ComputedTestFieldItemList.php

<?php

namespace Drupal\entity_test\Plugin\Field;

use Drupal\Core\TypedData\ComputedItemListTrait;
use Drupal\Core\Field\FieldItemList;

/**
 * A computed field item list.
 */
class ComputedTestFieldItemList extends FieldItemList {
    use ComputedItemListTrait;
    
    /**
     * Compute the list property from state.
     */
    protected function computeValue() {
        // Count the number of times this method has been executed during the
        // lifecycle of an entity.
        $execution_count = \Drupal::state()->get('computed_test_field_execution', 0);
        \Drupal::state()->set('computed_test_field_execution', ++$execution_count);
        foreach (\Drupal::state()->get('entity_test_computed_field_item_list_value', []) as $delta => $item) {
            $this->list[$delta] = $this->createItem($delta, $item);
        }
    }

}