Add Notification extender beforeSending method (#2533)

src/Notification/NotificationSyncer.php

@@ -43,6 +43,11 @@ class NotificationSyncer
      */
     protected static $notificationDrivers = [];
 
+    /**
+     * @var array
+     */
+    protected static $beforeSendingCallbacks = [];
+
     /**
      * Sync a notification so that it is visible to the specified users, and not
      * visible to anyone else. If it is being made visible for the first time,
@@ -94,6 +99,10 @@ class NotificationSyncer
             $this->setDeleted($toUndelete, false);
         }
 
+        foreach (static::$beforeSendingCallbacks as $callback) {
+            $newRecipients = $callback($blueprint, $newRecipients);
+        }
+
         // Create a notification record, and send an email, for all users
         // receiving this notification for the first time (we know because they
         // didn't have a record in the database). As both operations can be
@@ -176,4 +185,12 @@ class NotificationSyncer
     {
         return static::$notificationDrivers;
     }
+
+    /**
+     * @param callable|string $callback
+     */
+    public static function beforeSending($callback): void
+    {
+        static::$beforeSendingCallbacks[] = $callback;
+    }
 }