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 +1,79 @@
+/*
+ * 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._callbacks = Object.create(null);
+};
+
+exports.EventEmitter.prototype = {
+  /**
+   * Adds a callback for the specified event name.
+   *
+   * @param {string}   name
+   * @param {function} callback
+   */
+  on: function(name, callback)
+  {
+    if (name in this._callbacks)
+      this._callbacks[name].push(callback);
+    else
+      this._callbacks[name] = [callback];
+  },
+
+  /**
+   * Removes a callback for the specified event name.
+   *
+   * @param {string}   name
+   * @param {function} callback
+   */
+  off: function(name, callback)
+  {
+    let callbacks = this._callbacks[name];
+    if (callbacks)

[kzar, 2016/03/18 15:49:59]

I guess if the last callback for a name is removed we should also remove the
array?

[Sebastian Noack, 2016/03/18 18:15:49]

I had a similar discussion with Wladimir about the implementation of
messaging.Port for Firefox. And he pointed out that we currently never remove
messages listeners (that is true for preferences here as well), and if in the
future we use temporary listeners we don't have to create a new array at least
when they are added again.

[kzar, 2016/03/18 18:44:14]

Acknowledged.

+    {
+      let idx = callbacks.indexOf(callback);
+      if (idx != -1)
+        callbacks.splice(idx, 1);
+    }
+  },
+
+  /**
+   * Calls all previously added callbacks for the given event name.
+   *
+   * @param {string} name
+   * @param {...*}   [arg]
+   */
+  emit: function(name)
+  {
+    let callbacks = this._callbacks[name];
+    if (callbacks)
+    {
+      for (let callback of callbacks)
+        callbacks.apply(null, Array.prototype.slice.call(arguments, 1));
+    }
+  }
+};