platform/util/src/com/intellij/util/messages/MessageBus.java

/*
 * Copyright 2000-2012 JetBrains s.r.o.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.intellij.util.messages;

import com.intellij.openapi.Disposable;
import org.jetbrains.annotations.NotNull;
import org.jetbrains.annotations.Nullable;

/**
 * Core of IntelliJ IDEA messaging infrastructure. Basic functions:
 * <pre>
 * <ul>
 *   <li>allows to {@link #syncPublisher(Topic) push messages};</li>
 *   <li>
 *     allows to {@link #connect() create connections} for further {@link MessageBusConnection#subscribe(Topic, Object) subscriptions};
 *   </li>
 * </ul>
 * </pre>
 * <p/>
 * Use <code>'com.intellij.openapi.components.ComponentManager#getMessageBus()'</code> to obtain one.
 * <p/>
 * Please see <a href="Wiki">http://confluence.jetbrains.net/display/IDEADEV/IntelliJ+IDEA+Messaging+infrastructure</a>.
 */
public interface MessageBus {

  /**
   * Messages buses can be organised into hierarchies. That allows facilities {@link Topic#getBroadcastDirection() broadcasting}.
   * <p/>
   * Current method exposes parent bus (if any is defined).
   *
   * @return parent bus (if defined)
   */
  @Nullable
  MessageBus getParent();

  /**
   * Allows to create new connection that is not bound to any {@link Disposable}.
   *
   * @return newly created connection
   */
  @NotNull
  MessageBusConnection connect();

  /**
   * Allows to create new connection that is bound to the given {@link Disposable}. That means that returned connection
   * will be automatically {@link MessageBusConnection#dispose() released} if given {@link Disposable disposable parent} is collected.
   *
   * @param parentDisposable target parent disposable to which life cycle newly created connection shall be bound
   * @return newly created connection which life cycle is bound to the given disposable parent
   */
  @NotNull
  MessageBusConnection connect(@NotNull Disposable parentDisposable);

  /**
   * Allows to retrieve an interface for publishing messages to the target topic.
   * <p/>
   * Basically, the whole processing looks as follows:
   * <pre>
   * <ol>
   *   <li>
   *     Messaging clients create new {@link MessageBusConnection connections} within the target message bus and
   *     {@link MessageBusConnection#subscribe(Topic, Object) subscribe} to the target {@link Topic topics};
   *   </li>
   *   <li>
   *     Every time somebody wants to send a message for particular topic, he or she calls current method and receives object
   *     that conforms to the {@link Topic#getListenerClass() business interface} of the target topic. Every method call on that
   *     object is dispatched by the messaging infrastructure to the subscribers.
   *     {@link Topic#getBroadcastDirection() broadcasting} is performed if necessary as well;
   *   </li>
   * </ol>
   * </pre>
   * <p/>
   * It's also very important to understand message processing strategy in case of <b>nested dispatches</b>.
   * Consider the following situation:
   * <pre>
   * <ol>
   *   <li>
   *     <code>Subscriber<sub>1</sub></code> and <code>subscriber<sub>2</sub></code> are registered for the same topic within
   *     the same message bus;
   *   </li>
   *   <li><code>Message<sub>1</sub></code> is sent to that topic within the same message bus;</li>
   *   <li>Queued messages delivery starts;</li>
   *   <li>Queued messages delivery ends as there are no messages queued but not dispatched;</li>
   *   <li><code>Message<sub>1</sub></code> is queued for delivery to both subscribers;</li>
   *   <li>Queued messages delivery starts;</li>
   *   <li><code>Message<sub>1</sub></code> is being delivered to the <code>subscriber<sub>1</sub></code>;</li>
   *   <li><code>Subscriber<sub>1</sub></code> sends <code>message<sub>2</sub></code> to the same topic within the same bus;</li>
   *   <li>Queued messages delivery starts;</li>
   *   <li>
   *     <b>Important:</b> <code>subscriber<sub>2</sub></code> is being notified about all queued but not delivered messages,
   *     i.e. its callback is invoked for the message<sub>1</sub>;
   *   </li>
   *   <li>Queued messages delivery ends because all subscribers have been notified on the <code>message<sub>1</sub></code>;</li>
   *   <li><code>Message<sub>2</sub></code> is queued for delivery to both subscribers;</li>
   *   <li>Queued messages delivery starts;</li>
   *   <li><code>Subscriber<sub>1</sub></code> is notified on <code>message<sub>2</sub></code></li>
   *   <li><code>Subscriber<sub>2</sub></code> is notified on <code>message<sub>2</sub></code></li>
   * </ol>
   * </pre>
   * <p/>
   * <b>Thread-safety.</b>
   * All subscribers are notified sequentially from the calling thread.
   * <p/>
   * <b>Memory management.</b>
   * Returned objects are very light-weight and stateless, so, they are cached by the message bus in <code>'per-topic'</code> manner.
   * That means that caller of this method is not obliged to keep returned reference along with the reference to the message for
   * further publishing. It's enough to keep reference to the message bus only and publish
   * like {@code 'messageBus.syncPublisher(targetTopic).targetMethod()'}.
   *
   * @param topic target topic
   * @param <L>   {@link Topic#getListenerClass() business interface} of the target topic
   * @return publisher for target topic
   */
  @NotNull
  <L> L syncPublisher(@NotNull Topic<L> topic);

  /**
   * @deprecated use {@link #syncPublisher(Topic)} instead
   */
  @NotNull
  @Deprecated
  <L> L asyncPublisher(@NotNull Topic<L> topic);

  /**
   * Disposes current bus, i.e. drops all queued but not delivered messages (if any) and disallows further
   * {@link #connect(Disposable) connections}.
   */
  void dispose();
}