#3216713:Add formatter and static caching

Closes #3216713

modules/promotion/src/Plugin/Field/FieldFormatter/UsageLimitFormatter.php

+<?php
+
+namespace Drupal\commerce_promotion\Plugin\Field\FieldFormatter;
+
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\FormatterBase;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Plugin implementation of the 'commerce_usage_limit' formatter.
+ *
+ * @FieldFormatter(
+ *   id = "commerce_usage_limit",
+ *   label = @Translation("Usage limit"),
+ *   field_types = {
+ *     "integer",
+ *   },
+ * )
+ */
+class UsageLimitFormatter extends FormatterBase {
+
+  /**
+   * The usage.
+   *
+   * @var \Drupal\commerce_promotion\PromotionUsageInterface
+   */
+  protected $usage;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    $instance = parent::create($container, $configuration, $plugin_id, $plugin_definition);
+    $instance->usage = $container->get('commerce_promotion.usage');
+    return $instance;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function viewElements(FieldItemListInterface $items, $langcode) {
+    $elements = [];
+    $field_name = $items->getName();
+    $entity = $items->getEntity();
+
+    // Get usage of promotions/coupons from the static cache
+    // populated in PromotionUsage service.
+    $current_usage = $entity->getEntityTypeId() == 'commerce_promotion' ?
+      $this->usage->load($entity) :
+      $this->usage->loadByCoupon($entity);
+
+    // For the "usage_limit" output actual usage / limit.
+    if ($field_name == 'usage_limit') {
+      $usage_limit = $entity->getUsageLimit();
+      $usage_limit = $usage_limit ?: $this->t('Unlimited');
+      $usage = $current_usage . ' / ' . $usage_limit;
+    }
+    else {
+      // Output the actual limit (if not empty), otherwise output "Unlimited".
+      $usage = $current_usage ?: $this->t('Unlimited');
+    }
+
+    $elements[0] = [
+      '#markup' => $usage,
+    ];
+
+    return $elements;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function isApplicable(FieldDefinitionInterface $field_definition) {
+    $entity_type = $field_definition->getTargetEntityTypeId();
+    $field_name = $field_definition->getName();
+    $applicable_entity_type = in_array($entity_type, ['commerce_promotion', 'commerce_promotion_coupon']);
+    $applicable_field_name = in_array($field_name, ['usage_limit', 'usage_limit_customer']);
+    return $applicable_entity_type && $applicable_field_name;
+  }
+
+}