#3281556 add plugin for Diff

src/Plugin/diff/Field/StateFieldBuilder.php

+<?php
+
+namespace Drupal\state_machine\Plugin\diff\Field;
+
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\diff\Plugin\diff\Field\CoreFieldBuilder;
+
+/**
+ * Plugin to compare state fields.
+ *
+ * @FieldDiffBuilder(
+ *   id = "state_diff_builder",
+ *   label = @Translation("State Field Diff"),
+ *   field_types = {
+ *     "state"
+ *   },
+ * )
+ */
+class StateFieldBuilder extends CoreFieldBuilder {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(FieldItemListInterface $field_items) {
+    $result = [];
+
+    foreach ($field_items as $field_key => $field_item) {
+      if (!$field_item->isEmpty()) {
+        $value = $field_item->view(['label' => 'hidden', 'type' => 'default']);
+        $rendered_value = $this->renderer->renderPlain($value);
+        $result[$field_key][] = $rendered_value;
+      }
+    }
+
+    return $result;
+  }
+
+}