At the beginning of each iteration, the status of all inputs and outputs is unknown. Upon firing an actor, the status of its output signals may become known. Once the status of a signal becomes known, it cannot be changed back to unknown in the iteration. This monotonicity constraint ensures the existence and uniqueness of the fixed point. During an iteration, the prefire() and fire() methods of the controlled actors may be repeatedly invoked, but the postfire() method will be invoked exactly once after the fixed point has been found. The postfire() methods of the contained actors are invoked only in the postfire() method of this director, and they are invoked in arbitrary order.
If the prefire() method of an actor returns false, then this director assumes that all the outputs of the actor are absent. The actor has declined to fire.
Although this director does not require any specific ordering of actor firings, a scheduler is used to choose an efficient ordering.
By default, actors are strict, which means that all their input signals must be known before the actor can be fired. Here, what we mean by "fired" is that prefire() is invoked, and if it returns true, then fire() is invoked. Such actors will be fired only once in an iteration. A non-strict actor can be fired regardless of the status of its inputs, and may be fired repeatedly in an iteration if some of the inputs are unknown. Once an actor is fired with all its inputs known, it will not be fired again in the same iteration. A composite actor containing this director is a non-strict actor.
For an actor to be used under the control of this director, it must either be strict, or if it is non-strict, it must be monotonic. Montonicity implies two constraints on the actor. First, if prefire() ever returns true during an iteration, then it will return true on all subsequent invocations in the same iteration(). Second, if either prefire() or fire() call clear() on an output port, then no subsequent invocation in the same iteration can call put() on the port. If prefire() or fire() call put() on an output port with some token, then no subsequent invocation in the same iteration can call clear() or put() with a token with a different value. These constraints ensure determinacy.
If synchronizeToRealTime is set to true, then the postfire() method stalls until the real time elapsed since the model started matches the current time. This ensures that the director does not get ahead of real time. However, of course, this does not ensure that the director keeps up with real time. Note that this synchronization occurs after actors have been fired, but before they have been postfired.
This class is based on the original SRDirector, written by Paul Whitaker. @author Haiyang Zheng and Edward A. Lee @version $Id: FixedPointDirector.java,v 1.37.4.2 2008/03/25 23:11:30 cxh Exp $ @since Ptolemy II 5.2 @Pt.ProposedRating Green (hyzheng) @Pt.AcceptedRating Yellow (eal)
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