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.beam.sdk.transforms;
  
  import java.io.Serializable;
  import org.apache.beam.sdk.annotations.Internal;
  import org.apache.beam.sdk.values.PCollection;
  import org.apache.beam.sdk.values.PCollectionView;
  import org.apache.beam.sdk.values.TypeDescriptor;
  
  /**
   * <b><i>For internal use only; no backwards-compatibility guarantees.</i></b>
   *
   * <p>A function to adapt a primitive "view" of a {@link PCollection} - some materialization
   * specified in the Beam model and implemented by the runner - to a user-facing view type for side
   * input.
   *
   * <p>Both the underlying primitive view and the user-facing view are immutable.
   *
   * <p>The most common case is using the {@link View} transforms to prepare a {@link PCollection} for
   * use as a side input to {@link ParDo}. See {@link View#asSingleton()}, {@link View#asIterable()},
   * and {@link View#asMap()} for more detail on specific views available in the SDK.
   *
   * @param <PrimitiveViewT> the type of the underlying primitive view required
   * @param <ViewT> the type of the value(s) accessible via this {@link PCollectionView}
   */
  @Internal
  public abstract class ViewFn<PrimitiveViewT, ViewT> implements Serializable {
    /** Gets the materialization of this {@link ViewFn}. */
    public abstract Materialization<PrimitiveViewT> getMaterialization();
  
    /** A function to adapt a primitive view type to a desired view type. */
    public abstract ViewT apply(PrimitiveViewT primitiveViewT);
  
    /** Return the {@link TypeDescriptor} describing the output of this fn. */
    public abstract TypeDescriptor<ViewT> getTypeDescriptor();
  }
  