/* Glazed Lists (c) 2003-2006 */
/* http://publicobject.com/glazedlists/ publicobject.com,*/
/* O'Dell Engineering Ltd.*/
package ca.odell.glazedlists.event;
import ca.odell.glazedlists.impl.adt.IdentityMultimap;
import ca.odell.glazedlists.EventList;
import java.util.*;
/**
* Manage listeners, firing events, and making sure that events arrive in order.
*
* This manages listeners across multiple objects in a pipeline of observables
* and their listeners. It implements Martin Fowler's
* EventAggregator
* design.
*
*
To guarantee a safe notification order, this class makes sure that all an
* object's dependencies have been notified of a particular event before that
* object is itself notified. This is tricky because it requires us to interrupt
* the event flow and control its flow. In this class, event flow is controlled
* by queueing events and not necessarily firing them during the {@link #fireEvent}
* method.
*
* @author Jesse Wilson
*/
final class SequenceDependenciesEventPublisher implements ListEventPublisher {
/** keep track of how many times the fireEvent() method is on the stack */
private int reentrantFireEventCount = 0;
/** subject to cleanup when this event is completely distributed */
private Map subjectsToCleanUp = new IdentityHashMap();
/** for proper dependency management, when a listener and subject aren't the same identity */
private Map listenersToRelatedSubjects = new IdentityHashMap();
/** the last listener notified, the next one will be beyond it in the list */
private int nextToNotify;
/**
* A mix of different subjects and listeners pairs in a deliberate order.
* We should be careful not to make changes to this list directly and instead
* create a copy as necessary
*/
private List subjectAndListeners = Collections.EMPTY_LIST;
/**
* We use copy-on-write on the listeners list. This is a copy of the
* listeners list as it looked immediately before the current change
* started. If there is no change going on (reentrantFireEventCount == 0),
* then this should be null.
*/
private List subjectsAndListenersForCurrentEvent = null;
/**
* Rebuild the subject and listeners list so that all required invariants
* are met with respect to notification order. That is, for any listener
* T, all of the subjects S that T listens to have been updated before T
* receives a change event from any S.
*
* This implementation still has some problems and work left to do:
*
it's big! Can we optimize it? Perhaps shortcutting all the graph
* work for simple cases (the 99% case)
* it's complex! Can we simplify it?
* could we keep the datastructures around? it may be wasteful to
* reconstruct them every single time a listener is added
*/
private List orderSubjectsAndListeners(List subjectsAndListeners) {
// since we're regenerating the subjectAndListeners list, clear it
// and re-add the elements
List result = new ArrayList();
// HashMaps of unprocessed elements, keyed by both source and target
IdentityMultimap sourceToPairs = new IdentityMultimap();
IdentityMultimap targetToPairs = new IdentityMultimap();
// everything that has all of its listeners already notified in subjectAndListeners
Map satisfied = new IdentityHashMap();
// everything that has a listener already notified
List satisfiedToDo = new ArrayList();
// prepare the initial collections: maps that show how each element is
// used as source and target in directed edges, plus a list of nodes
// that have no incoming edges
for(int i = 0, size = subjectsAndListeners.size(); i < size; i++) {
SubjectAndListener subjectAndListener = subjectsAndListeners.get(i);
Object source = subjectAndListener.subject;
Object target = getRelatedSubject(subjectAndListener.listener);
sourceToPairs.addValue(source, subjectAndListener);
targetToPairs.addValue(target, subjectAndListener);
satisfied.remove(target);
if(targetToPairs.count(source) == 0) {
satisfied.put(source, Boolean.TRUE);
}
}
// start with the initial set of sources that don't have dependencies
satisfiedToDo.addAll(satisfied.keySet());
// We have a subject which has all of its dependencies satisfied.
// ie. all edges where this subject is a target are already in
// subjectAndListeners. Now we want to find further edges from this
// subject to further objects. We find all of its listeners, and look
// for one of them where all of its dependencies are in the
// satisfied list. If we find such a listener, add all its edges
// to subjectAndListeners and enque it to find it's listeners
// iteratively
while(!satisfiedToDo.isEmpty()) {
// for everything that's not a target,
Object subject = satisfiedToDo.remove(0);
// get all listeners to this subject, we try this set because
// we know at least one of their edges is satisfied, and
// we hope that all of their edges is satisfied.
List sourceTargets = (List) sourceToPairs.get(subject);
// can we satisfy this target?
tryEachTarget:
for(int t = 0, targetsSize = sourceTargets.size(); t < targetsSize; t++) {
Object sourceTarget = getRelatedSubject(sourceTargets.get(t).listener);
// make sure we can satisfy this if all its sources are in satisfiedSources
List allSourcesForSourceTarget = (List) targetToPairs.get(sourceTarget);
// we've since processed this entire target, we shouldn't process it twice
if(allSourcesForSourceTarget.size() == 0) continue;
for(int s = 0, sourcesSize = allSourcesForSourceTarget.size(); s < sourcesSize; s++) {
SubjectAndListener sourceAndTarget = allSourcesForSourceTarget.get(s);
if(!satisfied.containsKey(sourceAndTarget.subject)) {
continue tryEachTarget;
}
}
// we know we can satisfy this target, add all its edges
result.addAll(allSourcesForSourceTarget);
targetToPairs.remove(sourceTarget);
// this target is no longer considered a target, since all
// its dependencies are satisfied
satisfiedToDo.add(sourceTarget);
satisfied.put(sourceTarget, Boolean.TRUE);
}
}
// if there's remaining targets, we never covered everything
if(!targetToPairs.isEmpty()) {
throw new IllegalStateException("Listener cycle detected, " + targetToPairs.values());
}
// success!
return result;
}
private Object getRelatedSubject(Object listener) {
Object subject = listenersToRelatedSubjects.get(listener);
if(subject == null) return listener;
return subject;
}
/**
* Register the specified listener to receive events from the specified
* subject whenever they are fired.
*/
public synchronized void addListener(Subject subject, Listener listener, EventFormat eventFormat) {
List unordered = updateListEventListeners(subject, listener, null, eventFormat);
subjectAndListeners = orderSubjectsAndListeners(unordered);
}
/**
* Deregister the specified listener from recieving events from the specified
* subject.
*/
public synchronized void removeListener(Object subject, Object listener) {
subjectAndListeners = updateListEventListeners(subject, null, listener, null);
}
/**
* Support method for adding and removing listeners, that also cleans up
* stale listeners, such as those from weak references.
*
* @param listenerToAdd a listener to be added, or null
* @param listenerToRemove a listener to be removed, or null
*/
private List updateListEventListeners(Subject subject, Listener listenerToAdd, Listener listenerToRemove, EventFormat eventFormat) {
// we'll want to output a copy of all the listeners
int anticipatedSize = this.subjectAndListeners.size() + (listenerToAdd == null ? - 1 : 1);
List result = new ArrayList(anticipatedSize);
// walk through, adding all the old listeners to the new listeners list,
// unless a particular listener is slated for removal for some reaosn
for(int i = 0; i < subjectAndListeners.size(); i++) {
final SubjectAndListener originalSubjectAndListener = subjectAndListeners.get(i);
// if we're supposed to remove this listener, skip it
if(originalSubjectAndListener.listener == listenerToRemove && originalSubjectAndListener.subject == subject) {
listenerToRemove = null;
continue;
}
// if this listener is stale, skip it
if(originalSubjectAndListener.eventFormat.isStale(originalSubjectAndListener.subject, originalSubjectAndListener.listener)) {
continue;
}
// this listener's still good, keep it!
result.add(originalSubjectAndListener);
}
// sanity check to ensure we found the listener we were asked to remove, if any
if(listenerToRemove != null) {
throw new IllegalArgumentException("Cannot remove nonexistent listener " + listenerToRemove);
}
// add the listener we were asked to add, if any
if(listenerToAdd != null) {
result.add(new SubjectAndListener(subject, listenerToAdd, eventFormat));
}
return result;
}
/** {@inheritDoc} */
public void setRelatedListener(Object subject, Object relatedListener) {
// force the dependency by adding a listener that just doesn't
// do anything. This will make sure that subject is always after
// related listener in the dependencies graph
addListener(relatedListener, subject, NoOpEventFormat.INSTANCE);
}
/** {@inheritDoc} */
public void clearRelatedListener(Object subject, Object relatedListener) {
removeListener(relatedListener, subject);
}
/** {@inheritDoc} */
public void addDependency(EventList dependency, ListEventListener listener) {
// do nothing
}
/** {@inheritDoc} */
public void removeDependency(EventList dependency, ListEventListener listener) {
// do nothing
}
/** {@inheritDoc} */
public void setRelatedSubject(Object listener, Object relatedSubject) {
if(relatedSubject != null) {
listenersToRelatedSubjects.put(listener, relatedSubject);
} else {
listenersToRelatedSubjects.remove(listener);
}
}
/** {@inheritDoc} */
public void clearRelatedSubject(Object listener) {
listenersToRelatedSubjects.remove(listener);
}
/**
* Get all listeners of the specified object.
*/
public List getListeners(Object subject) {
List result = new ArrayList();
for(int i = 0, size = subjectAndListeners.size(); i < size; i++) {
SubjectAndListener subjectAndListener = subjectAndListeners.get(i);
if(subjectAndListener.subject != subject) continue;
result.add(subjectAndListener.listener);
}
return result;
}
/**
* Notify all listeners of the specified subject of the specified event.
*
* @param subject the event's source
* @param event the event to send to all listeners
* @param eventFormat the mechanism to notify listeners of the event, also
* used for a callback when this event is complete
*/
public void fireEvent(Subject subject, Event event, EventFormat eventFormat) {
// keep the subjects and listeners as they are at the beginning of
// the topmost event, the list won't change because we copy on write
if(reentrantFireEventCount == 0) {
subjectsAndListenersForCurrentEvent = subjectAndListeners;
nextToNotify = Integer.MAX_VALUE;
}
// keep track of whether this method is being reentered because one
// event caused another event. If so, we'll fire later
reentrantFireEventCount++;
try {
// record this subject as firing an event, so we can clean up later
EventFormat previous = subjectsToCleanUp.put(subject, eventFormat);
if(previous != null) throw new IllegalStateException("Reentrant fireEvent() by \"" + subject + "\"");
// Mark the listeners who need this event
int subjectAndListenersSize = subjectsAndListenersForCurrentEvent.size();
// was i = lastNotified + 1
for(int i = 0; i < subjectAndListenersSize; i++) {
SubjectAndListener subjectAndListener = subjectsAndListenersForCurrentEvent.get(i);
if(subjectAndListener.subject != subject) continue;
if(i < nextToNotify) nextToNotify = i;
subjectAndListener.addPendingEvent(event);
}
// If this method is reentrant, let someone higher up the stack handle this
if(reentrantFireEventCount != 1) return;
// remember any runtime exceptions thrown to rethrow later
RuntimeException toRethrow = null;
// fire events to listeners in order
while(true) {
SubjectAndListener nextToFire = null;
// find the next listener still pending
for(int i = nextToNotify; i < subjectAndListenersSize; i++) {
SubjectAndListener subjectAndListener = subjectsAndListenersForCurrentEvent.get(i);
if(subjectAndListener.hasPendingEvent()) {
nextToFire = subjectAndListener;
nextToNotify = i + 1;
break;
}
}
// there's nobody to notify, we're done firing events
if(nextToFire == null) break;
// notify this listener
try {
nextToFire.firePendingEvent();
} catch(RuntimeException e) {
if(toRethrow == null) toRethrow = e;
}
}
// clean up all the subjects now that we're done firing events
for(Iterator> i = subjectsToCleanUp.entrySet().iterator(); i.hasNext(); ) {
Map.Entry subjectAndEventFormat = i.next();
try {
subjectAndEventFormat.getValue().postEvent(subjectAndEventFormat.getKey());
} catch(RuntimeException e) {
if(toRethrow == null) toRethrow = e;
}
}
subjectsToCleanUp.clear();
// this event is completely finished
subjectsAndListenersForCurrentEvent = null;
// rethrow any exceptions
if(toRethrow != null) throw toRethrow;
} finally {
reentrantFireEventCount--;
}
}
/**
* Adapt any observer-style interface to a common format.
*/
public interface EventFormat {
/**
* Fire the specified event to the specified listener.
*/
void fire(Subject subject, Event event, Listener listener);
/**
* A callback made only after all listeners of the specified subject
* have been notified of the specified event. This can be used as
* a hook to clean up temporary datastructures for that event.
*/
void postEvent(Subject subject);
/**
* Whether the listener is still valid. Usually a listener becomes stale
* when a weak reference goes out of scope. If this method returns true,
* the listener will be silently removed and no longer receive events.
*/
boolean isStale(Subject subject, Listener listener);
}
/**
* An EventFormat used to specify explicit dependencies, but that doesn't
* actually fire events.
*
* @see {@link ListEventPublisher#setRelatedListener}
*/
private static class NoOpEventFormat implements SequenceDependenciesEventPublisher.EventFormat {
public static final SequenceDependenciesEventPublisher.EventFormat INSTANCE = new NoOpEventFormat();
public void fire(Object subject, Object event, Object listener) {
throw new UnsupportedOperationException();
}
public void postEvent(Object subject) {
throw new UnsupportedOperationException();
}
public boolean isStale(Object subject, Object listener) {
return false;
}
}
/**
* Manage a subject/listener pair, plus a possible event that is queued to
* be fired to the listener from the subject.
*/
private static class SubjectAndListener {
private final Subject subject;
private final Listener listener;
private final EventFormat eventFormat;
private Event pendingEvent;
public SubjectAndListener(Subject subject, Listener listener, EventFormat eventFormat) {
this.subject = subject;
this.listener = listener;
this.eventFormat = eventFormat;
}
public boolean hasPendingEvent() {
return pendingEvent != null;
}
public void addPendingEvent(Event pendingEvent) {
if(this.pendingEvent != null) throw new IllegalStateException();
if(pendingEvent == null) throw new IllegalStateException();
this.pendingEvent = pendingEvent;
}
public void firePendingEvent() {
assert(pendingEvent != null);
try {
eventFormat.fire(subject, pendingEvent, listener);
} finally {
pendingEvent = null;
}
}
public String toString() {
String separator = hasPendingEvent() ? ">>>" : "-->";
return subject + separator + listener;
}
}
}
