001/*
002 *  Copyright 2013 Anyware Services
003 *
004 *  Licensed under the Apache License, Version 2.0 (the "License");
005 *  you may not use this file except in compliance with the License.
006 *  You may obtain a copy of the License at
007 *
008 *      http://www.apache.org/licenses/LICENSE-2.0
009 *
010 *  Unless required by applicable law or agreed to in writing, software
011 *  distributed under the License is distributed on an "AS IS" BASIS,
012 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 *  See the License for the specific language governing permissions and
014 *  limitations under the License.
015 */
016package org.ametys.core.observation;
017
018import java.util.Map;
019
020/**
021 * Observer for consuming events.
022 */
023public interface Observer
024{
025    /** Minimum priority. */
026    public static final int MIN_PRIORITY = Integer.MAX_VALUE;
027    /** Maximum priority. */
028    public static final int MAX_PRIORITY = 0;
029    
030    /**
031     * Checks if the event is supported. If true, the observe(Event) method will be called.
032     * @param event the event.
033     * @return <code>true</code> for observing this event, <code>false</code> otherwise.
034     */
035    boolean supports(Event event);
036    
037    /**
038     * Retrieves the priority to observe this event.<br>
039     * This can be used to process a supported event before others observers.
040     * @param event the event.
041     * @return the priority where 0 the max priority and Integer.MAX_VALUE the min priority.
042     */
043    int getPriority(Event event);
044    
045    /**
046     * Observes an event.
047     * @param event the event.
048     * @param transientVars transientVars passed from one Observer to another when processing a single Event. 
049     * This may allow optimizations between observers.
050     * @throws Exception if an error occurs. 
051     * All exceptions will be logged but not propagated, as the observation mechanism should never fail.
052     */
053    void observe(Event event, Map<String, Object> transientVars) throws Exception;
054}