Proper Fennec support

lib/hooks.js

 /* 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/. */
 
-function hook(obj, name, func)
+function hook(obj, name, func, cleanup)
 {
   let orig = obj[name];
   let origGet = obj.__lookupGetter__(name);
   let origSet = obj.__lookupSetter__(name);
   let dumbOverrideAttempt = false;
 
   let newFunc = function()
   {
+    let params = arguments;
     try
     {
-      func.apply(this, arguments);
+      let result = func.apply(this, params);
+      if (typeof result == "object")
+        params = result;
     }
     catch(e)
     {
       Cu.reportError(e);
     }
-    orig.apply(this, arguments);
+
+    try
+    {
+      orig.apply(this, params);
+    }
+    finally
+    {
+      if (typeof cleanup == "function")
+        cleanup();
+    }
   };
   newFunc.toString = function()
   {
     dumbOverrideAttempt = true;
     return orig.toString();
   };
 
-  obj.__defineGetter__(name, function() {
+  obj.__defineGetter__(name, function()
+  {
     dumbOverrideAttempt = false;
     return newFunc;
   });
 
-  obj.__defineSetter__(name, function(value) {
+  obj.__defineSetter__(name, function(value)
+  {
     if (dumbOverrideAttempt)
     {
       orig = value;
     }
     else
     {
       delete obj[name];
       obj[name] = value;
     }
   });
 
   return function()
   {
     delete obj[name];
     obj[name] = orig;
     if (origGet)
     {
       obj.__defineGetter__(name, origGet);
     }
     if (origSet)
     {
       obj.__defineSetter__(name, origSet);
     }
   };
 }
 exports.hook = hook;