001// Copyright 2006, 2008, 2010 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
015package org.apache.tapestry5.services;
016
017import org.apache.tapestry5.annotations.HeartbeatDeferred;
018
019/**
020 * Allows for deferred execution of logic, useful when trying to get multiple components to coordinate behavior. A
021 * component may add a command to be executed "{@linkplain #end() at the end of the heartbeat}". The classic example of
022 * 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
023 * field's id is correct until after both have rendered.
024 * <p/>
025 * The Heartbeat is injected into components via the {@link org.apache.tapestry5.annotations.Environmental} annotation.
026 */
027public interface Heartbeat
028{
029    /**
030     * Begins a new Heartbeat. Heartbeats nest. Every call to begin() should be matched by a call to {@link #end()}.
031     */
032    void begin();
033
034    /**
035     * Executes all commands since the most recent {@link #begin()}.
036     */
037    void end();
038
039    /**
040     * Adds a new command to the current Heartbeat. The command will be executed by {@link #end()}.
041     * 
042     * @param command
043     *            command to be executed at the end of the heartbeat
044     * @see HeartbeatDeferred
045     */
046    void defer(Runnable command);
047}