MDL-63058 core_favourites: add existence checks to the service layer

This allows someone using the user_favourite_service to check whether
an item is already marked as a favourite.

favourites/classes/local/service/user_favourite_service.php

@@ -135,4 +135,25 @@ public function delete_favourite(string $component, string $itemtype, int $itemi
 
         $this->repo->delete($favourite->id);
     }
+
+    /**
+     * Check whether an item has been marked as a favourite in the respective area.
+     *
+     * @param string $component the frankenstyle component name.
+     * @param string $itemtype the type of the favourited item.
+     * @param int $itemid the id of the item which was favourited (not the favourite's id).
+     * @param \context $context the context of the item which was favourited.
+     * @return bool true if the item is favourited, false otherwise.
+     */
+    public function favourite_exists(string $component, string $itemtype, int $itemid, \context $context) : bool {
+        return $this->repo->exists_by(
+            [
+                'userid' => $this->userid,
+                'component' => $component,
+                'itemtype' => $itemtype,
+                'itemid' => $itemid,
+                'contextid' => $context->id
+            ]
+        );
+    }
 }

favourites/tests/service_test.php

@@ -126,6 +126,19 @@ protected function get_mock_repository(array $mockstore) {
                 return array_key_exists($id, $mockstore);
             })
         );
+        $mockrepo->expects($this->any())
+            ->method('exists_by')
+            ->will($this->returnCallback(function(array $criteria) use (&$mockstore) {
+                // Check the mockstore for all objects with properties matching the key => val pairs in $criteria.
+                foreach ($mockstore as $index => $mockrow) {
+                    $mockrowarr = (array)$mockrow;
+                    if (array_diff($criteria, $mockrowarr) == []) {
+                        return true;
+                    }
+                }
+                return false;
+            })
+        );
         $mockrepo->expects($this->any())
             ->method('delete')
             ->will($this->returnCallback(function(int $id) use (&$mockstore) {
@@ -305,4 +318,38 @@ public function test_delete_favourite_basic() {
         $this->expectException(\moodle_exception::class);
         $service->delete_favourite('core_course', 'course', $course1context->instanceid, $course1context);
     }
+
+    /**
+     * Test confirming the behaviour of the favourite_exists() method.
+     */
+    public function test_favourite_exists() {
+        list($user1context, $user2context, $course1context, $course2context) = $this->setup_users_and_courses();
+
+        // Get a user_favourite_service for the user.
+        $repo = $this->get_mock_repository([]);
+        $service = new \core_favourites\local\service\user_favourite_service($user1context, $repo);
+
+        // Favourite a course.
+        $fav1 = $service->create_favourite('core_course', 'course', $course1context->instanceid, $course1context);
+
+        // Verify we can check existence of the favourite.
+        $this->assertTrue(
+            $service->favourite_exists(
+                'core_course',
+                'course',
+                $course1context->instanceid,
+                $course1context
+            )
+        );
+
+        // And one that we know doesn't exist.
+        $this->assertFalse(
+            $service->favourite_exists(
+                'core_course',
+                'someothertype',
+                $course1context->instanceid,
+                $course1context
+            )
+        );
+    }
 }