Bug 508850 - XPCOMUtils should provide a convenient way to define lazy getters

r=sayrer
sr=bsmedberg

js/src/xpconnect/loader/XPCOMUtils.jsm

@@ -1,4 +1,5 @@
-/*
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: sw=2 ts=2 sts=2 et filetype=javascript
  * ***** BEGIN LICENSE BLOCK *****
  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
  *
@@ -103,6 +104,7 @@
 
 var EXPORTED_SYMBOLS = [ "XPCOMUtils" ];
 
+const Cc = Components.classes;
 const Ci = Components.interfaces;
 const Cr = Components.results;
 
@@ -227,6 +229,47 @@ var XPCOMUtils = {
     };
   },
 
+  /**
+   * Defines a getter on a specified object that will be created upon first use.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aName
+   *        The name of the getter to define on aObject.
+   * @param aLambda
+   *        A function that returns what the getter should return.  This will
+   *        only ever be called once.
+   */
+  defineLazyGetter: function XPCU_defineLazyGetter(aObject, aName, aLambda)
+  {
+    aObject.__defineGetter__(aName, function() {
+      delete aObject[aName];
+      return aObject[aName] = aLambda();
+    });
+  },
+
+  /**
+   * Defines a getter on a specified object for a service.  The service will not
+   * be obtained until first use.
+   *
+   * @param aObject
+   *        The object to define the lazy getter on.
+   * @param aName
+   *        The name of the getter to define on aObject for the service.
+   * @param aContract
+   *        The contract used to obtain the service.
+   * @param aInterfaceName
+   *        The name of the interface to query the service to.
+   */
+  defineLazyServiceGetter: function XPCU_defineLazyServiceGetter(aObject, aName,
+                                                                 aContract,
+                                                                 aInterfaceName)
+  {
+    this.defineLazyGetter(aObject, aName, function XPCU_serviceLambda() {
+      return Cc[aContract].getService(Ci[aInterfaceName]);
+    });
+  },
+
   /**
    * Convenience access to category manager
    */

js/src/xpconnect/tests/unit/test_xpcomutils.js

@@ -1,23 +1,131 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: sw=4 ts=4 sts=4 et
+ * ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is Necko Test Code.
+ *
+ * The Initial Developer of the Original Code is
+ * Mozilla Corporation.
+ * Portions created by the Initial Developer are Copyright (C) 2009
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *   Boris Zbarsky <bzbarsky@mit.edu> (Original Author)
+ *   Shawn Wilsher <me@shawnwilsher.com>
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+/**
+ * This file tests the methods on XPCOMUtils.jsm.
+ */
+
 Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");
 
-var x = {
-    QueryInterface: XPCOMUtils.generateQI([Components.interfaces.nsIClassInfo,
-					   "nsIDOMNode"])
-};
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+
+////////////////////////////////////////////////////////////////////////////////
+//// Tests
+
+function test_generateQI_string_names()
+{
+    var x = {
+        QueryInterface: XPCOMUtils.generateQI([
+            Components.interfaces.nsIClassInfo,
+            "nsIDOMNode"
+        ])
+    };
 
-function run_test() {
     try {
-	x.QueryInterface(Components.interfaces.nsIClassInfo);
+        x.QueryInterface(Components.interfaces.nsIClassInfo);
     } catch(e) {
-	do_throw("Should QI to nsIClassInfo");
+        do_throw("Should QI to nsIClassInfo");
     }
     try {
-	x.QueryInterface(Components.interfaces.nsIDOMNode);
+        x.QueryInterface(Components.interfaces.nsIDOMNode);
     } catch(e) {
-	do_throw("Should QI to nsIDOMNode");
+        do_throw("Should QI to nsIDOMNode");
     }
     try {
-	x.QueryInterface(Components.interfaces.nsIDOMDocument);
-	do_throw("QI should not have succeeded!");
+        x.QueryInterface(Components.interfaces.nsIDOMDocument);
+        do_throw("QI should not have succeeded!");
     } catch(e) {}
 }
+
+function test_defineLazyGetter()
+{
+    let accessCount = 0;
+    let obj = { };
+    const TEST_VALUE = "test value";
+    XPCOMUtils.defineLazyGetter(obj, "foo", function() {
+        accessCount++;
+        return TEST_VALUE;
+    });
+    do_check_eq(accessCount, 0);
+
+    // Get the property, making sure the access count has increased.
+    do_check_eq(obj.foo, TEST_VALUE);
+    do_check_eq(accessCount, 1);
+
+    // Get the property once more, making sure the access count has not
+    // increased.
+    do_check_eq(obj.foo, TEST_VALUE);
+    do_check_eq(accessCount, 1);
+}
+
+function test_defineLazyServiceGetter()
+{
+    let obj = { };
+    XPCOMUtils.defineLazyServiceGetter(obj, "service",
+                                       "@mozilla.org/consoleservice;1",
+                                       "nsIConsoleService");
+    let service = Cc["@mozilla.org/consoleservice;1"].
+                  getService(Ci.nsIConsoleService);
+
+    // Check that the lazy service getter and the actual service have the same
+    // properties.
+    for (let prop in obj.service)
+        do_check_true(prop in service);
+    for (let prop in service)
+        do_check_true(prop in obj.service);
+}
+
+////////////////////////////////////////////////////////////////////////////////
+//// Test Runner
+
+let tests = [
+    test_generateQI_string_names,
+    test_defineLazyGetter,
+    test_defineLazyServiceGetter,
+];
+
+function run_test()
+{
+    tests.forEach(function(test) {
+        print("Running next test: " + test.name);
+        test();
+    });
+}