/*
 * 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;

/**
 * <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, provided by the runner
 *        {@code <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 contents);
}