Add WC_Queue and WC()->queue() wrapper for accessing it

To take care of instantiating a canonical job queue for use across
all of WooCommerce, and by 3rd party code.

Also use this new API for enqueuing webhooks instead of raw Action
Scheduler APIs.

includes/class-woocommerce.php

@@ -335,6 +335,7 @@ final class WooCommerce {
 		include_once WC_ABSPATH . 'includes/class-wc-shortcodes.php';
 		include_once WC_ABSPATH . 'includes/class-wc-logger.php';
 		include_once WC_ABSPATH . 'includes/queue/class-wc-action-queue.php';
+		include_once WC_ABSPATH . 'includes/queue/class-wc-queue.php';
 
 		/**
 		 * Data stores - used to store and retrieve CRUD object data from the database.
@@ -658,6 +659,15 @@ final class WooCommerce {
 		}
 	}
 
+	/**
+	 * Get queue instance.
+	 *
+	 * @return WC_Queue_Interface
+	 */
+	public function queue() {
+		return WC_Queue::instance();
+	}
+
 	/**
 	 * Get Checkout Class.
 	 *

includes/queue/class-wc-queue.php

@@ -0,0 +1,78 @@
+<?php
+/**
+ * WC Queue
+ *
+ * @version 3.5.0
+ * @package WooCommerce/Interface
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit; // Exit if accessed directly.
+}
+
+/**
+ * WC Queue
+ *
+ * Singleton for managing the WC queue instance.
+ *
+ * @version 3.5.0
+ */
+class WC_Queue {
+
+	/**
+	 * The single instance of the queue.
+	 *
+	 * @var WC_Queue_Interface|null
+	 */
+	protected static $instance = null;
+
+	/**
+	 * The default queue class to initialize
+	 *
+	 * @var string
+	 */
+	protected static $default_cass = 'WC_Action_Queue';
+
+	/**
+	 * @return WC_Queue_Interface
+	 */
+	final public static function instance() {
+
+		if ( is_null( self::$instance ) ) {
+			$class = self::get_class();
+			self::$instance = new $class();
+			self::$instance = self::validate_instance( self::$instance );
+		}
+		return self::$instance;
+	}
+
+	/**
+	 * Get class to instantiate
+	 *
+	 * And make sure 3rd party code has the chance to attach a custom queue class.
+ 	 *
+	 * @return string
+	 */
+	protected static function get_class() {
+		if ( ! did_action( 'plugins_loaded' ) ) {
+			wc_doing_it_wrong( __FUNCTION__, __( 'This function should not be called before plugins_loaded.', 'woocommerce' ), '3.5.0' );
+		}
+
+		return apply_filters( 'woocommerce_queue_class', self::$default_cass );
+	}
+
+	/**
+	 * Enforce a WC_Queue_Interface is 
+ 	 *
+	 * @return WC_Queue_Interface
+	 */
+	protected static function validate_instance( $instance ) {
+		if ( false === ( $instance instanceof WC_Queue_Interface ) ) {
+			$default_class = self::$default_cass;
+			wc_doing_it_wrong( __FUNCTION__, sprintf( __( 'The class attached to the "woocommerce_queue_class" does not implement the WC_Queue_Interface interface. The default %s class will be used instead.', 'woocommerce' ), $default_class ), '3.5.0' );
+			$instance = new $default_class();
+		}
+
+		return $instance;
+	}
+}

includes/wc-webhook-functions.php

@@ -21,7 +21,7 @@ function wc_webhook_process_delivery( $webhook, $arg ) {
 	// user who triggered it.
 	if ( apply_filters( 'woocommerce_webhook_deliver_async', true, $webhook, $arg ) ) {
 		// Deliver in background.
-		wc_schedule_single_action( time(), 'woocommerce_deliver_webhook_async', array( 'webhook_id' => $webhook->get_id(), 'arg' => $arg ), 'woocommerce-webhooks' );
+		WC()->queue()->add( 'woocommerce_deliver_webhook_async', array( 'webhook_id' => $webhook->get_id(), 'arg' => $arg ), 'woocommerce-webhooks' );
 	} else {
 		// Deliver immediately.
 		$webhook->deliver( $arg );