content: /*
   *  Licensed to the Apache Software Foundation (ASF) under one
   *  or more contributor license agreements.  See the NOTICE file
   *  distributed with this work for additional information
   *  regarding copyright ownership.  The ASF 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
   *
   *        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 org.apache.causeway.core.metamodel.context;
  
  import java.util.Optional;
  import java.util.concurrent.atomic.AtomicReference;
  
  import org.jspecify.annotations.Nullable;
  
  import org.apache.causeway.applib.annotation.Programmatic;
  import org.apache.causeway.applib.services.i18n.TranslationService;
  import org.apache.causeway.commons.internal.assertions._Assert;
  import org.apache.causeway.commons.internal.exceptions._Exceptions;
  
  /**
   * @since 2.0
   */
  @Programmatic
  public abstract class MetaModelContext implements HasMetaModelContext {
  
      @Override
      public final MetaModelContext getMetaModelContext() {
          return this;
      }
  
      // -- INSTANCE (SINGLETON)
  
      public static Optional<MetaModelContext> instance() {
          return Optional.ofNullable(INSTANCE_HOLDER.get());
      }
  
      @Nullable
      public static MetaModelContext instanceNullable() {
          return instance().orElse(null);
      }
  
      public static MetaModelContext instanceElseFail() {
          return instance()
                  .orElseThrow(()->_Exceptions.noSuchElement("MetaModelContext not yet or no longer available."));
      }
  
      // -- UTILITY
  
      public static  TranslationService translationServiceOrFallback() {
          return MetaModelContext.instance()
                  .map(MetaModelContext::getTranslationService)
                  .orElseGet(TranslationService::identity);
      }
  
      // -- HELPER
  
      protected static final AtomicReference<MetaModelContext> INSTANCE_HOLDER = new AtomicReference<>();
  
      /**
       * Internal: clears the singleton instance reference
       */
      protected static void clear() {
          INSTANCE_HOLDER.set(null);
      }
  
      /**
       * Internal: set singleton. Override NOT allowed.
       */
      protected static void set(final MetaModelContext mmc) {
          _Assert.assertNull(MetaModelContext.INSTANCE_HOLDER.get(),
                  ()->"MetaModelContext singelton is already instantiated, override is not allowed!");
          setOrReplace(mmc);
      }
  
      /**
       * Internal: set or replace singleton. Override allowed.
       */
      public static void setOrReplace(final MetaModelContext mmc) {
          MetaModelContext.INSTANCE_HOLDER.set(mmc);
      }
  
  }
  