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package stdlib;
import java.util.ArrayList;
import java.util.List;
/**
* A helper class which can be used to manage an object's subscriptions.
*
* @param <T> the type of the publisher
* @param <U> the type of the optional data argument
* @see Subscriber
*/
public final class Subscriptions<T, U> {
private final T publisher;
private final List<Subscriber<T, U>> subscribers;
private boolean changed = false;
/**
* Initializes a new instance of the {@link Subscriptions} class with
* zero subscribers.
*
* @param publisher the object being publisher
* @throws IllegalArgumentException if publisher is <code>null</code>
*/
public Subscriptions(T publisher) {
if (publisher == null)
throw new IllegalArgumentException("publisher cannot be null");
this.publisher = publisher;
this.subscribers = new ArrayList<Subscriber<T, U>>();
}
/**
* Adds an subscriber to the set of subscribers for the publisher object,
* provided that it is not the same as some subscriber already in the set.
*
* @param subscriber
* @throws IllegalArgumentException if the parameter subscriber is <code>null</code>
*/
public void addSubscriber(Subscriber<T, U> subscriber) {
if (subscriber == null)
throw new IllegalArgumentException("subscriber cannot be null");
if (!subscribers.contains(subscriber)) {
subscribers.add(subscriber);
}
}
/**
* Indicates that the publisher object has no longer changed, or that it has
* already notified all of its subscribers of its most recent change, so that
* {@link #hasChanged()} will now return <code>false</code>. This method
* is called automatically by the {@link #notifySubscribers} methods.
*
*/
public void clearChanged() {
changed = false;
}
/**
* Returns the number of subscribers of publisher object.
*
* @return the number of subscribers of publisher object
*/
public int countSubscribers() {
return subscribers.size();
}
/**
* Deletes an subscriber from the set of subscribers. Passing <code>null</code>
* to this method will have no effect.
*
* @param subscriber the {@link Subscriber} to be deleted
*/
public void deleteSubscriber(Subscriber<T, U> subscriber) {
subscribers.remove(subscriber);
}
/**
* Clears the subscriber list so that the publisher object no longer has any
* subscribers.
*/
public void deleteSubscribers() {
this.subscribers.clear();
}
/**
* Returns <code>true</code> if the publisher object has changed;
* otherwise, <code>false</code>.
*
* @return <code>true</code> if the publisher object has changed;
* otherwise, <code>false</code>
*/
public boolean hasChanged() {
return changed;
}
/**
* If this object has changed, as indicated by the {@link #hasChanged()},
* then notify all of its subscribers and then clear the changed state. This
* method is equivalent to calling <code>notifySubscribers(null)</code>.
*/
public void notifySubscribers() {
notifySubscribers(null);
}
/**
* If this object has changed, as indicated by the {@link #hasChanged()},
* then notify all of its subscribers and then clear the changed state.
*
* @param data optional event specific data which will be passed to the subscribers
*/
public void notifySubscribers(U data) {
if (hasChanged()) {
for (Subscriber<T, U> subscriber : subscribers) {
subscriber.update(publisher, data);
}
clearChanged();
}
}
/**
* Flags the publisher object as having changed.
*/
public void setChanged() {
changed = true;
}
}
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