    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}