Fwd: Create a new object.

From: "Rob Van Pamel" <[hidden email]>

Date: November 27, 2006 2:55:45 AM PST

To: "'Joshua Gargus'" <[hidden email]>

Subject: RE: [croquet] Create a new object.

Indeed you are right. It happens when I do a mouseover the new created

object. The space is the space that I created 

I get a reference to the space like this 

newspace := TSpace new.

newspace registerGlobal: #mainEntry.

space := newspace.

I created a global variabele which represents that space? Perhaps there it

goes wrong? 

The Stepper i created in a new process with a delay indeed. 

I have added a Fileout in this message from my class. 

I have the following classes Redcube, NavigatorWorld, NavigatorMaster,

Greencube and redcube (the last 2 are not important because it's the same as

a Redcube)

In navigatorworld I initialize the world( TSpace and objects(Redcubes) ,

create the stepper which updates the world.

To create an object, i give it some specific values like a unique

objectName(which comes from the DB). Read out the x,y,z values. and then i

add it to a list. When all the objects are added to the list i create them.

using the addChild method. This is the same for the initialisation or the

update procedure. 

The stepper is a whole class (which you can see in the myCroquet.st) 

I hope this is fine. 

I hope you understand my code. 

Thanx for the quick response!!

-----Oorspronkelijk bericht-----

Van: Joshua Gargus [[hidden email]] 

Verzonden: maandag 27 november 2006 11:25

Aan: Rob Van Pamel

CC: [hidden email]

Onderwerp: Re: [croquet] Create a new object.

It seems likely that you're not adding object properly.  How are you  

getting a reference to the space that you want to add to?  Where is  

the stepper running? (on a Croquet object? on a Morphic object? in a  

Process using Delay>>wait?)

I suspect that you're running the stepper from an object "inside the  

island", which is normally fine, but not if you're triggering a non- 

replicated side-effect like reading an object from a database and  

adding it to the space.  Think of it this way:

Succinctly put, the basis of Croquet's replication model is "If you  

have two identical replicas of a world, and they subsequently process  

the identical events in the same order, then the replicas will remain  

identical".  (putting it even more succinctly: "determinism").

 From your rather sketchy description, I can imagine that things  

might go something like this:

- you start off with 2 synced replicas

- every 5 seconds, a stepper (in both replicas) checks if there is  

something new in the database

If this is correct, then you are already in trouble, because you have  

introduced non-determinism.  There is no guarantee that both replicas  

will get the same result from the database lookup.  This is  

especially true if only one of the machines attempts the database  

connection and creates the object in the space (how can the other  

replica possibly know that an object has been created?), but is also  

true if both machines are connecting to the same (perhaps  

distributed) database.

Here's one way that the above scenario might produce symptoms like  

the stack trace you observed...

If you mouse over the newly created object, all replicas of the  

object should be notified so that they can respond appropriately.   

This notification occurs by sending a replicated message via the  

TFarRef to the object.  The mouse-over processing occurs during  

picking, which happens during rendering (i.e. between ticks of the  

island clock).  The pick operation attempts to find the TFarRef  

corresponding to the picked object, but fails because of the way that  

the object was created (i.e. via a non-replicated action on one  

island replica).

Does this guess sound close?  Could you describe the structure of  

your code in a bit more detail?

Cheers,

Josh

On Nov 27, 2006, at 1:47 AM, Rob Van Pamel wrote:

Hello everybody

I sometimes get an error when i try to add an object to the Tspace.

I do this as following.

-----------

  CODE

-----------

temp := Redcube new.

cube translationX:(cube x)asInteger y:(cube y)asInteger z:(cube z) 

asInteger.

space addChild:cube.

Now this works fine. But i am  working with a database which reads  

out data and that data represents the objects that i add to the  

TSpace. I do this every 5 seconds with a stepper, and when i found  

out that an object is in the database but isn't yet in the Tspace i  

add it in the same way i add it before.

It works but sometimes i get an error like this

Error : This object has no name.

But when i try to debug it, i don't see my code anywhere. Not even  

in the Full Stack. It's about TFarref ,TMutexSet and so on ...

Anybody who has had something like this ?

Thanx in advantage

  Rob