1 files changed, 150 insertions, 0 deletions
diff --git a/services/common/observers.js b/services/common/observers.js
new file mode 100644
index 0000000000..c0b7710482
--- /dev/null
+++ b/services/common/observers.js
@@ -0,0 +1,150 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+this.EXPORTED_SYMBOLS = ["Observers"];
+
+var Cc = Components.classes;
+var Ci = Components.interfaces;
+var Cr = Components.results;
+var Cu = Components.utils;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+/**
+ * A service for adding, removing and notifying observers of notifications.
+ * Wraps the nsIObserverService interface.
+ *
+ * @version 0.2
+ */
+this.Observers = {
+  /**
+   * Register the given callback as an observer of the given topic.
+   *
+   * @param   topic       {String}
+   *          the topic to observe
+   *
+   * @param   callback    {Object}
+   *          the callback; an Object that implements nsIObserver or a Function
+   *          that gets called when the notification occurs
+   *
+   * @param   thisObject  {Object}  [optional]
+   *          the object to use as |this| when calling a Function callback
+   *
+   * @returns the observer
+   */
+  add: function(topic, callback, thisObject) {
+    let observer = new Observer(topic, callback, thisObject);
+    this._cache.push(observer);
+    this._service.addObserver(observer, topic, true);
+
+    return observer;
+  },
+
+  /**
+   * Unregister the given callback as an observer of the given topic.
+   *
+   * @param topic       {String}
+   *        the topic being observed
+   *
+   * @param callback    {Object}
+   *        the callback doing the observing
+   *
+   * @param thisObject  {Object}  [optional]
+   *        the object being used as |this| when calling a Function callback
+   */
+  remove: function(topic, callback, thisObject) {
+    // This seems fairly inefficient, but I'm not sure how much better
+    // we can make it.  We could index by topic, but we can't index by callback
+    // or thisObject, as far as I know, since the keys to JavaScript hashes
+    // (a.k.a. objects) can apparently only be primitive values.
+    let [observer] = this._cache.filter(v => v.topic      == topic    &&
+                                             v.callback   == callback &&
+                                             v.thisObject == thisObject);
+    if (observer) {
+      this._service.removeObserver(observer, topic);
+      this._cache.splice(this._cache.indexOf(observer), 1);
+    }
+  },
+
+  /**
+   * Notify observers about something.
+   *
+   * @param topic   {String}
+   *        the topic to notify observers about
+   *
+   * @param subject {Object}  [optional]
+   *        some information about the topic; can be any JS object or primitive
+   *
+   * @param data    {String}  [optional] [deprecated]
+   *        some more information about the topic; deprecated as the subject
+   *        is sufficient to pass all needed information to the JS observers
+   *        that this module targets; if you have multiple values to pass to
+   *        the observer, wrap them in an object and pass them via the subject
+   *        parameter (i.e.: { foo: 1, bar: "some string", baz: myObject })
+   */
+  notify: function(topic, subject, data) {
+    subject = (typeof subject == "undefined") ? null : new Subject(subject);
+       data = (typeof    data == "undefined") ? null : data;
+    this._service.notifyObservers(subject, topic, data);
+  },
+
+  _service: Cc["@mozilla.org/observer-service;1"].
+            getService(Ci.nsIObserverService),
+
+  /**
+   * A cache of observers that have been added.
+   *
+   * We use this to remove observers when a caller calls |remove|.
+   *
+   * XXX This might result in reference cycles, causing memory leaks,
+   * if we hold a reference to an observer that holds a reference to us.
+   * Could we fix that by making this an independent top-level object
+   * rather than a property of this object?
+   */
+  _cache: []
+};
+
+
+function Observer(topic, callback, thisObject) {
+  this.topic = topic;
+  this.callback = callback;
+  this.thisObject = thisObject;
+}
+
+Observer.prototype = {
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
+  observe: function(subject, topic, data) {
+    // Extract the wrapped object for subjects that are one of our wrappers
+    // around a JS object.  This way we support both wrapped subjects created
+    // using this module and those that are real XPCOM components.
+    if (subject && typeof subject == "object" &&
+        ("wrappedJSObject" in subject) &&
+        ("observersModuleSubjectWrapper" in subject.wrappedJSObject))
+      subject = subject.wrappedJSObject.object;
+
+    if (typeof this.callback == "function") {
+      if (this.thisObject)
+        this.callback.call(this.thisObject, subject, data);
+      else
+        this.callback(subject, data);
+    }
+    else // typeof this.callback == "object" (nsIObserver)
+      this.callback.observe(subject, topic, data);
+  }
+}
+
+
+function Subject(object) {
+  // Double-wrap the object and set a property identifying the wrappedJSObject
+  // as one of our wrappers to distinguish between subjects that are one of our
+  // wrappers (which we should unwrap when notifying our observers) and those
+  // that are real JS XPCOM components (which we should pass through unaltered).
+  this.wrappedJSObject = { observersModuleSubjectWrapper: true, object: object };
+}
+
+Subject.prototype = {
+  QueryInterface: XPCOMUtils.generateQI([]),
+  getScriptableHelper: function() {},
+  getInterfaces: function() {}
+};