Issue #3521036: added diff field builder plugin for viewsreference fields

Closes #3521036

src/Plugin/diff/Field/ViewsReferenceFieldBuilder.php

+<?php
+
+namespace Drupal\viewsreference\Plugin\diff\Field;
+
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\diff\FieldDiffBuilderBase;
+
+/**
+ * Plugin to diff viewsreference fields.
+ *
+ * @FieldDiffBuilder(
+ *   id = "viewsreference_field_diff_builder",
+ *   label = @Translation("Views Reference Field Diff"),
+ *   field_types = {
+ *     "viewsreference"
+ *   },
+ * )
+ */
+class ViewsReferenceFieldBuilder extends FieldDiffBuilderBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(FieldItemListInterface $field_items) {
+    $result = [];
+
+    // Every item from $field_items is of type FieldItemInterface.
+    foreach ($field_items as $field_key => $field_item) {
+      if (!$field_item->isEmpty()) {
+        $values = $field_item->getValue();
+
+        if (isset($values['target_id'])) {
+          $result[$field_key][] = $this->t('View ID: :view_id', [
+            ':view_id' => $values['target_id'] ?: $this->t('Empty'),
+          ]);
+        }
+
+        if (isset($values['display_id'])) {
+          $result[$field_key][] = $this->t('View Display ID: :display_id', [
+            ':display_id' => $values['display_id'] ?: $this->t('Empty'),
+          ]);
+        }
+
+        if (isset($values['data'])) {
+          $data_values = $values['data'] ?
+            (array) unserialize($values['data'], ['allowed_classes' => FALSE]) :
+            [];
+
+          $data_values_filtered = array_filter($data_values, function ($value) {
+            return isset($value) && $value !== '';
+          });
+          if (empty($data_values_filtered)) {
+            $result[$field_key][] = $this->t('View Data: Empty');
+          }
+          else {
+            foreach ($data_values as $data_prop => $data_value) {
+              $result[$field_key][] = $this->t('View Data: @data_prop: @data_value', [
+                '@data_prop' => $data_prop,
+                '@data_value' => $data_value ?: $this->t('Empty'),
+              ]);
+            }
+          }
+        }
+      }
+    }
+
+    return $result;
+  }
+
+}