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   /*
    * Copyright 2018 The Netty Project
    *
    * The Netty Project licenses this file to you 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:
    *
    *   https://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 io.netty.channel;
  
  import io.netty.util.ReferenceCountUtil;
  import io.netty.util.internal.TypeParameterMatcher;
  
  /**
   * {@link ChannelInboundHandlerAdapter} which allows to conveniently only handle a specific type of user events.
   *
   * For example, here is an implementation which only handle {@link String} user events.
   *
   * <pre>
   *     public class StringEventHandler extends
   *             {@link SimpleUserEventChannelHandler}&lt;{@link String}&gt; {
   *
   *         {@code @Override}
   *         protected void eventReceived({@link ChannelHandlerContext} ctx, {@link String} evt)
   *                 throws {@link Exception} {
   *             System.out.println(evt);
   *         }
   *     }
   * </pre>
   *
   * Be aware that depending of the constructor parameters it will release all handled events by passing them to
   * {@link ReferenceCountUtil#release(Object)}. In this case you may need to use
   * {@link ReferenceCountUtil#retain(Object)} if you pass the object to the next handler in the {@link ChannelPipeline}.
   */
  public abstract class SimpleUserEventChannelHandler<I> extends ChannelInboundHandlerAdapter {
  
      private final TypeParameterMatcher matcher;
      private final boolean autoRelease;
  
      /**
       * see {@link #SimpleUserEventChannelHandler(boolean)} with {@code true} as boolean parameter.
       */
      protected SimpleUserEventChannelHandler() {
          this(true);
      }
  
      /**
       * Create a new instance which will try to detect the types to match out of the type parameter of the class.
       *
       * @param autoRelease   {@code true} if handled events should be released automatically by passing them to
       *                      {@link ReferenceCountUtil#release(Object)}.
       */
      protected SimpleUserEventChannelHandler(boolean autoRelease) {
          matcher = TypeParameterMatcher.find(this, SimpleUserEventChannelHandler.class, "I");
          this.autoRelease = autoRelease;
      }
  
      /**
       * see {@link #SimpleUserEventChannelHandler(Class, boolean)} with {@code true} as boolean value.
       */
      protected SimpleUserEventChannelHandler(Class<? extends I> eventType) {
          this(eventType, true);
      }
  
      /**
       * Create a new instance
       *
       * @param eventType      The type of events to match
       * @param autoRelease    {@code true} if handled events should be released automatically by passing them to
       *                       {@link ReferenceCountUtil#release(Object)}.
       */
      protected SimpleUserEventChannelHandler(Class<? extends I> eventType, boolean autoRelease) {
          matcher = TypeParameterMatcher.get(eventType);
          this.autoRelease = autoRelease;
      }
  
      /**
       * Returns {@code true} if the given user event should be handled. If {@code false} it will be passed to the next
       * {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
       */
      protected boolean acceptEvent(Object evt) throws Exception {
          return matcher.match(evt);
      }
  
      @Override
      public final void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
          boolean release = true;
          try {
              if (acceptEvent(evt)) {
                  @SuppressWarnings("unchecked")
                  I ievt = (I) evt;
                  eventReceived(ctx, ievt);
             } else {
                 release = false;
                 ctx.fireUserEventTriggered(evt);
             }
         } finally {
             if (autoRelease && release) {
                 ReferenceCountUtil.release(evt);
             }
         }
     }
 
     /**
      * Is called for each user event triggered of type {@link I}.
      *
      * @param ctx the {@link ChannelHandlerContext} which this {@link SimpleUserEventChannelHandler} belongs to
      * @param evt the user event to handle
      *
      * @throws Exception is thrown if an error occurred
      */
     protected abstract void eventReceived(ChannelHandlerContext ctx, I evt) throws Exception;
 }