This is a complex one. Summary:
I believe I'm not confused with try/catch, but am surprised that the EventDispatcher assumes our code is reentrant. Reentrant like the EventDispatcher is dispatching a system Event in the middle of me handling other events. So we're vulnerable to reentrant callbacks when we call EventDispatcher.dispatchEvent. Details: Normally when an null dereference happens an Error is propagated up the call stack to a catch handler. The bug is this: if in the middle of this call stack there happens to be an EventDispatcher.dispatchEvent call and the null pointer happens somewhere inside one of the listeners, then the Flash VM runtime thingy / implementation of dispatchEvent() itself seems to catch & handle the Error, and takes the opportunity to dispatch a pending Events. In my case I've seen a ProgressEvent.SOCKET_DATA being dispatched because my server pumped out some data to me really quickly, though I assume any pending Event could be dispatched. This really causes havoc because our methods aren't reentrant; flash isn't multithreaded afaik and we don't think we have to write everything thread-safe. Here's an illustration I will walk through. At the start of the stack our DTVSocket.onSocketConnect listens for an ProgressEvent.SOCKET_DATA, and just got one. Notice for now the EventDispatcher [no source] in the middle of the stack trace. Capping off the stack trace, see our TextUtil.setText method ... it is doing something bad, and has just triggered a null pointer exception! dtv.util::TextUtil$/setText (BANG! null pointer exception) MarbachViewer/MarbachViewer::createChannelCanvas MarbachViewer/MarbachViewer::subscribeToChannel MarbachViewer/MarbachViewer::onConnected flash.events::EventDispatcher/flash.events:EventDispatcher::dispatchEventFunction [no source] flash.events::EventDispatcher/dispatchEvent [no source] dtv.util::DTVSocket/dtv.util:DTVSocket::dispatchOnConnectedEvent dtv.util::DTVSocket/dtv.util:DTVSocket::onReceivedSessionTicket Function/http://adobe.com/AS3/2006/builtin::call [no source] dtv.util::DTVSocket/dtv.util:DTVSocket::onJSON dtv.util::DTVSocket/dtv.util:DTVSocket::onData dtv.util::DTVSocket/dtv.util:DTVSocket::onSocketData Now notice in this second shorter trace the stack has been unwound up to the dispatchEvent, but no further. onSocketData was just passed a second, new ProgressEvent.SOCKET_DATA. It must be that EventDispatcher.dispatchEvent caught my setText's null pointer exception and dispatched some additional socket data to my ProgressEvent.SOCKET_DATA listener. onSocketData is not thread safe. and bad stuff will happen very soon after in our code. dtv.util::DTVSocket/dtv.util:DTVSocket::onSocketData (WHOA! Not good) flash.events::EventDispatcher/flash.events:EventDispatcher::dispatchEventFunction [no source] flash.events::EventDispatcher/dispatchEvent [no source] dtv.util::DTVSocket/dtv.util:DTVSocket::dispatchOnConnectedEvent dtv.util::DTVSocket/dtv.util:DTVSocket::onReceivedSessionTicket Function/http://adobe.com/AS3/2006/builtin::call [no source] dtv.util::DTVSocket/dtv.util:DTVSocket::onJSON dtv.util::DTVSocket/dtv.util:DTVSocket::onData dtv.util::DTVSocket/dtv.util:DTVSocket::onSocketData So the deal is this: Beware of event listener entry-points being called in a reentrant fashion if your app is handling an exception. Kind of like someone hitting you when you're down, flash is asking for your app to do stuff when it's crippled. A solution is to wrap the guts of all entry points in try/catch handlers. Or maybe only for high-frequency entry points. I looked thru these links for insight, perhaps hints indicating this kind of behavior but didn't find it: http://livedocs.macromedia.com/flex/2/docs/wwhelp/wwhimpl/js/html/wwhelp.htm?href=00001890.html http://livedocs.macromedia.com/flex/2/langref/statements.html#try..catch..finally Happy November! Nick -- Flexcoders Mailing List FAQ: http://groups.yahoo.com/group/flexcoders/files/flexcodersFAQ.txt Search Archives: http://www.mail-archive.com/flexcoders%40yahoogroups.com Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/flexcoders/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/flexcoders/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
