Add SingletonDecorator

Refactor PurchaseUnitSanitizer service registration to SingletonDecorator

lib/packages/Pattern/SingletonDecorator.php

@@ -0,0 +1,61 @@
+<?php
+
+namespace WooCommerce\PayPalCommerce\Vendor\Pattern;
+
+class SingletonDecorator {
+
+	/**
+	 * The callable with the executing code
+	 *
+	 * @var callable
+	 */
+	private $callable;
+
+	/**
+	 * The execution result
+	 *
+	 * @var mixed
+	 */
+	private $result;
+
+	/**
+	 * Indicates if the callable is resolved
+	 *
+	 * @var bool
+	 */
+	private $executed = false;
+
+	/**
+	 * SingletonDecorator constructor.
+	 *
+	 * @param callable $callable
+	 */
+	public function __construct( callable $callable ) {
+		$this->callable = $callable;
+	}
+
+	/**
+	 * The make constructor.
+	 *
+	 * @param callable $callable
+	 * @return self
+	 */
+	public static function make( callable $callable ): self {
+		return new static( $callable );
+	}
+
+	/**
+	 * Invokes a callable once and returns the same result on subsequent invokes.
+	 *
+	 * @param mixed ...$args Arguments to be passed to the callable.
+	 * @return mixed
+	 */
+	public function __invoke( ...$args ) {
+		if ( ! $this->executed ) {
+			$this->result   = call_user_func_array( $this->callable, $args );
+			$this->executed = true;
+		}
+
+		return $this->result;
+	}
+}