001    // Copyright 2004, 2005 The Apache Software Foundation
002    //
003    // Licensed under the Apache License, Version 2.0 (the "License");
004    // you may not use this file except in compliance with the License.
005    // You may obtain a copy of the License at
006    //
007    //     http://www.apache.org/licenses/LICENSE-2.0
008    //
009    // Unless required by applicable law or agreed to in writing, software
010    // distributed under the License is distributed on an "AS IS" BASIS,
011    // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012    // See the License for the specific language governing permissions and
013    // limitations under the License.
014    
015    package org.apache.tapestry.event;
016    
017    /**
018     * May observe changes in an object's properties.  This is a "weak" variation
019     * on JavaBean's style bound properties.  It is used when there will be at most
020     * a single listener on property changes, and that the listener is not interested
021     * in the old value.
022     *
023     * @author Howard Lewis Ship
024     * 
025     **/
026    
027    public interface ChangeObserver
028    {
029        /**
030         *  Sent when the observed object changes a property.  The event identifies
031         *  the object, the property and the new value.
032         *
033         **/
034    
035        void observeChange(ObservedChangeEvent event);
036        
037        /**
038         * Checks if the current state changes have already been committed.
039         * 
040         * @return True if state changes are locked, false otherwise.
041         */
042        boolean isLocked();
043    }