/*******************************************************************************
* CogTool Copyright Notice and Distribution Terms
* CogTool 1.3, Copyright (c) 2005-2013 Carnegie Mellon University
* This software is distributed under the terms of the FSF Lesser
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package edu.cmu.cs.hcii.cogtool.util;
import java.util.ArrayList;
import java.util.EventObject;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
/**
* Standard implementation for providing semantic notification to an observer
* of a specific change to an observed subject instance.
*
* @author mlh
* @see IAlerter
*/
public class Alerter implements IAlerter
{
/**
* Support class for enumerating <code>AlertHandlerEntry</code> instances
* such that the <code>eventClass</code> value of each returned instance
* will "inherit" or "be assignable to" the given <code>eventClass</code>.
* <p>
* Enumeration progresses by examining each entry associated with the
* given object to determine if it subsumes the given
* <code>eventClass</code>. That is, the given <code>alertType</code>
* parameter must "inherit" (or "be assignable to") the
* <code>eventClass</code> value of each <code>AlertHandlerEntry</code>
* instance enumerated. When one is found during <code>hasNext</code>,
* it is "remembered" and returned by the subsequent <code>next</code>
* call.
*
* @author mlh
*/
protected static class AlertHandlerIterator implements Iterator<Alerter.AlertHandlerEntry>
{
/**
* The Java class that represents the semantic change;
* enumerated <code>AlertHandlerEntry</code> instances must have a
* <code>eventClass</code> value that subsumes this class.
*/
protected Class<? extends EventObject> alertClass;
/**
* An iterator that will enumerate all handlers associated with
* the <code>Alerter</code> object.
*/
protected Iterator<Alerter.AlertHandlerEntry> allHandlers;
/**
* The last found <code>AlertHandlerEntry</code> instance
* that satisfies the constraint that its <code>eventClass</code>
* value subsumes <code>alertClass</code>.
*/
protected Alerter.AlertHandlerEntry nextEntry;
/**
* Initialize the iteration.
*
* @param alertType the class of the semantic change of interest
* @param alertAllHandlers the iterator of all handlers associated
* with the <code>Alerter</code> object
* @author mlh
*/
public AlertHandlerIterator(Class<? extends EventObject> alertType,
Iterator<Alerter.AlertHandlerEntry> alertAllHandlers)
{
alertClass = alertType;
allHandlers = alertAllHandlers;
nextEntry = null;
}
/**
* Determine if another <code>AlertHandlerEntry</code> instances
* that satisfies the constraint is yet to be enumerated.
*
* @return true iff an instance has been found and not yet
* fetched by a call to <code>next</code> or another
* satisfying instance is enumerated by the iterator
* that generates all handler entries
* @author mlh
*/
public boolean hasNext()
{
// Only need to search if the last valid entry was consumed.
if (nextEntry == null) {
while (allHandlers.hasNext()) {
Alerter.AlertHandlerEntry entry = allHandlers.next();
// Try to find another handler whose associated class
// subsumes the given <code>alertClass</code>.
// That is, the semantic change class "inherits" or
// "is assignable to" the class associated with the
// alert handler.
if (entry.eventClass.isAssignableFrom(alertClass)) {
nextEntry = entry;
return true; // Found the next one
}
}
return false; // No more left
}
return true; // Haven't consumed the last one yet
}
/**
* Returns the next <code>AlertHandlerEntry</code> instance
* containing a handler that would be invoked if a semantic change
* of the given class would be made.
* <p>
* According to the specification of <code>Iterator</code>,
* this method raises <code>NoSuchElementException</code>
* if there are no more entries to enumerate. Thus, <code>next</code>
* should only be called after a successful (that is,
* <code>true</code>) return of a call to <code>hasNext</code>.
*
* @return the next <code>AlertHandlerEntry</code> instance containing
* a handler that would be invoked if a semantic change
* of the given class would be made
* @exception <code>NoSuchElementException</code> if no more entries
* exist to enumerate
* @author mlh
* @see java.util.Iterator
*/
public Alerter.AlertHandlerEntry next()
{
if (hasNext()) {
Alerter.AlertHandlerEntry returnValue = nextEntry;
nextEntry = null; // indicate this one has been consumed
return returnValue;
}
throw new NoSuchElementException();
}
/**
* Remove the last enumerated <code>AlertHandlerEntry</code> instance
* from the associated collection; NOT IMPLEMENTED.
*
* @exception <code>UnsupportedOperationException</code> as this
* method is not implemented.
* @author mlh
* @see java.util.Iterator
*/
public void remove()
{
// Not implemented!
throw new UnsupportedOperationException();
}
}
/**
* When enumerating the objects that will handle the notification
* of a semantic change (specific or not), the objects generated
* are of this type. It associates the handler object with
* the subclass of <code>EventObject</code> that reflects the
* semantic change of interest.
*
* @author mlh
*/
public static class AlertHandlerEntry
{
public Object observer;
public Class<? extends EventObject> eventClass;
public AlertHandler handler;
/**
* Initialize the association of handler with the class of the semantic
* change in which it is interested.
*
* @param obs the observer of the semantic change
* @param evtClass the class of the semantic change of interest
* @param h the handler to be notified of the change
* @author mlh
*/
public AlertHandlerEntry(Object obs,
Class<? extends EventObject> evtClass,
AlertHandler h)
{
observer = obs;
eventClass = evtClass;
handler = h;
}
}
/**
* To hold the handlers that will respond to semantic changes in this
* object.
*/
protected List<Alerter.AlertHandlerEntry> handlers = null;
/**
* Add a handler to observe semantic changes raised by this instance
* of the specified semantic type (a subclass of <code>EventObject</code>).
* <p>
* The same handler may be added more than once; if so, it will be
* notified more than once.
* <p>
* Handlers will be invoked in the same order as they were registered.
*
* @param observer the object interested in the semantic event
* @param eventClass the class of the semantic change of interest
* @param handler the handler to be notified of the change
* @param noDuplicates whether to prevent duplicate entries
* @author mlh
*/
public void addHandler(Object observer,
Class<? extends EventObject> eventClass,
AlertHandler handler,
boolean noDuplicates)
{
if (handlers == null) {
handlers = new ArrayList<Alerter.AlertHandlerEntry>();
}
else if (noDuplicates) {
for (int i = 0; i < handlers.size(); i++) {
Alerter.AlertHandlerEntry entry =
handlers.get(i);
if ((entry.observer == observer) &&
(entry.eventClass == eventClass) &&
(entry.handler == handler))
{
return;
}
}
}
handlers.add(new Alerter.AlertHandlerEntry(observer,
eventClass,
handler));
}
/**
* Add a handler to observe semantic changes raised by this instance
* of the specified semantic type (a subclass of <code>EventObject</code>).
* <p>
* The same handler may be added more than once; if so, it will be
* notified more than once.
* <p>
* Handlers will be invoked in the same order as they were registered.
* <p>
* The list holding <code>AlertHandlerEntry</code> instances
* is created only when needed.
* <p>
* Equivalent to invoking 4-parameter addHandler with noDuplicates = true.
*
* @param observer the object interested in the semantic event
* @param eventClass the class of the semantic change of interest
* @param handler the handler to be notified of the change
* @author mlh
*/
public void addHandler(Object observer,
Class<? extends EventObject> eventClass,
AlertHandler handler)
{
addHandler(observer, eventClass, handler, true);
}
/**
* Remove the first registration of the given handler as specified
* to observe semantic changes raised by this instance
* of the specified semantic type (a subclass of <code>EventObject</code>).
* <p>
* To remove all registrations, repeat the call to
* <code>removeHandler</code> until it returns <code>false</code>.
*
* @param eventClass the class of the semantic change of interest
* @param handler the handler to be notified of the change
* @return true iff the remove was successful
* @author mlh
*/
public boolean removeHandler(Class<? extends EventObject> eventClass, AlertHandler handler)
{
Iterator<Alerter.AlertHandlerEntry> allHandlers = getHandlers(); // enumerate all handlers
while (allHandlers.hasNext()) {
Alerter.AlertHandlerEntry entry = allHandlers.next();
if ((entry.eventClass == eventClass) && (entry.handler == handler))
{
// Removal works because all handlers are enumerated
allHandlers.remove();
return true;
}
}
return false;
}
// TODO Currently it's easy to accidentally leak AlertHandlers, since
// the code that installs them has to be careful to arrange to
// remove them. It seems likely that using weak collections here
// might make it possible for them to be automagically GCed. This
// should be investigated.
/**
* Remove all registered handlers for observing semantic changes
* associated with the given observer.
*
* @param observer the object interested in the semantic event
* @author mlh
*/
public void removeAllHandlers(Object observer)
{
Iterator<Alerter.AlertHandlerEntry> allHandlers = getHandlers(); // enumerate all handlers
while (allHandlers.hasNext()) {
Alerter.AlertHandlerEntry entry = allHandlers.next();
if (entry.observer == observer) {
allHandlers.remove();
}
}
}
/**
* Enumerate all the handler-type pairs registered on this instance.
* <p>
* The returned iterator instance will enumerate
* <code>AlertHandlerEntry</code> instances.
*
* @return an iterator that enumerates all handlers registered
* on this instance
* @author mlh
*/
public Iterator<Alerter.AlertHandlerEntry> getHandlers()
{
if (handlers != null) {
return handlers.iterator();
}
// Used because there are no handlers
return new EmptyIterator<Alerter.AlertHandlerEntry>();
}
/**
* Enumerate all the handler-type pairs registered on this instance
* that would be invoked if a semantic change of the given type occurred.
* <p>
* The returned iterator instance will enumerate
* <code>AlertHandlerEntry</code> instances.
* <p>
* Note that the given <code>eventClass</code> parameter will "inherit"
* (or "be assignable to") the <code>eventClass</code> value of each
* <code>AlertHandlerEntry</code> instance enumerated.
*
* @param eventClass the class of the semantic change of interest
* @return an iterator that enumerates handlers registered
* to be notified of semantic changes of the
* given "type"
* @author mlh
*/
public Iterator<Alerter.AlertHandlerEntry> getHandlers(Class<? extends EventObject> eventClass)
{
if (handlers != null) {
// We need to cache all applicable handlers based on the requested
// eventClass in case some handler is removed from the set of
// handlers associated with this object by one of the "earlier"
// handler executions.
List<Alerter.AlertHandlerEntry> applicableHandlers = new ArrayList<Alerter.AlertHandlerEntry>();
Iterator<Alerter.AlertHandlerEntry> allHandlers = handlers.iterator();
while (allHandlers.hasNext()) {
Alerter.AlertHandlerEntry entry = allHandlers.next();
// Try to find another handler whose associated class
// subsumes the given <code>alertClass</code>.
// That is, the semantic change class "inherits" or
// "is assignable to" the class associated with the
// alert handler.
if (entry.eventClass.isAssignableFrom(eventClass)) {
applicableHandlers.add(entry);
}
}
return applicableHandlers.iterator();
// return new AlertHandlerIterator(eventClass,
// this.handlers.iterator());
}
// Used because there are no handlers
return new EmptyIterator<Alerter.AlertHandlerEntry>();
}
/**
* Notify all handlers that are interested in the given semantic change.
* <p>
* Only those handlers associated with <code>EventObject</code> subclasses
* that the given <code>alert</code> "inherits" (or "could be assigned to")
* will be notified.
* <p>
* Each handler is notified by invoking its <code>handleAlert</code>
* method with the given semantic change data.
*
* @param alert the data reflecting the semantic change
* @author mlh
*/
public void raiseAlert(EventObject alert)
{
Iterator<Alerter.AlertHandlerEntry> alertHandlers = getHandlers(alert.getClass());
while (alertHandlers.hasNext()) {
Alerter.AlertHandlerEntry entry = alertHandlers.next();
entry.handler.handleAlert(alert);
}
}
}