ptolemy.domains.ptides.kernel

Class PtidesDirector

java.lang.Object

ptolemy.kernel.util.NamedObj

ptolemy.kernel.util.Attribute

ptolemy.actor.Director

ptolemy.domains.de.kernel.DEDirector

ptolemy.domains.ptides.kernel.PtidesDirector

All Implemented Interfaces:

java.lang.Cloneable, Executable, Initializable, SuperdenseTimeDirector, Changeable, Debuggable, DebugListener, Decorator, Derivable, ModelErrorHandler, MoMLExportable, Moveable, Nameable

public class PtidesDirector
extends DEDirector
implements Decorator

This director implements the Ptides programming model, which is used for the design of distributed real-time systems.

This director can only be used inside the PtidesPlatform and if the PtidesPlatform is dragged and dropped from the library, it already contains a PtidesDirector. A PtidesPlatform must be embedded in a timed director such as DE or Continuous. The time of this enclosing director simulates physical time. The localClock of this PtidesDirector simulates platformTime.

The Ptides director is based on the DE director. Like the DE director, this director maintains a totally ordered set of events. Event timestamps are given in logical time. The logical time is decoupled from the platformTime. The model time of the director is the platformTime unless an actor is fired; then the model time is the timestamp of the event that caused the actor firing.

Unlike the DE Director, this director can process events out of timestamp order if they are not causally related. Whether events can be processed is checked in a safe-to-process analysis. This analysis returns a boolean to indicate whether an event can be processed without violating Ptides semantics, based on information such as events currently in the event queue, their model time relationship with each other, as well as the current platform physical time. In this particular version of the Ptides scheduler, the director takes all events from the event queue, and compares their timestamp with the current platform time + a pre-computed offset (call this the delayOffset). If the platform time is larger, than this event is safe to process. Otherwise, we wait for platform time to pass until this event becomes safe, at which point it is processed. Other, smarter kinds of safe-to-process analysis can be implemented in future versions.

Currently, only the DE director can be used as the enclosing director. One reason for using the DE director is that time cannot go backwards in DE, which is an important physical time property. More importantly, the fire method of this director changes the persistent state of actors, which means this director cannot be used inside of an actor that performs fix point iteration, which includes (currently), Continuous, CT and SR. For more details, please refer to Edward A. Lee, Haiyang Zheng. Leveraging Synchronous Language Principles for Heterogeneous Modeling and Design of Embedded Systems, Proceedings of the 7th ACM & IEEE international conference on Embedded software, ACM, 114-123, 2007.

This director provides a set of features to address both the distributed and the real-time aspects of system design. To address the distributed aspect, each PtidesPlatform simulates a computation platform (e.g., a microprocessor), while the enclosing director simulates the physical world. Actors under the Ptides director then communicate to the outside via SensorPorts, ActuatorPorts, or network ports (NetworkReceivers, NetworkTransmitters).

This director allows for simulation of execution time. If the PtidesPlatform contains ResourceSchedulers, the scheduling of actors is performed by these. Actors must specify in parameters which ResourceSchedulers they are assigned to and the executionTime. The passage of execution time equals the passage of platformTime. Execution time has no influence on the event timestamps.

In a Ptides environment, all platforms are assumed to be synchronized within a bounded error.

The platform time is used in the following situations: generating timestamps for sensor events, enforcing deadlines for actuation events, and to setup the wake-up time for timed interrupts. Also, the Ptides operational semantics assumes a bound in the time synchronization error. This error is captured in the parameter clockSynchronizationErrorBound. If the actual error exceeds this bound, the safe-to-process analysis could produce an incorrect result. The demo PtidesNetworkLatencyTest illustrates this error.

The implementation is based on the operational semantics of Ptides, as described in: Jia Zou, Slobodan Matic, Edward A. Lee, Thomas Huining Feng, Patricia Derler. Execution Strategies for Ptides, a Programming Model for Distributed Embedded Systems, 15th IEEE Real-Time and Embedded Technology and Applications Symposium, 2009, IEEE Computer Society, 77-86, April, 2009.

Since:

Ptolemy II 10.0

Version:

$Id$

Author:

Patricia Derler, Edward A. Lee, Slobodan Matic, Mike Zimmer, Jia Zou

Pt.AcceptedRating:

Red (derler)

Pt.ProposedRating:

Red (derler)

Nested Class Summary

Nested classes/interfaces inherited from class ptolemy.kernel.util.NamedObj

NamedObj.ContainedObjectsIterator

Field Summary

Fields

Modifier and Type	Field and Description
protected java.util.Map<TypedIOPort,java.util.Set<TypedIOPort>>	_inputPortGroups Map an input port to a set which is its input port group.
protected java.util.List<TypedIOPort>	_inputPorts List of all input ports in the model (actuator and network transmitter ports are also considered input ports).
protected Time	_nextFireTime The earliest time this director should be refired.
protected java.util.Map<TypedIOPort,java.util.Map<TypedIOPort,SuperdenseDependency>>	_superdenseDependencyPair Store the superdense dependency between pairs of input ports using nested Maps.
Parameter	autoThrottling Auto throttling of local sources.
SharedParameter	clockSynchronizationErrorBound Bound on clock synchronization error across all platforms.

Fields inherited from class ptolemy.domains.de.kernel.DEDirector

_actorsFinished, _actorsInExecution, _disabledActors, _eventQueue, _eventQueueLock, _isInitializing, _microstep, _noMoreActorsToFire, _stopFireRequested, binCountFactor, enforceMicrostepSemantics, isCQAdaptive, minBinCount, stopWhenQueueIsEmpty, synchronizeToRealTime

Fields inherited from class ptolemy.actor.Director

_actorsFinishedExecution, _aspectForActor, _aspectsPresent, _defaultMicrostep, _executionAspects, _finishRequested, _initializables, _nextScheduleTime, _stopRequested, _tokenSentToCommunicationAspect, _zeroTime, localClock, startTime, stopTime

Fields inherited from class ptolemy.kernel.util.NamedObj

_changeListeners, _changeLock, _changeRequests, _debugging, _debugListeners, _deferChangeRequests, _elementName, _isPersistent, _verbose, _workspace, ATTRIBUTES, CLASSNAME, COMPLETE, CONTENTS, DEEP, FULLNAME, LINKS

Fields inherited from interface ptolemy.actor.Executable

COMPLETED, NOT_READY, STOP_ITERATING

Constructor Summary

Constructors

Constructor and Description
PtidesDirector(CompositeEntity container, java.lang.String name) Construct a director in the given container with the given name.

Method Summary

Modifier and Type	Method and Description
protected boolean	_checkForNextEvent() Model time is only used for correct execution of actors and the scheduler will determine whether another event can be fired in the current firing of the platform, so this method isn't needed.
protected void	_enqueueTriggerEvent(IOPort ioPort, Token token, Receiver receiver) Put a trigger event into the event queue.
protected static java.lang.Double	_getDoubleParameterValue(NamedObj object, java.lang.String parameterName) Return the value stored in a parameter associated with the NamedObj.
protected static java.lang.Integer	_getIntParameterValue(NamedObj object, java.lang.String parameterName) Return the value stored in a parameter associated with the NamedObj.
protected Actor	_getNextActorToFire() Return the actor to fire in this iteration, or null if no actor should be fired.
protected void	_issueExecutionAspectWarning() In DE, a warning is issued when execution aspects are used because these might change the DE semantics of the execution.
void	addInputEvent(PtidesPort sourcePort, PtidesEvent event, double deviceDelay) Add a new event to the input queue.
void	attributeChanged(Attribute attribute) Update the director parameters when attributes are changed.
java.lang.Object	clone(Workspace workspace) Clone the object into the specified workspace.
DecoratorAttributes	createDecoratorAttributes(NamedObj target) Create and return the decorated attributes for the PtidesDirector.
java.util.List<NamedObj>	decoratedObjects() Return local sources contained by the composite of this director.
Dependency	defaultDependency() Return the default dependency between input and output ports, which for the Ptides domain is a SuperdenseDependency.
void	fire() Before super.fire() is called, transfer all input events that are ready are transferred.
Time	fireAt(Actor actor, Time time, int index) Add a pure event to the queue of pure events.
Time	getCurrentSourceTimestamp() Return the source timestamp of the event that is currently being processed.
Time	getDeadline(NamedObj actor, Time timestamp) Compute the deadline for an actor that requests a firing at time timestamp.
int	getIndex() Return a superdense time index for the current time, where the index is equal to the microstep.
int	getMicrostep() Return the current microstep or the microstep of the event, if an actor is currently executing.
Time	getModelTime() Return the local time or, (i) if an actor is executing or (ii) an input token is read, (i) the timestamp of the event that caused the actor execution or (ii) the timestamp of the input event.
java.util.Map<TypedIOPort,java.util.Map<TypedIOPort,SuperdenseDependency>>	getSuperdenseDependencyPair() Return the superdense dependency hashmap This is used for the code generation in order to fill the generated director hashmap.
void	initialize() Initialize all the actors and variables.
boolean	isGlobalDecorator() Returns false, as this director only decorates local sources immediately contained by the PtidesDirector, thus it should not cross opaque hierarchy boundaries.
Receiver	newReceiver() Return a new receiver of the type PtidesReceiver.
boolean	postfire() Return false if there are no more actors to be fired or the stop() method has been called.
boolean	prefire() Override the base class to not set model time to that of the enclosing director.
void	preinitialize() Call the preinitialize of the super class and create new event Queue.
void	resumeActor(NamedObj actor) Resume the execution of an actor that was previously blocked because it didn't have all the resources it needed for execution.
void	setContainer(NamedObj container) Override the base class to first set the container, then establish a connection with any decorated objects it finds in scope in the new container.

Methods inherited from class ptolemy.domains.de.kernel.DEDirector

_actorFired, _disableActor, _enqueueEvent, _enqueueTriggerEvent, _fire, _getDepthOfActor, _getDepthOfIOPort, _noActorToFire, _requestFiring, _schedule, addDebugListener, cancelFireAt, cancelFireAt, describePriorities, fireAt, fireAtCurrentTime, fireAtRelativeTime, getCausalityInterface, getModelNextIterationTime, getNextEventTime, getStartTime, getStopTime, invalidateSchedule, mutexLockObject, removeDebugListener, setIndex, stop, stopFire, suggestedModalModelDirectors, transferInputs, wrapup

Methods inherited from class ptolemy.actor.Director

_actorFinished, _consultTimeRegulators, _description, _isEmbedded, _isTopLevel, _transferInputs, _transferOutputs, addInitializable, createSchedule, delayDependency, elapsedTimeSinceStart, finish, fireAt, fireContainerAt, fireContainerAt, getCurrentTime, getEnvironmentTime, getExecutionAspect, getGlobalTime, getModelStartTime, getModelStopTime, getNextIterationTime, getTimeResolution, implementsStrictActorSemantics, initialize, invalidateResolvedTypes, isEmbedded, isFireFunctional, isStopRequested, isStrict, iterate, notifyTokenSentToCommunicationAspect, preinitialize, removeInitializable, requestInitialization, resume, scheduleContainedActors, setCurrentTime, setEmbedded, setModelTime, setTimeResolution, supportMultirateFiring, suspend, terminate, transferOutputs, transferOutputs

Methods inherited from class ptolemy.kernel.util.Attribute

_checkContainer, _getContainedObject, _propagateExistence, getContainer, moveDown, moveToFirst, moveToIndex, moveToLast, moveUp, setName, updateContent

Methods inherited from class ptolemy.kernel.util.NamedObj

_addAttribute, _adjustOverride, _attachText, _cloneFixAttributeFields, _containedDecorators, _copyChangeRequestList, _debug, _debug, _debug, _debug, _debug, _executeChangeRequests, _exportMoMLContents, _getIndentPrefix, _isMoMLSuppressed, _markContentsDerived, _notifyHierarchyListenersAfterChange, _notifyHierarchyListenersBeforeChange, _propagateValue, _removeAttribute, _splitName, _stripNumericSuffix, _validateSettables, addChangeListener, addHierarchyListener, attributeDeleted, attributeList, attributeList, attributeTypeChanged, clone, containedObjectsIterator, decorators, deepContains, depthInHierarchy, description, description, event, executeChangeRequests, exportMoML, exportMoML, exportMoML, exportMoML, exportMoML, exportMoMLPlain, getAttribute, getAttribute, getAttributes, getChangeListeners, getClassName, getDecoratorAttribute, getDecoratorAttributes, getDerivedLevel, getDerivedList, getDisplayName, getElementName, getFullName, getModelErrorHandler, getName, getName, getPrototypeList, getSource, handleModelError, isDeferringChangeRequests, isOverridden, isPersistent, lazyContainedObjectsIterator, message, notifyOfNameChange, propagateExistence, propagateValue, propagateValues, removeAttribute, removeChangeListener, removeHierarchyListener, requestChange, setClassName, setDeferringChangeRequests, setDerivedLevel, setDisplayName, setModelErrorHandler, setPersistent, setSource, sortContainedObjects, toplevel, toString, uniqueName, validateSettables, workspace

Methods inherited from class java.lang.Object

equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface ptolemy.kernel.util.Nameable

description, getContainer, getDisplayName, getFullName, getName, getName, setName

Field Detail

clockSynchronizationErrorBound

public SharedParameter clockSynchronizationErrorBound

Bound on clock synchronization error across all platforms.

autoThrottling

public Parameter autoThrottling

Auto throttling of local sources. This parameter is only visible in expert mode and defaults to the boolean value true.

_inputPorts

protected java.util.List<TypedIOPort> _inputPorts

List of all input ports in the model (actuator and network transmitter ports are also considered input ports).

_inputPortGroups

protected java.util.Map<TypedIOPort,java.util.Set<TypedIOPort>> _inputPortGroups

Map an input port to a set which is its input port group.

_nextFireTime

protected Time _nextFireTime

The earliest time this director should be refired.

_superdenseDependencyPair

protected java.util.Map<TypedIOPort,java.util.Map<TypedIOPort,SuperdenseDependency>> _superdenseDependencyPair

Store the superdense dependency between pairs of input ports using nested Maps. Providing the source input as a key will return a Map value, where the destination input port can be used as a key to return the superdense dependency.

Constructor Detail

PtidesDirector

public PtidesDirector(CompositeEntity container,
                      java.lang.String name)
               throws IllegalActionException,
                      NameDuplicationException

Construct a director in the given container with the given name. The container argument must not be null, or a NullPointerException will be thrown. If the name argument is null, then the name is set to the empty string. Increment the version number of the workspace.

Parameters:

container - Container of the director.

name - Name of this director.

Throws:

IllegalActionException - If the director is not compatible with the specified container.

NameDuplicationException - If the container not a CompositeActor and the name collides with an entity in the container.

Method Detail

addInputEvent

public void addInputEvent(PtidesPort sourcePort,
                          PtidesEvent event,
                          double deviceDelay)
                   throws IllegalActionException

Add a new event to the input queue. Compute the time when this input can be consumed and store in queue. The time depends on the device delay.

Parameters:

sourcePort - the source port.

event - New input event.

deviceDelay - The device delay.

Throws:

IllegalActionException - If device delay parameter cannot be computed.

attributeChanged

public void attributeChanged(Attribute attribute)
                      throws IllegalActionException

Update the director parameters when attributes are changed.

Overrides:

attributeChanged in class DEDirector

Parameters:

attribute - The changed parameter.

Throws:

IllegalActionException - If the parameter set is not valid. Not thrown in this class.

clone

public java.lang.Object clone(Workspace workspace)
                       throws java.lang.CloneNotSupportedException

Clone the object into the specified workspace. The new object is not added to the directory of that workspace (you must do this yourself if you want it there). The result is an attribute with no container.

Overrides:

clone in class DEDirector

Parameters:

workspace - The workspace for the cloned object.

Returns:

The new Attribute.

Throws:

java.lang.CloneNotSupportedException - Not thrown in this base class

See Also:

NamedObj.exportMoML(Writer, int, String), NamedObj.setDeferringChangeRequests(boolean)

createDecoratorAttributes

public DecoratorAttributes createDecoratorAttributes(NamedObj target)

Create and return the decorated attributes for the PtidesDirector. The director decorates local sources with throttling attributes that allow for specification of how far ahead in logical time these actors can execute or how many tokens they can produce at a time.

Specified by:

createDecoratorAttributes in interface Decorator

Parameters:

target - The NamedObj that will be decorated.

Returns:

The decorated attributes for the target NamedObj, or null if the specified NamedObj is not decorated by this decorator.

defaultDependency

public Dependency defaultDependency()

Return the default dependency between input and output ports, which for the Ptides domain is a SuperdenseDependency.

Overrides:

defaultDependency in class Director

Returns:

The default dependency that describes a time delay of 0.0, and a index delay of 0 between ports.

See Also:

Dependency, CausalityInterface, Actor.getCausalityInterface()

decoratedObjects

public java.util.List<NamedObj> decoratedObjects()

Return local sources contained by the composite of this director.

Specified by:

decoratedObjects in interface Decorator

Returns:

List of entities.

fire

public void fire()
          throws IllegalActionException

Before super.fire() is called, transfer all input events that are ready are transferred. After super.fire() is called, transfer all output events that are ready are transferred.

Specified by:

fire in interface Executable

Overrides:

fire in class DEDirector

Throws:

IllegalActionException - If we couldn't process an event or if an event of smaller timestamp is found within the event queue.

See Also:

DEDirector._fire()

fireAt

public Time fireAt(Actor actor,
                   Time time,
                   int index)
            throws IllegalActionException

Add a pure event to the queue of pure events.

Overrides:

fireAt in class DEDirector

Parameters:

actor - Actor to fire.

time - Time the actor should be fired at.

index - Microstep the actor should be fired at.

Returns:

The time the actor requested to be refired at.

Throws:

IllegalActionException - If firing of the container doesn't succeed.

See Also:

DEDirector.fireAtCurrentTime(Actor)

getCurrentSourceTimestamp

public Time getCurrentSourceTimestamp()

Return the source timestamp of the event that is currently being processed. If no event is being processed, (i.e. event is analyzed for safe to process, actor is fired, ...) this method can return null or the timestamp of the previous event. This method should not be called if no event is currently being processed.

Returns:

The current source timestamp.

getDeadline

public Time getDeadline(NamedObj actor,
                        Time timestamp)
                 throws IllegalActionException

Compute the deadline for an actor that requests a firing at time timestamp.

Overrides:

getDeadline in class Director

Parameters:

actor - The actor that requests firing.

timestamp - The time when the actor wants to be fired.

Returns:

The deadline for the actor.

Throws:

IllegalActionException - If time objects cannot be created.

getIndex

public int getIndex()

Return a superdense time index for the current time, where the index is equal to the microstep.

Specified by:

getIndex in interface SuperdenseTimeDirector

Overrides:

getIndex in class DEDirector

Returns:

A superdense time index.

See Also:

DEDirector.setIndex(int), SuperdenseTimeDirector

getModelTime

public Time getModelTime()

Return the local time or, (i) if an actor is executing or (ii) an input token is read, (i) the timestamp of the event that caused the actor execution or (ii) the timestamp of the input event.

Overrides:

getModelTime in class Director

Returns:

The local time or the semantic time.

See Also:

Director.setModelTime(Time)

getMicrostep

public int getMicrostep()

Return the current microstep or the microstep of the event, if an actor is currently executing.

Overrides:

getMicrostep in class DEDirector

Returns:

microstep of the current time.

See Also:

DEDirector.getIndex(), DEDirector.setIndex(int), SuperdenseTimeDirector

getSuperdenseDependencyPair

public java.util.Map<TypedIOPort,java.util.Map<TypedIOPort,SuperdenseDependency>> getSuperdenseDependencyPair()

Return the superdense dependency hashmap This is used for the code generation in order to fill the generated director hashmap.

Returns:

The Superdense dependency hashmap

initialize

public void initialize()
                throws IllegalActionException

Initialize all the actors and variables. Perform static analysis on superdense dependencies between input ports in the topology.

Specified by:

initialize in interface Initializable

Overrides:

initialize in class DEDirector

Throws:

IllegalActionException - If any of the methods contained in initialize() throw it.

isGlobalDecorator

public boolean isGlobalDecorator()

Returns false, as this director only decorates local sources immediately contained by the PtidesDirector, thus it should not cross opaque hierarchy boundaries.

Specified by:

isGlobalDecorator in interface Decorator

Returns:

false.

newReceiver

public Receiver newReceiver()

Return a new receiver of the type PtidesReceiver.

Overrides:

newReceiver in class DEDirector

Returns:

A new PtidesReceiver.

postfire

public boolean postfire()
                 throws IllegalActionException

Return false if there are no more actors to be fired or the stop() method has been called.

Specified by:

postfire in interface Executable

Overrides:

postfire in class DEDirector

Returns:

True If this director will be fired again.

Throws:

IllegalActionException - If the stopWhenQueueIsEmpty parameter does not contain a valid token, or refiring can not be requested.

prefire

public boolean prefire()
                throws IllegalActionException

Override the base class to not set model time to that of the enclosing director. This method always returns true, deferring the decision about whether to fire an actor to the fire() method.

Specified by:

prefire in interface Executable

Overrides:

prefire in class DEDirector

Returns:

True.

Throws:

IllegalActionException

preinitialize

public void preinitialize()
                   throws IllegalActionException

Call the preinitialize of the super class and create new event Queue.

Specified by:

preinitialize in interface Initializable

Overrides:

preinitialize in class DEDirector

Throws:

IllegalActionException - If the preinitialize() method of the container or one of the deeply contained actors throws it, or the parameters, minBinCount, binCountFactor, and isCQAdaptive, do not have valid tokens.

_issueExecutionAspectWarning

protected void _issueExecutionAspectWarning()

In DE, a warning is issued when execution aspects are used because these might change the DE semantics of the execution. In Ptides, this is not the case.

Overrides:

_issueExecutionAspectWarning in class DEDirector

resumeActor

public void resumeActor(NamedObj actor)
                 throws IllegalActionException

Description copied from class: DEDirector

Resume the execution of an actor that was previously blocked because it didn't have all the resources it needed for execution. This method puts an event into the queue for the current time.

Overrides:

resumeActor in class DEDirector

Parameters:

actor - The actor that resumes execution.

Throws:

IllegalActionException - Not thrown here but in derived classes.

setContainer

public void setContainer(NamedObj container)
                  throws IllegalActionException,
                         NameDuplicationException

Override the base class to first set the container, then establish a connection with any decorated objects it finds in scope in the new container.

Overrides:

setContainer in class Director

Parameters:

container - The container to attach this attribute to..

Throws:

IllegalActionException - If this attribute is not of the expected class for the container, or it has no name, or the attribute and container are not in the same workspace, or the proposed container would result in recursive containment.

NameDuplicationException - If the container already has an attribute with the name of this attribute.

See Also:

Attribute.getContainer()

_checkForNextEvent

protected boolean _checkForNextEvent()

Model time is only used for correct execution of actors and the scheduler will determine whether another event can be fired in the current firing of the platform, so this method isn't needed. By always returning true, _getNextActorToFire() will be called which runs the scheduler.

Overrides:

_checkForNextEvent in class DEDirector

Returns:

true Always.

_enqueueTriggerEvent

protected void _enqueueTriggerEvent(IOPort ioPort,
                                    Token token,
                                    Receiver receiver)
                             throws IllegalActionException

Put a trigger event into the event queue.

The trigger event has the same timestamp as that of the director. The microstep of this event is always equal to the current microstep of this director. The depth for the queued event is the depth of the destination IO port. Finally, the token and the destination receiver are also stored in the event.

If the event queue is not ready or the actor that contains the destination port is disabled, do nothing.

Parameters:

ioPort - The destination IO port.

token - The token associated with this event.

receiver - The destination receiver.

Throws:

IllegalActionException - If the time argument is not the current time, or the depth of the given IO port has not be calculated, or the new event can not be enqueued.

_getDoubleParameterValue

protected static java.lang.Double _getDoubleParameterValue(NamedObj object,
                                                           java.lang.String parameterName)
                                                    throws IllegalActionException

Return the value stored in a parameter associated with the NamedObj.

Parameters:

object - The object that has the parameter.

parameterName - The name of the parameter to be retrieved.

Returns:

the value of the named parameter if the parameter is not null. Otherwise return null.

Throws:

IllegalActionException - If thrown while getting the value of the parameter.

_getIntParameterValue

protected static java.lang.Integer _getIntParameterValue(NamedObj object,
                                                         java.lang.String parameterName)
                                                  throws IllegalActionException

Return the value stored in a parameter associated with the NamedObj.

Parameters:

object - The object that has the parameter.

parameterName - The name of the parameter to be retrieved.

Returns:

the value of the named parameter if the parameter is not null. Otherwise return null.

Throws:

IllegalActionException - If thrown while getting the value of the parameter.

_getNextActorToFire

protected Actor _getNextActorToFire()
                             throws IllegalActionException

Return the actor to fire in this iteration, or null if no actor should be fired. Since _checkForNextEvent() always returns true, this method will keep being called until it returns null.

Overrides:

_getNextActorToFire in class DEDirector

Returns:

The next actor to be fired, which can be null.

Throws:

IllegalActionException - If _isSafeToProcess() throws it.