This technique is used to export and import individual objects or subsets of objects in a simulation. When an object is dearchived with this technique, a new instance is created for the saved object and loaded into a running simulation. This approach is in contrast to the method described in Saving and Loading the Entire State of a Simulation the section called “Saving and Loading the Entire State of a Simulation”, which creates an entirely new simulation from an archived world. The steps described here also apply to sending and receiving objects over a network, as described in Sending and Receiving breve Objects the section called “Sending and Receiving breve Objects”.
In a "simple" object archive, only a single object is encoded. If the
object has object variables or
pointer variables, they will be set to 0
when the object is dearchived. All other variable types will be
restored to their pre-archived values. For simple object archiving and
dearchiving, no additional code or preparation is required.
Variables of type object and pointer cannot be automatically maintained without
special treatment. This is because an object may reference another, which in turn
references several others, and so forth. If all object variables were maintained, then a huge chain
reaction would result every time an instance was saved.
To avoid this, saving objects does not include an object variable unless the the object being saved has declared it as a dependency, using the Object method add-dependency. If you define an object, Y, as a dependency of X, then saving X will also save Y. Loading the saved instance X will also recreate the instance Y.
Because adding dependencies also has the potential to lead to chain-reactions of archiving, it should be used with care. Generally, an object should only add dependencies on objects it has created and for which it is exclusively responsible. So while a simulated creature might legitimately add a dependency on an object containing its genome or an object which determines its movement (a "brain"), it would be inappropriate for the object to add a dependency on the controller object, or other creatures in the world.
When an object is to be archived using this technique, the
user-supplied method archive is executed
for the object. In your archive method,
you should include code that might be needed to prepare the object for
archiving such as updating variables which will be required when the
object is dearchived. Your archive method must, on success, return
1. Any other return value is considered failure and will
abort the archive.
When an object is to be dearchived using this method, the user-supplied
method dearchive is executed for the
object. In your dearchive method, you
should include code that might be needed to restore the state of the
object, or to inform the rest of the simulation of its presence if
necessary. Your dearchive method must, on
success, return 1. Any other return value is considered
failure and will abort the archive.
Once dependencies, archive methods and dearchive methods (all of which may prove to be unnecessary for most straightforward classes) are addressed, you may initiate an archive using the Object method archive-as-xml.
To dearchive an object previously archived with this technique, use the Control method dearchive-xml. Note that this creates new instances of the objects in the archived file, instead of "filling" an existing object.