Bug 1254194: Add XPCOMUtils.defineLazyPreferenceGetter. r=billm

MozReview-Commit-ID: JBVelZ0Xg2H

--HG--
extra : rebase_source : 3909e75aa6b24ed2a1d2663c11be69c27f4de1e7

js/xpconnect/loader/XPCOMUtils.jsm

@@ -283,6 +283,63 @@ this.XPCOMUtils = {
     });
   },
 
+  /**
+   * Defines a getter on a specified object for preference value. The
+   * preference is read the first time that the property is accessed,
+   * and is thereafter kept up-to-date using a preference observer.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aName
+   *        The name of the getter property to define on aObject.
+   * @param aPreference
+   *        The name of the preference to read.
+   * @param aDefaultValue
+   *        The default value to use, if the preference is not defined.
+   */
+  defineLazyPreferenceGetter: function XPCU_defineLazyPreferenceGetter(
+                                   aObject, aName, aPreference, aDefaultValue = null)
+  {
+    // Note: We need to keep a reference to this observer alive as long
+    // as aObject is alive. This means that all of our getters need to
+    // explicitly close over the variable that holds the object, and we
+    // cannot define a value in place of a getter after we read the
+    // preference.
+    let observer = {
+      QueryInterface: this.generateQI([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
+
+      value: undefined,
+
+      observe(subject, topic, data) {
+        if (data == aPreference) {
+          this.value = undefined;
+        }
+      },
+    }
+
+    let defineGetter = get => {
+      Object.defineProperty(aObject, aName, {
+        configurable: true,
+        enumerable: true,
+        get,
+      });
+    };
+
+    function lazyGetter() {
+      if (observer.value === undefined) {
+        observer.value = Preferences.get(aPreference, aDefaultValue);
+      }
+      return observer.value;
+    }
+
+    defineGetter(() => {
+      Services.prefs.addObserver(aPreference, observer, true);
+
+      defineGetter(lazyGetter);
+      return lazyGetter();
+    });
+  },
+
   /**
    * Convenience access to category manager
    */
@@ -382,6 +439,11 @@ this.XPCOMUtils = {
   },
 };
 
+XPCOMUtils.defineLazyModuleGetter(this, "Preferences",
+                                  "resource://gre/modules/Preferences.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Services",
+                                  "resource://gre/modules/Services.jsm");
+
 /**
  * Helper for XPCOMUtils.generateQI to avoid leaks - see bug 381651#c1
  */

js/xpconnect/tests/unit/test_xpcomutils.js

@@ -8,6 +8,7 @@
  * This file tests the methods on XPCOMUtils.jsm.
  */
 
+Components.utils.import("resource://gre/modules/Preferences.jsm");
 Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");
 
 const Cc = Components.classes;
@@ -116,6 +117,44 @@ add_test(function test_defineLazyServiceGetter()
 });
 
 
+add_test(function test_defineLazyPreferenceGetter()
+{
+    const PREF = "xpcomutils.test.pref";
+
+    let obj = {};
+    XPCOMUtils.defineLazyPreferenceGetter(obj, "pref", PREF, "defaultValue");
+
+
+    equal(obj.pref, "defaultValue", "Should return the default value before pref is set");
+
+    Preferences.set(PREF, "currentValue");
+
+
+    do_print("Create second getter on new object");
+
+    obj = {};
+    XPCOMUtils.defineLazyPreferenceGetter(obj, "pref", PREF, "defaultValue");
+
+
+    equal(obj.pref, "currentValue", "Should return the current value on initial read when pref is already set");
+
+    Preferences.set(PREF, "newValue");
+
+    equal(obj.pref, "newValue", "Should return new value after preference change");
+
+    Preferences.set(PREF, "currentValue");
+
+    equal(obj.pref, "currentValue", "Should return new value after second preference change");
+
+
+    Preferences.reset(PREF);
+
+    equal(obj.pref, "defaultValue", "Should return default value after pref is reset");
+
+    run_next_test();
+});
+
+
 add_test(function test_categoryRegistration()
 {
   const CATEGORY_NAME = "test-cat";