1 /*
   2  * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package javafx.beans;
  27 
  28 import java.lang.ref.WeakReference;
  29 import javafx.beans.NamedArg;
  30 
  31 /**
  32  * A {@code WeakInvalidationListener} can be used if an {@link Observable}
  33  * should only maintain a weak reference to the listener. This helps to avoid
  34  * memory leaks that can occur if observers are not unregistered from observed
  35  * objects after use.
  36  * <p>
  37  * A {@code WeakInvalidationListener} is created by passing in the original
  38  * {@link InvalidationListener}. The {@code WeakInvalidationListener} should
  39  * then be registered to listen for changes of the observed object.
  40  * <p>
  41  * Note: You have to keep a reference to the {@code InvalidationListener} that
  42  * was passed in as long as it is in use, otherwise it can be garbage collected
  43  * too soon.
  44  *
  45  * @see InvalidationListener
  46  * @see Observable
  47  *
  48  *
  49  * @since JavaFX 2.0
  50  */
  51 public final class WeakInvalidationListener implements InvalidationListener, WeakListener {
  52 
  53     private final WeakReference<InvalidationListener> ref;
  54 
  55     /**
  56      * The constructor of {@code WeakInvalidationListener}.
  57      *
  58      * @param listener
  59      *            The original listener that should be notified
  60      */
  61     public WeakInvalidationListener(@NamedArg("listener") InvalidationListener listener) {
  62         if (listener == null) {
  63             throw new NullPointerException("Listener must be specified.");
  64         }
  65         this.ref = new WeakReference<InvalidationListener>(listener);
  66     }
  67 
  68     /**
  69      * {@inheritDoc}
  70      */
  71     @Override
  72     public boolean wasGarbageCollected() {
  73         return (ref.get() == null);
  74     }
  75 
  76     /**
  77      * {@inheritDoc}
  78      */
  79     @Override
  80     public void invalidated(Observable observable) {
  81         InvalidationListener listener = ref.get();
  82         if (listener != null) {
  83             listener.invalidated(observable);
  84         } else {
  85             // The weakly reference listener has been garbage collected,
  86             // so this WeakListener will now unhook itself from the
  87             // source bean
  88             observable.removeListener(this);
  89         }
  90     }
  91 }