Create service for transient cache

[MAILPOET-3646]

lib/Cache/TransientCache.php

@@ -0,0 +1,73 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Cache;
+
+use MailPoet\WP\Functions as WPFunctions;
+use MailPoetVendor\Carbon\Carbon;
+
+class TransientCache {
+  public const SUBSCRIBERS_STATISTICS_COUNT_KEY = 'mailpoet_subscribers_statistics_count_cache';
+
+  /** @var WPFunctions */
+  private $wp;
+
+  public function __construct(WPFunctions $wp) {
+    $this->wp = $wp;
+  }
+
+  public function getItem(string $key, int $id): ?array {
+    $items = $this->getItems($key);
+    return $items[$id]['item'] ?? null;
+  }
+
+  public function getOldestCreatedAt(string $key): ?\DateTime {
+    $oldest = $this->getOldestItem($key);
+    return $oldest['created_at'] ?? null;
+  }
+
+  public function getOldestItem(string $key): ?array {
+    $items = $this->getItems($key);
+    $oldest = null;
+    foreach ($items as $item) {
+      if ($oldest === null || $item['created_at'] < $oldest['created_at']) {
+        $oldest = $item;
+      }
+    }
+    return $oldest;
+  }
+
+  public function setItem(string $key, array $item, int $id): void {
+    $items = $this->getItems($key) ?? [];
+    $items[$id] = [
+      'item' => $item,
+      'created_at' => Carbon::now(),
+    ];
+    $this->setItems($key, $items);
+  }
+
+  public function invalidateItem(string $key, int $id): void {
+    $items = $this->getItems($key);
+    unset($items[$id]);
+    if (count($items)) {
+      $this->setItems($key, $items);
+    } else {
+      $this->deleteItems($key);
+    }
+  }
+
+  public function invalidateItems(string $key): void {
+    $this->deleteItems($key);
+  }
+
+  private function deleteItems(string $key): void {
+    $this->wp->deleteTransient($key);
+  }
+
+  private function setItems(string $key, array $items): void {
+    $this->wp->setTransient($key, $items);
+  }
+
+  private function getItems(string $key): array {
+    return $this->wp->getTransient($key) ?: [];
+  }
+}

lib/DI/ContainerConfigurator.php

@@ -387,7 +387,8 @@ class ContainerConfigurator implements IContainerConfigurator {
     // Third party classes
     $container->autowire(\MailPoetVendor\CSS::class)->setClass(\MailPoetVendor\CSS::class)->setPublic(true);
     $container->autowire(\MailPoetVendor\csstidy::class)->setClass(\MailPoetVendor\csstidy::class);
-
+    // Cache
+    $container->autowire(\MailPoet\Cache\TransientCache::class)->setPublic(true);
     return $container;
   }