001    // Copyright 2006, 2008 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.tapestry5.services;
016    
017    /**
018     * Allows for deferred execution of logic, useful when trying to get multiple components to coordinate behavior. A
019     * component may add a command to be executed "{@linkplain #end() at the end of the heartbeat}". The classic example of
020     * this is a Label and the field it labels; since we don't know which order the two will render, we can't tell if the
021     * field's id is correct until after both have rendered.
022     * <p/>
023     * The Heartbeat is injected into components via the {@link org.apache.tapestry5.annotations.Environmental} annotation.
024     */
025    public interface Heartbeat
026    {
027        /**
028         * Begins a new Heartbeat. Heartbeats nest. Every call to begin() should be matched by a call to {@link #end()}.
029         */
030        void begin();
031    
032        /**
033         * Executes all commands since the most recent {@link #begin()}.
034         */
035        void end();
036    
037        /**
038         * Adds a new command to the current Heartbeat. The command will be executed by {@link #end()}.
039         *
040         * @param command command to be executed at the end of the heartbeat
041         */
042        void defer(Runnable command);
043    }