    001// Licensed under the Apache License, Version 2.0 (the "License");
    002// you may not use this file except in compliance with the License.
    003// You may obtain a copy of the License at
    004//
    005// http://www.apache.org/licenses/LICENSE-2.0
    006//
    007// Unless required by applicable law or agreed to in writing, software
    008// distributed under the License is distributed on an "AS IS" BASIS,
    009// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    010// See the License for the specific language governing permissions and
    011// limitations under the License.
    012
    013package org.apache.tapestry5.services;
    014
    015import org.apache.tapestry5.annotations.HeartbeatDeferred;
    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 *
    023 * The Heartbeat is injected into components via the {@link org.apache.tapestry5.annotations.Environmental} annotation.
    024 */
    025public 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
    041     *            command to be executed at the end of the heartbeat
    042     * @see HeartbeatDeferred
    043     */
    044    void defer(Runnable command);
    045}