Computed field

/**
 * Implements hook_entity_base_field_info().
 *
 * - Provides a base field that displays the current workflow state on nodes.
 *   This field is intended to be used to expose this information.
 */
function state_machine_workflow_entity_base_field_info(EntityTypeInterface $entity_type) {
  $fields = [];

  /** @var \Drupal\state_machine_workflow\WorkflowHelper $workflowHelper */
  $workflowHelper = \Drupal::service('state_machine_workflow.workflow.helper');

  if ($entity_type->id() === 'node') {
    $fields['current_workflow_state'] = BaseFieldDefinition::create('current_workflow_state')
      ->setLabel(new TranslatableMarkup('Current workflow state'))
      ->setClass('\Drupal\state_machine_workflow\CurrentWorkflowStateFieldItemList')
      ->setDisplayOptions('form', ['type' => 'hidden'])
      ->setTranslatable(TRUE)
      ->setDisplayOptions('view', [
        'label' => 'hidden',
        'type' => 'hidden',
        'weight' => 0,
      ])
      ->setDisplayConfigurable('form', TRUE)
      ->setDisplayConfigurable('view', TRUE)
      ->setComputed(TRUE);
  }

  return $fields;
}

/**
 * Implements hook_field_formatter_info_alter().
 *
 * Make sure the standard formatter for string can be used for the current
 * workflow state computed field.
 */
function state_machine_workflow_field_formatter_info_alter(array &$info) {
  $info['string']['field_types'][] = 'current_workflow_state';
}

/**
 * Implements hook_views_data_alter().
 */
function state_machine_workflow_views_data_alter(array &$data) {
  if (isset($data['node'])) {
    // Add the current company computed field to Views.
    $data['node']['current_workflow_state'] = [
      'title' => t('Current workflow state'),
      'field' => [
        'id' => 'state_machine_workflow_view_current_workflow_state',
      ],
    ];
  }
}

================================================

<?php

namespace Drupal\state_machine_workflow;

use Drupal\Core\Field\FieldItemList;

/**
 * Item list for the current workflow state.
 *
 * @see \Drupal\state_machine_workflow\Plugin\Field\FieldType\CurrentWorkflowStateItem
 */
class CurrentWorkflowStateFieldItemList extends FieldItemList {

  /**
   * {@inheritdoc}
   */
  public function getIterator() {
    $this->ensureLoaded();
    return new \ArrayIterator($this->list);
  }

  /**
   * {@inheritdoc}
   */
  public function getValue($include_computed = FALSE) {
    $this->ensureLoaded();
    return parent::getValue($include_computed);
  }

  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    $this->ensureLoaded();
    return parent::isEmpty();
  }

  /**
   * Makes sure that the item list is never empty.
   *
   * For 'normal' fields that use database storage the field item list is
   * initially empty, but since this is a computed field this always has a
   * value.
   * Make sure the item list is always populated, so this field is not skipped
   * for rendering in EntityViewDisplay and friends.
   *
   * This trick has been borrowed from issue #2846554 which does the same for
   * the PathItem field.
   *
   * @see https://www.drupal.org/node/2846554
   */
  protected function ensureLoaded() {
    if (!isset($this->list[0])) {
      $this->list[0] = $this->createItem(0);
    }
  }

}


================================================

<?php

namespace Drupal\state_machine_workflow\Plugin\Field\FieldType;

use Drupal\Core\Field\Plugin\Field\FieldType\StringItem;
use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\TypedData\DataDefinitionInterface;
use Drupal\Core\TypedData\TypedDataInterface;

/**
 * Computed field that shows the current workflow state.
 *
 * @FieldType(
 *   id = "current_workflow_state",
 *   label = @Translation("Current workflow state"),
 *   description = @Translation("Computed field that shows the current workflow state."),
 *   no_ui = TRUE,
 *   default_widget = "current_workflow_state_widget",
 *   default_formatter = "current_workflow_state_formatter"
 * )
 */
class CurrentWorkflowStateItem extends StringItem {

  /**
   * The workflow helper service.
   *
   * @var \Drupal\state_machine_workflow\WorkflowHelperInterface
   */
  protected $workflowHelper;

  /**
   * The revision manager service.
   *
   * @var \Drupal\state_machine_workflow\RevisionManagerInterface
   */
  protected $revisionManager;

  /**
   * Whether or not the value has been calculated.
   *
   * @var bool
   */
  protected $isCalculated = FALSE;

  /**
   * {@inheritdoc}
   */
  public function __construct(DataDefinitionInterface $definition, $name = NULL, TypedDataInterface $parent = NULL) {
    parent::__construct($definition, $name, $parent);
    $this->workflowHelper = \Drupal::service('state_machine_workflow.workflow.helper');
    $this->revisionManager = \Drupal::service('state_machine_workflow.revision_manager');
  }

  /**
   * {@inheritdoc}
   */
  public function __get($name) {
    $this->ensureCalculated();
    return parent::__get($name);
  }

  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    $this->ensureCalculated();
    return parent::isEmpty();
  }

  /**
   * {@inheritdoc}
   */
  public function getValue() {
    $this->ensureCalculated();
    return parent::getValue();
  }

  /**
   * Makes sure that the value is populated.
   *
   * Normal fields get their data from the database and are populated if there
   * is data available. Since this is a computed field we need to make sure
   * there is always data available ourselves.
   *
   * This trick has been borrowed from issue #2846554 which does the same for
   * the PathItem field.
   *
   * @todo Remove this when issue #2392845 is fixed.
   *
   * @see https://www.drupal.org/node/2392845
   * @see https://www.drupal.org/node/2846554
   */
  protected function ensureCalculated() {
    if (!$this->isCalculated) {
      $entity = $this->getEntity();
      if (!$entity->isNew()) {
        $latest_revision = $this->revisionManager->loadLatestRevision($entity);
        $latest_revision = \Drupal::service('entity.repository')->getTranslationFromContext($latest_revision);
        if ($this->workflowHelper->hasEntityStateField($latest_revision)) {
          $field = $this->workflowHelper->getEntityStateField($latest_revision);
          $state_id = $field->getValue()['value'];
          $value = $field->getWorkflow()->getState($state_id)->getLabel();
          $this->setValue($value);
        }
      }
      $this->isCalculated = TRUE;
    }
  }

  /**
   * Checks access to the given entity.
   *
   * By default, entity 'view' access is checked. However, a subclass can choose
   * to exclude certain items from entity access checking by immediately
   * granting access.
   *
   * @param \Drupal\Core\Entity\EntityInterface $entity
   *    The entity to check.
   *
   * @return bool|\Drupal\Core\Access\AccessResult
   *   A cacheable access result.
   */
  protected function checkAccess(EntityInterface $entity) {
    return $entity->access('edit', NULL, FALSE);
  }

}


================================================

<?php

namespace Drupal\state_machine_workflow\Plugin\Field\FieldWidget;

use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\state_machine_workflow\WorkflowHelperInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Plugin implementation of the 'current_workflow_state_widget' widget.
 *
 * @FieldWidget(
 *   id = "current_workflow_state_widget",
 *   label = @Translation("Plain text"),
 *   field_types = {
 *     "current_workflow_state"
 *   }
 * )
 */
class CurrentWorkflowStateWidget extends WidgetBase implements ContainerFactoryPluginInterface {

  /**
   * The workflow helper service.
   *
   * @var \Drupal\state_machine_workflow\WorkflowHelperInterface
   */
  protected $workflowHelper;

  /**
   * Constructs a SearchWidget object.
   *
   * @param string $plugin_id
   *   The plugin_id for the widget.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
   *   The definition of the field to which the widget is associated.
   * @param array $settings
   *   The widget settings.
   * @param array $third_party_settings
   *   Any third party settings.
   * @param \Drupal\state_machine_workflow\WorkflowHelperInterface $workflow_helper
   *   The workflow helper service.
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings, WorkflowHelperInterface $workflow_helper) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings);

    $this->workflowHelper = $workflow_helper;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $plugin_id,
      $plugin_definition,
      $configuration['field_definition'],
      $configuration['settings'],
      $configuration['third_party_settings'],
      $container->get('state_machine_workflow.workflow.helper')
    );
  }

  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'title' => 'Current workflow state',
      'title_display' => 'before',
      'show_for_new_entities' => FALSE,
    ] + parent::defaultSettings();
  }

  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $elements = parent::settingsForm($form, $form_state);

    $elements['title'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Label'),
      '#default_value' => $this->getSetting('title'),
    ];
    $elements['title_display'] = [
      '#type' => 'radios',
      '#title' => $this->t('Display label'),
      '#default_value' => $this->getSetting('title_display'),
      '#options' => [
        'before' => $this->t('Label goes before the element'),
        'after' => $this->t('Label goes after the element'),
        'invisible' => $this->t('Label is there but is made invisible using CSS'),
        'attribute' => $this->t('Make it the title attribute (hover tooltip)'),
      ],
    ];
    $elements['show_for_new_entities'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Show when creating a new entity'),
      '#description' => $this->t('If unchecked, the widget is shown only on forms where an existing entity is being edited.'),
      '#default_value' => $this->getSetting('show_for_new_entities'),
    ];

    return $elements;
  }

  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [
      $this->t('Label: @title', [
        '@title' => $this->getSetting('title'),
      ]),
      $this->t('Display label: @title_display', [
        '@title_display' => $this->getSetting('title_display'),
      ]),
    ];

    if ($this->getSetting('show_for_new_entities')) {
      $summary[] = $this->t('Show when creating a new entity');
    }

    return $summary;
  }

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    /** @var \Drupal\Core\Entity\FieldableEntityInterface $entity */
    $entity = $items->getEntity();
    $field = $this->workflowHelper->getEntityStateField($entity);
    $state_id = $field->getValue()['value'];

    $element['#title'] = $this->getSetting('title');
    $element['#title_display'] = $this->getSetting('title_display');
    $element['#type'] = 'item';
    $element['#element_validate'][] = [get_class($this), 'validateFormElement'];

    $element['current_workflow_state']['#type'] = 'container';
    $element['current_workflow_state']['#attributes'] = ['class' => ['current-workflow-state']];
    $element['current_workflow_state']['label'] = [
      '#plain_text' => $field->getWorkflow()->getState($state_id)->getLabel(),
    ];

    // Show the widget only when the entity is not new, or when the specific
    // setting is turned on.
    $element['#access'] = !$entity->isNew() || $this->getSetting('show_for_new_entities');

    return $element;
  }

  /**
   * Form element validation handler.
   *
   * @param array $element
   *   The form element.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form state.
   */
  public static function validateFormElement(array &$element, FormStateInterface $form_state) {
    // We have nothing to validate, this data is coming from a trusted source.
    // Just set the value for the element directly.
    $form_state->setValueForElement($element['current_workflow_state'], $element['current_workflow_state']['label']['#plain_text']);
  }

}