Bug Report: Removing a FrameListener null ref exception

Clay

21-03-2008 23:45:56

I've run into a problem... Say you add a FrameListener:

root.FrameStarted += new FrameListener.FrameStartedHandler(TestListener);
From this frame listener if you to remove itself, you get a NullReferenceException. Here is the code:

        static bool TestListener(FrameEvent evt)

        {

            Mogre.Root.Singleton.FrameStarted -= new FrameListener.FrameStartedHandler(TestListener);

            return true;

        }

This gives a NullReferenceException with this callstack:

   at Mogre.Root.remove_FrameStarted(FrameStartedHandler hnd)

   at MogrePython.Program.TestListener(FrameEvent evt) in C:\Users\clay\Documents\Visual Studio 2008\Projects\MogrePython\MogrePython\Program.cs:line 34

   at Mogre.Root.raise_FrameStarted(FrameEvent evt)

   at Mogre.Root.OnFrameStarted(FrameEvent evt)

   at Mogre.FrameListener_Director.frameStarted(FrameListener_Director* , FrameEvent* evt)

   at Ogre.Root.renderOneFrame(Root* )

   at Mogre.Root.RenderOneFrame()

   at MogrePython.OgreWindow.Go() in C:\Users\clay\Documents\Visual Studio 2008\Projects\MogrePython\MogrePython\OgreWindow.cs:line 81

   at MogrePython.Program.Main() in C:\Users\clay\Documents\Visual Studio 2008\Projects\MogrePython\MogrePython\Program.cs:line 21

   at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)

   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)

   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()

   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)

   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)

   at System.Threading.ThreadHelper.ThreadStart()

Marioko, could you take a look at this? I can't quite tell what the problem is from looking at the C++/CLI code.

Thanks,
-Clay

Clay

22-03-2008 04:06:53

mmm first of all, why do you need self remove callback??? why instead use some other callback to kill it???
I'm trying to enable a very specific scenario... I have a decorator function in python which shields Ogre from exceptions thrown by the FrameListener:

def SafeFrameListener(listener):

	def InnerFunction(evt):

		try:

			return listener(evt)

		except Exception, e:

			clr.AddReferenceByPartialName("System.Windows.Forms")

			from System.Windows.Forms import MessageBox

			MessageBox.Show(str(e), "An exception occurred!");

			

			Ogre.Root.Singleton.FrameStarted -= InnerFunction

			Ogre.Root.Singleton.FrameEnded -= InnerFunction

		

			return True

	return InnerFunction

Let's say there was a function like this:

def Listener(evt):

    Error()  # Let's say this throws an exception

    return True



Mogre.Root.Singleton.FrameStarted += Listener

If you ran the above code, and added this listener, it would take down your application... Instead, if you apply the above decorator you would get a MessageBox that pops up telling you an error occurred, what the error was, then removes the function so it stops throwing exceptions. The only difference would be this:

@SafeFrameListener

def Listener(evt):

    Error()  # Let's say this throws an exception

    return True



Mogre.Root.Singleton.FrameStarted += Listener

As long as you add @SafeFrameListener to decorate any Listener, you don't have to worry about an exception unwinding the stack. This is important when you are using a scripting language because a simple typo causes an unrecoverable error in a FrameListener...

I could rewrite it to display the error message once then silently handle/ignore the exception, but that's ugly, difficult, and hacky. =)