1 files changed, 69 insertions, 0 deletions
diff --git a/shared/js/settings_listener.js b/shared/js/settings_listener.js
new file mode 100644
index 0000000..272729c
--- /dev/null
+++ b/shared/js/settings_listener.js
@@ -0,0 +1,69 @@
+/* -*- Mode: Java; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- /
+/* vim: set shiftwidth=2 tabstop=2 autoindent cindent expandtab: */
+
+'use strict';
+
+var SettingsListener = {
+  /* Timer to remove the lock. */
+  _timer: null,
+
+  /* lock stores here */
+  _lock: null,
+
+  /**
+   * getSettingsLock: create a lock or retrieve one that we saved.
+   * mozSettings.createLock() is expensive and lock should be reused
+   * whenever possible.
+   */
+
+  getSettingsLock: function sl_getSettingsLock() {
+    // Each time there is a getSettingsLock call, we pospone the removal
+    clearTimeout(this._timer);
+    this._timer = setTimeout((function removeLock() {
+      this._lock = null;
+    }).bind(this), 0);
+
+    // If there is a lock present we return that
+    if (this._lock) {
+      return this._lock;
+    }
+
+    // If there isn't we return one.
+    var settings = window.navigator.mozSettings;
+
+    return (this._lock = settings.createLock());
+  },
+
+  observe: function sl_observe(name, defaultValue, callback) {
+    var settings = window.navigator.mozSettings;
+    if (!settings) {
+      window.setTimeout(function() { callback(defaultValue); });
+      return;
+    }
+
+    var req;
+    try {
+      req = this.getSettingsLock().get(name);
+    } catch (e) {
+      // It is possible (but rare) for getSettingsLock() to return
+      // a SettingsLock object that is no longer valid.
+      // Until https://bugzilla.mozilla.org/show_bug.cgi?id=793239
+      // is fixed, we just catch the resulting exception and try
+      // again with a fresh lock
+      console.warn('Stale lock in settings_listener.js.',
+                   'See https://bugzilla.mozilla.org/show_bug.cgi?id=793239');
+      this._lock = null;
+      req = this.getSettingsLock().get(name);
+    }
+
+    req.addEventListener('success', (function onsuccess() {
+      callback(typeof(req.result[name]) != 'undefined' ?
+        req.result[name] : defaultValue);
+    }));
+
+    settings.addObserver(name, function settingChanged(evt) {
+      callback(evt.settingValue);
+    });
+  }
+};
+