Every instance of Actor has a behavior object associated with it.

Receiving and processing messages

Every user-level message sent to an Actor must be a request carrying a Message instance. The message is sent to the actor’s behavior object, and the reply is relayed to the calling actor.

Methods taking blocks or ActorProxy values often demand special treatment; see the section on the weaknesses of the library’s design for more information.

Ways of responding to messages

Simple replies

Methods on a behavior object can simply return their result, like any other method on any other object, and it will be relayed to the caller.

Replying with a promise

They may also choose to return a promise of an eventual answer. The caller will get their reply when the promise resolves or is rejected.

Suspending the caller

Finally, they may suspend the decision about how to reply to the caller.

Calling the method Actor class >> #caller retrieves and detaches the request object that the actor is working on right now. The actor can then store the request in a variable, making its reply using #resolveWith: or #rejectWith: later. Calling Actor class >> #caller a second time will return nil, since the request was detached on the first call.

The actor only automatically replies to a request if Actor class >> #caller has not been called. Making a call to Actor class >> #caller signals that the request will be taken care of manually.

The Barrier tutorial shows an example of this technique.

Class ActorBehavior

Any Smalltalk object can serve as a behavior object, but inheriting from ActorBehavior offers a number of convenient features:

  • ActorBehavior class >> #spawn is a convenient abbreviation for Actor class >> #bootProxy:. See ways of constructing an actor.

  • ActorBehavior >> #log: and #logAll: produce log message events using the tracing mechanism. This allows log messages to be recorded in the correct order with respect to surrounding events when tracing is active. (By default, these messages go to the Transcript; see the section on tracing for details.)

  • ActorBehavior >> #changed, #changed: and #changed:with: ensure that dependents of the behavior are updated in the UI process, rather than directly in the actor’s own process. This is important not only because Morphic dependents often rely on executing in the UI process, but also for robustness. If any of the update methods signals an exception, the offending dependent is simply removed, rather than killing the actor. An exception from one update method therefore will not prevent the other dependents of the changed object from running.