MDL-85433 navigation: Generalise secondary navigation for single format

Introduce flexibility by allowing course formats to define custom logic
for retrieving activity modules.

.upgradenotes/MDL-85433-2025060405592009.yml

@@ -0,0 +1,8 @@
+issueNumber: MDL-85433
+notes:
+  core_courseformat:
+    - message: >-
+        A new interface, `main_activity_interface`, is now available. Course
+        format plugins should implement it if they intend to display only a
+        single activity in the course page.
+      type: improved

course/format/classes/main_activity_interface.php

@@ -0,0 +1,35 @@
+<?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/>.
+
+namespace core_courseformat;
+
+/**
+ * Contains the interface that any course format plugin should implement
+ * when it wants to only display a single activity in the course page.
+ *
+ * @package    core_courseformat
+ * @copyright  2025 Sara Arjona <[email protected]>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+interface main_activity_interface {
+
+    /**
+     * Get the main activity of the course.
+     *
+     * @return \cm_info|null The main activity object or null if main activity is not set.
+     */
+    public function get_main_activity(): ?\cm_info;
+}

lib/classes/navigation/views/secondary.php

@@ -1108,10 +1108,16 @@ class secondary extends view {
         // the course in order to obtain the required module settings navigation.
         if ($page->context instanceof \context_course) {
             $this->page->set_secondary_active_tab($coursesecondarynode->key);
-            // Get the currently used module in the single activity course.
-            $module = current(array_filter(get_course_mods($course->id), function ($module) {
-                return $module->visible == 1;
-            }));
+            // Get the currently used module in course.
+            $format = course_get_format($course);
+            if ($format instanceof \core_courseformat\main_activity_interface) {
+                $module = $format->get_main_activity();
+            } else {
+                $module = current(array_filter(get_course_mods($course->id), function ($module) {
+                    return $module->visible == 1;
+                }));
+            }
+
             // If the default module for the single course format has not been set yet, skip displaying the module
             // related navigation in the secondary navigation.
             if (!$module) {