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   /*
    * Copyright 2017 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.resolver.dns;
  
  import io.netty.handler.codec.dns.DnsQuestion;
  
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  
  /**
   * Combines two {@link DnsQueryLifecycleObserverFactory} into a single {@link DnsQueryLifecycleObserverFactory}.
   */
  public final class BiDnsQueryLifecycleObserverFactory implements DnsQueryLifecycleObserverFactory {
      private final DnsQueryLifecycleObserverFactory a;
      private final DnsQueryLifecycleObserverFactory b;
  
      /**
       * Create a new instance.
       * @param a The {@link DnsQueryLifecycleObserverFactory} that will receive events first.
       * @param b The {@link DnsQueryLifecycleObserverFactory} that will receive events second.
       */
      public BiDnsQueryLifecycleObserverFactory(DnsQueryLifecycleObserverFactory a, DnsQueryLifecycleObserverFactory b) {
          this.a = checkNotNull(a, "a");
          this.b = checkNotNull(b, "b");
      }
  
      @Override
      public DnsQueryLifecycleObserver newDnsQueryLifecycleObserver(DnsQuestion question) {
          return new BiDnsQueryLifecycleObserver(a.newDnsQueryLifecycleObserver(question),
                                                 b.newDnsQueryLifecycleObserver(question));
      }
  }