/* * Copyright (c) 2010, 2018, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package javafx.beans.value; import javafx.beans.InvalidationListener; import javafx.beans.Observable; /** * An {@code ObservableValue} is an entity that wraps a value and allows to * observe the value for changes. In general this interface should not be * implemented directly but one of its sub-interfaces * ({@code ObservableBooleanValue} etc.). *
* The value of the {@code ObservableValue} can be requested with * {@link #getValue()}. *
* An implementation of {@code ObservableValue} may support lazy evaluation, * which means that the value is not immediately recomputed after changes, but * lazily the next time the value is requested. All bindings and properties in * this library support lazy evaluation. *
* An {@code ObservableValue} generates two types of events: change events and * invalidation events. A change event indicates that the value has changed. An * invalidation event is generated if the current value is not valid anymore. * This distinction becomes important if the {@code ObservableValue} supports * lazy evaluation, because for a lazily evaluated value one does not know if an * invalid value really has changed until it is recomputed. For this reason, * generating change events requires eager evaluation while invalidation events * can be generated for eager and lazy implementations. *
* Implementations of this class should strive to generate as few events as * possible to avoid wasting too much time in event handlers. Implementations in * this library mark themselves as invalid when the first invalidation event * occurs. They do not generate anymore invalidation events until their value is * recomputed and valid again. *
* Two types of listeners can be attached to an {@code ObservableValue}: * {@link InvalidationListener} to listen to invalidation events and * {@link ChangeListener} to listen to change events. *
* Important note: attaching a {@code ChangeListener} enforces eager computation
* even if the implementation of the {@code ObservableValue} supports lazy
* evaluation.
*
* @param
* Note that the same actual {@code ChangeListener} instance may be safely
* registered for different {@code ObservableValues}.
*
* The {@code ObservableValue} stores a strong reference to the listener
* which will prevent the listener from being garbage collected and may
* result in a memory leak. It is recommended to either unregister a
* listener by calling {@link #removeListener(ChangeListener)
* removeListener} after use or to use an instance of
* {@link WeakChangeListener} avoid this situation.
*
* @see #removeListener(ChangeListener)
*
* @param listener
* The listener to register
* @throws NullPointerException
* if the listener is null
*/
void addListener(ChangeListener super T> listener);
/**
* Removes the given listener from the list of listeners that are notified
* whenever the value of the {@code ObservableValue} changes.
*
* If the given listener has not been previously registered (i.e. it was
* never added) then this method call is a no-op. If it had been previously
* added then it will be removed. If it had been added more than once, then
* only the first occurrence will be removed.
*
* @see #addListener(ChangeListener)
*
* @param listener
* The listener to remove
* @throws NullPointerException
* if the listener is null
*/
void removeListener(ChangeListener super T> listener);
/**
* Returns the current value of this {@code ObservableValue}
*
* @return The current value
*/
T getValue();
}