Add Listeners utility class

Author: tpietzsch

src/main/java/org/scijava/listeners/Listeners.java

@@ -0,0 +1,127 @@
+package org.scijava.listeners;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.function.Consumer;
+
+/**
+ * A set of listeners of type {@code T}.
+ *
+ * @param <T>
+ *            listener type
+ */
+public interface Listeners< T >
+{
+	/**
+	 * Add a listener to this set.
+	 *
+	 * @param listener
+	 *            the listener to add.
+	 * @return {@code true} if the listener was added. {@code false} if it was
+	 *         already present.
+	 */
+	boolean add( final T listener );
+
+	/**
+	 * Removes a listener from this set.
+	 *
+	 * @param listener
+	 *            the listener to remove.
+	 * @return {@code true} if the listener was successfully removed.
+	 *         {@code false} if the listener was not present.
+	 */
+	boolean remove( final T listener );
+
+	default boolean addAll( final Collection< ? extends T > listeners )
+	{
+		if ( listeners.isEmpty() )
+			return false;
+		return listeners.stream().map( this::add ).reduce( Boolean::logicalOr ).get();
+	}
+
+	default boolean removeAll( final Collection< ? extends T > listeners )
+	{
+		if ( listeners.isEmpty() )
+			return false;
+		return listeners.stream().map( this::remove ).reduce( Boolean::logicalOr ).get();
+	}
+
+	/**
+	 * Implements {@link Listeners} using an {@link ArrayList}.
+	 */
+	class List< T > implements Listeners< T >
+	{
+		private final Consumer< T > onAdd;
+
+		public List( final Consumer< T > onAdd )
+		{
+			this.onAdd = onAdd;
+		}
+
+		public List()
+		{
+			this( o -> {} );
+		}
+
+		public final ArrayList< T > list = new ArrayList<>();
+
+		@Override
+		public boolean add( final T listener )
+		{
+			if ( !list.contains( listener ) )
+			{
+				list.add( listener );
+				onAdd.accept( listener );
+				return true;
+			}
+			return false;
+		}
+
+		@Override
+		public boolean remove( final T listener )
+		{
+			return list.remove( listener );
+		}
+
+		public ArrayList< T > listCopy()
+		{
+			return new ArrayList<>( list );
+		}
+	}
+
+	/**
+	 * Extends {@link Listeners.List}, making {@code add} and {@code remove}
+	 * methods synchronized.
+	 */
+	class SynchronizedList< T > extends List< T >
+	{
+		public SynchronizedList( final Consumer< T > onAdd )
+		{
+			super( onAdd );
+		}
+
+		public SynchronizedList()
+		{
+			super();
+		}
+
+		@Override
+		public synchronized boolean add( final T listener )
+		{
+			return super.add( listener );
+		}
+
+		@Override
+		public synchronized boolean remove( final T listener )
+		{
+			return super.remove( listener );
+		}
+
+		@Override
+		public synchronized ArrayList< T > listCopy()
+		{
+			return super.listCopy();
+		}
+	}
+}
+