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Ulm's Oberon Library:
Messages

NAME

SYNOPSIS

TYPE Object = Disciplines.Object;
TYPE Message = POINTER TO MessageRec;
TYPE MessageRec =
   RECORD
      (PersistentDisciplines.ObjectRec)
      processed: BOOLEAN;
      done: BOOLEAN;
      errors: RelatedEvents.Object;
   END;
TYPE Handler = PROCEDURE (object: Object; VAR message: Message);



PROCEDURE InstallHandler(object: Object; handler: Handler);
PROCEDURE Send(object: Object; VAR message: Message);
PROCEDURE Init(message: Message); (* doesn't call NEW *)

DESCRIPTION

Messages itself does not support the persistency of message handlers if objects are an extension of PersistentDisciplines.Object. Nevertheless, the persistency of the handlers may be achieved if they are all associated with the object or one of its persistent disciplines. In this case, InstallHandler has to be called by the create interface procedures of the object or its disciplines for PersistentObjects.

Modules which implement abstractions which are based on messages have to

• declare extended types of Message which represents the operations and additional components for the associated parameters.
• provide methods for PersistentObjects to create, read and write their message types. This is required to allow remote procedure calls and logging.
• define a handler of type Handler which processes all incoming messages and ignores those which are not recognized. If known message are processed, the processed component and, on success, the done component should be set to TRUE. If processed becomes TRUE, no further message handlers will be called for that message. Errors have to be related to errors.
• call InstallHandler for each object whose messages are to be processed.

All newly created messages must be initialized by Init (which itself doesn't call NEW). Init sets processed and done to FALSE and initializes errors to an object with an empty error queue (see RelatedEvents for details). Afterwards, additional components which represent the parameters of the operation may be set. Send allows then to send the message to the given object.

Messages supports Forwarders by installing a forwarding handler for dependent objects. Note that the installation of handlers is not affected by Forwarders.Forward, i.e. InstallHandler installs always handler for object independently from any dependency relations. Local handlers take always precedence over the handlers of the object which is depended on.

DIAGNOSTICS

SEE ALSO

Forwarders

general forwarding operation

PersistentObjects

input and output of persistent objects

PersistentDisciplines

input and output of attached data structures of persistent objects

PersistentEvents

input and output of events

RelatedEvents

error handling

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