MDL-69166 core_payment: helper methods to get cost and to deliver order

Also create the infrastructure to let components know when they have to
deliver what they sold. We are going to use namespace functions instead
of traditional callbacks.

lang/en/payment.php

@@ -22,6 +22,7 @@
  * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 
+$string['callbacknotimplemented'] = 'The callback is not implemented for component {$a}.';
 $string['nogateway'] = 'There is no payment gateway that can be used.';
 $string['nogatewayselected'] = 'You first need to select a payment gateway.';
 $string['selectpaymenttype'] = 'Select payment type';

payment/classes/helper.php

@@ -113,4 +113,40 @@ public static function gateways_modal_link_params(float $amount, string $currenc
             'data-description' => $description,
         ];
     }
+
+    /**
+     * Asks the cost from the related component.
+     *
+     * @param string $component Name of the component that the componentid belongs to
+     * @param int $componentid An internal identifier that is used by the component
+     * @return array['amount' => float, 'currency' => string]
+     * @throws \moodle_exception
+     */
+    public static function get_cost(string $component, int $componentid): array {
+        $cost = component_class_callback("$component\\payment\\provider", 'get_cost', [$componentid]);
+
+        if ($cost === null) {
+            throw new \moodle_exception('callbacknotimplemented', 'core_payment', '', $component);
+        }
+
+        return $cost;
+    }
+
+    /**
+     * Delivers what the user paid for.
+     *
+     * @param string $component Name of the component that the componentid belongs to
+     * @param int $componentid An internal identifier that is used by the component
+     * @return bool Whether successful or not
+     * @throws \moodle_exception
+     */
+    public static function deliver_order(string $component, int $componentid): bool {
+        $result = component_class_callback("$component\\payment\\provider", 'deliver_order', [$componentid]);
+
+        if ($result === null) {
+            throw new \moodle_exception('callbacknotimplemented', 'core_payment', '', $component);
+        }
+
+        return $result;
+    }
 }

payment/classes/local/callback/provider.php

@@ -0,0 +1,50 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * This file contains the \core_payment\local\local\callback\provider interface.
+ *
+ * Plugins should implement this if they use payment subsystem.
+ *
+ * @package core_payment
+ * @copyright 2020 Shamim Rezaie <shamim@moodle.com>
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace core_payment\local\callback;
+
+defined('MOODLE_INTERNAL') || die();
+
+/**
+ * The provider interface for plugins to provide callbacks which are needed by the payment subsystem.
+ *
+ * @copyright  2020 Shamim Rezaie <shamim@moodle.com>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+interface provider {
+
+    /**
+     * @param int $identifier An identifier that is known to the plugin
+     * @return array['amount' => float, 'currency' => string]
+     */
+    public static function get_cost(int $identifier): array;
+
+    /**
+     * @param int $identifier An identifier that is known to the plugin
+     * @return bool Whether successful or not
+     */
+    public static function deliver_order(int $identifier): bool;
+}