Issue 3826 - Filter preference change events

lib/events.js

Index: lib/events.js
===================================================================
new file mode 100644
--- /dev/null
+++ b/lib/events.js
@@ -0,0 +1,84 @@
+/*
+ * 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
+ * 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 Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @module events */
+
+"use strict";
+
+/**
+ * Registers and emits names events.
+ *
+ * @constructor
+ */
+exports.EventEmitter = function()
+{
+  this._listeners = Object.create(null);
+};
+
+exports.EventEmitter.prototype = {
+  /**
+   * Adds a listener for the specified event name.
+   *
+   * @param {string}   name
+   * @param {function} listener
+   */
+  on: function(name, listener)
+  {
+    if (name in this._listeners)
+      this._listeners[name].push(listener);
+    else
+      this._listeners[name] = [listener];
+  },
+
+  /**
+   * Removes a listener for the specified event name.
+   *
+   * @param {string}   name
+   * @param {function} listener
+   */
+  off: function(name, listener)
+  {
+    let listeners = this._listeners[name];
+    if (listeners)
+    {
+      let idx = listeners.indexOf(listener);
+      if (idx != -1)
+        listeners.splice(idx, 1);
+    }
+  },
+
+  /**
+   * Calls all previously added listeners for the given event name.
+   *
+   * @param {string} name
+   * @param {...*}   [arg]
+   */
+  emit: function(name)
+  {
+    let listeners = this._listeners[name];
+    if (listeners)
+    {
+      let args = [];

[Sebastian Noack, 2016/03/23 11:37:22]

Array.prototype.slice.call(arguments, 1), or passing arguments as array-like
object to another function, causes this function to be deoptimized in V8. So I
replaced it with a loop.

[kzar, 2016/03/23 11:56:01]

Acknowledged.

+      for (let i = 1; i < arguments.length; i++)
+        args.push(arguments[i]);
+
+      let currentListeners = listeners.slice();

[Sebastian Noack, 2016/03/23 11:37:22]

This is necessary to avoid side effects when a listener added/removed listeners
itself.

[kzar, 2016/03/23 11:56:01]

Acknowledged.

+      for (let listener of currentListeners)
+        listener.apply(null, args);
+    }
+  }
+};

[Sebastian Noack, 2016/03/23 11:37:22]

Moreover, I think "listener" is a more appropriate name than "callback" in the
context here. So I changed that as well.