Step 1: Override PluginSettingsBase::defaultSettings() in order to set the defaults

/**
 * {@inheritdoc}
 */
 public static function defaultSettings() {
  return [
    // Create the custom setting 'size', and
    // assign a default value of 60
    'size' => 60,
  ] + parent::defaultSettings();
}

Step 2: Create the configuration schema for the settings you've created

Configuration schema goes in the following file:

/[MODULE_NAME]/config/schema/[MODULE_NAME].schema.yml

In this file, you will describe the data type(s) of the setting(s) you've introduced in defaultSettings():

Step 1 created a setting named 'size', that stores an integer value. The schema for this would look like the following:

field.widget.settings.[WIDGET ID]:
  type: mapping
  label: 'WIDGET NAME widget settings'
  mapping:
    size:
      type: integer
      label: 'Size'

Step 3 - Create a form to allow users to change the settings

The form to allow for users to change the values for the settings is created by overriding WidgetBase::settingsForm()

/**
 * {@inheritdoc}
 */
public function settingsForm(array $form, FormStateInterface $form_state) {
  $element['size'] = [
    '#type' => 'number',
    '#title' => $this->t('Size of textfield'),
    '#default_value' => $this->getSetting('size'),
    '#required' => TRUE,
    '#min' => 1,
  ];

  return $element;
}

You can also list the selected settings in the summary for the widget as follows:

/**
 * {@inheritdoc}
 */
public function settingsSummary() {
  $summary = [];

  $summary[] = $this->t('Textfield size: @size', ['@size' => $this->getSetting('size')]);

  return $summary;
}

You can use the getSetting() method of the class to retrieve the setting, for use in the widget:

class BarWidget extends WidgetBase implements WidgetInterface {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element['value'] = $element + [
      '#type' => 'textfield',
      '#default_value' => isset($items[$delta]->value) ? $items[$delta]->value : NULL,
      '#size' => $this->getSetting('size'),
    ];

    return $element;
  }

}

Example Widget

The TextWidget from the field_example module under the examples project:

namespace Drupal\field_example\Plugin\Field\FieldWidget;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Plugin implementation of the 'field_example_text' widget.
 *
 * @FieldWidget(
 *   id = "field_example_text",
 *   module = "field_example",
 *   label = @Translation("RGB value as #ffffff"),
 *   field_types = {
 *     "field_example_rgb"
 *   }
 * )
 */
class TextWidget extends WidgetBase {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $value = isset($items[$delta]->value) ? $items[$delta]->value : '';
    $element += [
      '#type' => 'textfield',
      '#default_value' => $value,
      '#size' => 7,
      '#maxlength' => 7,
      '#element_validate' => [
        [static::class, 'validate'],
      ],
    ];
    return ['value' => $element];
  }

  /**
   * Validate the color text field.
   */
  public static function validate($element, FormStateInterface $form_state) {
    $value = $element['#value'];
    if (strlen($value) == 0) {
      $form_state->setValueForElement($element, '');
      return;
    }
    if (!preg_match('/^#([a-f0-9]{6})$/iD', strtolower($value))) {
      $form_state->setError($element, t("Color must be a 6-digit hexadecimal value, suitable for CSS."));
    }
  }

}