Issue 7096 - Make it possible to lazy initialize a filter's subscriptions

lib/filterClasses.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present 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 42 matching lines @@
 
 /**
  * Abstract base class for filters
  *
  * @param {string} text   string representation of the filter
  * @constructor
  */
 function Filter(text)
 {
   this.text = text;
-
-  /**
-   * Subscriptions to which this filter belongs.
-   * @type {(Subscription|Set.<Subscription>)?}
-   * @private
-   */
-  this._subscriptions = null;
 }
 exports.Filter = Filter;
 
 Filter.prototype =
 {
   /**
    * String representation of the filter
    * @type {string}
    */
   text: null,
 
   /**
    * Filter type as a string, e.g. "blocking".
    * @type {string}
    */
   get type()
   {
     throw new Error("Please define filter type in the subclass");
   },
 
   /**
+   * Subscriptions to which this filter belongs.
+   * @type {(Subscription|Set.<Subscription>)?}
+   * @private
+   */
+  _subscriptions: null,
+
+  /**
+   * Whether the filter's subscriptions have already been added to the filter.
+   * @type {boolean}
+   * @package
+   */
+  get subscriptionsAdded()
+  {
+    return this.hasOwnProperty("_subscriptions");
+  },
+  set subscriptionsAdded(value)
+  {
+    // Once set to true, this cannot be set back to false even if all
+    // subscriptions are removed.
+    if (value)
+      this._subscriptions = this._subscriptions;
+  },
+
+  /**
    * Yields subscriptions to which the filter belongs.
    * @yields {Subscription}
    */
   *subscriptions()
   {
     if (this._subscriptions)
     {
       if (this._subscriptions instanceof Set)
         yield* this._subscriptions;
       else
@@ skipping 1252 matching lines @@
 
   /**
    * Script that should be executed
    * @type {string}
    */
   get script()
   {
     return this.body;
   }
 });