Issue 3823 - Split up message responder code

lib/messaging.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 87 matching lines @@
    */
   off: function(name, callback)
   {
     let callbacks = this._callbacks[name];
     if (callbacks)
     {
       let idx = callbacks.indexOf(callback);
       if (idx != -1)
         callbacks.splice(idx, 1);
     }
+  },
+
+  /**
+   * Disables the port and makes it stop listening to incoming messages.
+   */
+  disconnect: function()
+  {
+    ext.onMessage.removeListener(this._onMessage);
   }
 };
 
 /**
  * The default port to receive messages.
  *
  * @type {Port}
  */
 exports.port = new Port();
 
 /**
- * Creates a new port that is shutdown when the given window is unloaded.
+ * Creates a new port that is disconnected when the given window is unloaded.
  *
  * @param {Window} window
  * @return {Port}
  */
 exports.getPort = function(window)
 {
   let port = new Port();
   window.addEventListener("unload", () =>
   {
-    ext.onMessage.removeListener(port._onMessage);
+    port.disconnect();
   });

[Wladimir Palant, 2016/03/21 15:50:13]

Please consider caching the port per window so that it won't be created multiple
times unnecessarily.

Also, maybe have Port.disconnect() method that's called here like in Firefox?
Note that I export Port constructor. This way the API allows you to create and
disconnect a port manually if necessary.

[Sebastian Noack, 2016/03/21 17:15:42]

Done.

   return port;
 };