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gaia/apps/settings/js/modules/settings_cache.js

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+/**
+ * SettingsCache is a singleton that caches mozSettings values for fast
+ * access.
+ *
+ * @module SettingsCache
+ */
+define(function() {
+  'use strict';
+  var _settings = window.navigator.mozSettings;
+
+  // Cache of all current settings values.  There's some large stuff
+  // in here, but not much useful can be done with the settings app
+  // without these, so we keep this around most of the time.
+  var _settingsCache = null;
+
+  // True when a request has already been made to fill the settings
+  // cache.  When this is true, no further get("*") requests should be
+  // made; instead, pending callbacks should be added to
+  // _pendingSettingsCallbacks.
+  var _settingsCacheRequestSent = null;
+
+  // There can be race conditions in which we need settings values,
+  // but haven't filled the cache yet.  This array tracks those
+  // listeners.
+  var _pendingSettingsCallbacks = [];
+
+  var _callbacks = [];
+
+  var _onSettingsChange = function sc_onSettingsChange(event) {
+    var key = event.settingName;
+    var value = event.settingValue;
+
+    // Always update the cache if it's present, even if the DOM
+    // isn't loaded yet.
+    if (_settingsCache) {
+      _settingsCache[key] = value;
+    }
+
+    _callbacks.forEach(function(callback) {
+      callback(event);
+    });
+  };
+
+  if (_settings) {
+    _settings.onsettingchange = _onSettingsChange;
+  }
+
+  /**
+   * Event reporting that a setting value is changed.
+   *
+   * @event module:SettingsCache#settingsChange
+   * @property {MozSettingsEvent} event
+   */
+  var SettingsCache = {
+    // the reset function is for unit tests
+    reset: function sc_reset() {
+      _settings = window.navigator.mozSettings;
+      if (_settings) {
+        _settings.onsettingchange = _onSettingsChange;
+      }
+      _settingsCache = null;
+      _settingsCacheRequestSent = null;
+      _pendingSettingsCallbacks = [];
+      _callbacks = [];
+    },
+
+    get cache() {
+      return _settingsCache;
+    },
+
+    /**
+     * Where callback is a function to be called with a request object for a
+     * successful fetch of settings values, when those values are ready.
+     *
+     * @alias module:SettingsCache#getSettings
+     * @param {Function} callback
+     */
+    getSettings: function sc_getSettings(callback) {
+      if (!_settings) {
+        return;
+      }
+
+      if (_settingsCache && callback) {
+        // Fast-path that we hope to always hit: our settings cache is
+        // already available, so invoke the callback now.
+        callback(_settingsCache);
+        return;
+      }
+
+      if (!_settingsCacheRequestSent && !_settingsCache) {
+        _settingsCacheRequestSent = true;
+        var lock = _settings.createLock();
+        var request = lock.get('*');
+        request.onsuccess = function(e) {
+          var result = request.result;
+          var cachedResult = {};
+          for (var attr in result) {
+            cachedResult[attr] = result[attr];
+          }
+          _settingsCache = cachedResult;
+          var cbk;
+          while ((cbk = _pendingSettingsCallbacks.pop())) {
+            cbk(result);
+          }
+        };
+      }
+      if (callback) {
+        _pendingSettingsCallbacks.push(callback);
+      }
+    },
+
+    /**
+     * @alias module:SettingsCache#addEventListener
+     * @param {String} eventName
+     * @param {Function} callback
+     */
+    addEventListener: function sc_addEventListener(eventName, callback) {
+      if (eventName !== 'settingsChange') {
+        return;
+      }
+      var index = _callbacks.indexOf(callback);
+      if (index === -1) {
+        _callbacks.push(callback);
+      }
+    },
+
+    /**
+     * @alias module:SettingsCache#removeEventListener
+     * @param {String} eventName
+     * @param {Function} callback
+     */
+    removeEventListener: function sc_removeEventListsner(eventName, callback) {
+      if (eventName !== 'settingsChange') {
+        return;
+      }
+      var index = _callbacks.indexOf(callback);
+      if (index !== -1) {
+        _callbacks.splice(index, 1);
+      }
+    }
+  };
+
+  // Make a request for settings to warm the cache, since we need it
+  // very soon in startup after the DOM is available.
+  SettingsCache.getSettings(null);
+  return SettingsCache;
+});