Critical Mass Modula-3: netobj/src/netobjrt/NetObjNotifier.i3

netobj/src/netobjrt/NetObjNotifier.i3

The NetObjNotifier interface allows the holder of a surrogate object to request notification of when the object's owner becomes inaccessible. This can be useful, for example, if it is necessary to remove surrogates from a table upon termination of the programs holding their corresponding concrete objects.

INTERFACE NetObjNotifier;

IMPORT NetObj;

TYPE
  OwnerState = {Dead, Failed};

  NotifierClosure = OBJECT METHODS
    notify(obj: NetObj.T; st: OwnerState);
  END;

PROCEDURE AddNotifier(obj: NetObj.T; cl: NotifierClosure);

Arrange that a call to cl.notify will be scheduled when obj becomes inaccessible. If obj is not a surrogate object then AddNotifier has no effect. If obj is already inaccessible at the time AddNotifier is called, then a call to cl.notify is scheduled immediately. \ttindex{NetObjNotifier.AddNotifier}


END NetObjNotifier.

The notify method of a NotifierClosure is invoked when the concrete object corresponding to the surrogate obj becomes inaccessible. The procedure AddNotifier must have been called to enable this notification. There may be more than one NotifierClosure for the same surrogate. At notification time, the st argument is Dead if and only if the object owner is known to be permanently inaccessible. Otherwise st is Failed. It is possible for notify to be called multiple times on the same object. Any invocations on obj are guaranteed to fail in a timely fashion subsequent to a closure notification with st = Dead.

In general, a surrogate object can still be collected if a notifier closure is registered for it. However, if the closure object contains a reference to the surrogate, then its registration might delay or prevent collection. Therefore this should be avoided.

Although this interface is organized to enable notification of owner death on a per object basis, in practice this is achieved by monitoring the state of the owner's address space. This means that death notification will be more or less simultaneous for all surrogates whose concrete objects have the same owner.