Error 2112 in Flex Using EarthBrowser’s Mac Middle Mouse Fix

We’re using the EarthBrowser Mac Mouse Wheel fix in our current Flex project. I was noticing some sporadic “SecurityError: Error #2112: Provided parameter LoaderContext.ApplicationDomain is from a disallowed domain.” pop up, but only when scrolling our TileList via the middle mouse. If you used the mouse on the scrollbar, it worked fine.

I found the cause of the problem was when the JavaScript called Flash Player via ExternalInterface, eventually the loader.load call in the Flex Image control would throw an exception. Since the Flex 3.3 SDK< doesn’t wrap the load with a try/catch, there is no way to catch this error.  If you’ve ever attempted to debug ApplicationDomain errors in ActionScript, you know it’s a time sink… and hard.  So, I never really found out the problem, but did find a solution.

The fix is to delay the MouseEvent.MOUSE_WHEEL dispatched by 1 frame using UIComponent’s built-in callLater function.  I wish I had time to find out the true cause, but at this point, all I have is conjectures.  My guess is that the call stack that JavaScript is initiating via ExternalInterface is sequestered in memory, perhaps in a temporary ApplicationDomain.  This then is perhaps why the ApplicationDomain.currentDomain differs from the usual one.  Since there is a lot of security around MouseEvents now (triggering File downloads/uploads, initiating full screen playback, etc.) it makes sense.

Anyway, in my testing, my fix seems to work pretty good; haven’t gotten an error yet.  We’ll be testing the updated code all week, so if I find any issues, I’ll update this post.  Here’s the code I modified in MacMouseWheelHandler.as on line 72:

static private function _externalMouseEvent(delta:Number):void
{
        // [jwarden 1.25.2010] Originally, it'd just dispatch the event immediately.
        // However, we were getting image loading errors when you scrolled on some tile pages.
        // I believe this is because when JavaScript executes, the stack is in a protected
        // memory space, and the ApplicationDomain.currentDomain utilized by the Image control
        // is different than normal.  This causes a 2112 error, and since the Flex 3.3
        // image control doesn't wrap loader.load with a try/catch, the exception blows
        // things up.  If I wait a frame via callLater, the problem goes away.
        // Still trying to find out why, but for now, she works.
        if(_currItem && _clonedEvent)
        {
                var mouseEvent:MouseEvent = new MouseEvent(MouseEvent.MOUSE_WHEEL, true, false,
                                                              _clonedEvent.localX, _clonedEvent.localY, _clonedEvent.relatedObject,
                                                              _clonedEvent.ctrlKey, _clonedEvent.altKey, _clonedEvent.shiftKey, _clonedEvent.buttonDown,
                                                              int(delta));

                if(_currItem is UIComponent)
                {

                        UIComponent(_currItem).callLater(_currItem.dispatchEvent, [mouseEvent]);
                }
                else
                {
                        _currItem.dispatchEvent(mouseEvent);
                }
        }
}

If you’re not using Flex, and just pure AS3, you might try providing a way for your components to wait a frame utilizing Event.ENTER_FRAME.