google/protobuf/SingleFieldBuilder.java

// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
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package com.google.protobuf;

/**
 * {@code SingleFieldBuilder} implements a structure that a protocol
 * message uses to hold a single field of another protocol message. It supports
 * the classical use case of setting an immutable {@link Message} as the value
 * of the field and is highly optimized around this.
 * <br>
 * It also supports the additional use case of setting a {@link Message.Builder}
 * as the field and deferring conversion of that {@code Builder}
 * to an immutable {@code Message}. In this way, it's possible to maintain
 * a tree of {@code Builder}'s that acts as a fully read/write data
 * structure.
 * <br>
 * Logically, one can think of a tree of builders as converting the entire tree
 * to messages when build is called on the root or when any method is called
 * that desires a Message instead of a Builder. In terms of the implementation,
 * the {@code SingleFieldBuilder} and {@code RepeatedFieldBuilder}
 * classes cache messages that were created so that messages only need to be
 * created when some change occurred in its builder or a builder for one of its
 * descendants.
 *
 * @param  the type of message for the field
 * @param  the type of builder for the field
 * @param  the common interface for the message and the builder
 *
 * @author jonp@google.com (Jon Perlow)
 */
public class SingleFieldBuilder
    <MType extends GeneratedMessage,
     BType extends GeneratedMessage.Builder,
     IType extends MessageOrBuilder>
    implements GeneratedMessage.BuilderParent {

  // Parent to send changes to.
  private GeneratedMessage.BuilderParent parent;

  // Invariant: one of builder or message fields must be non-null.

  // If set, this is the case where we are backed by a builder. In this case,
  // message field represents a cached message for the builder (or null if
  // there is no cached message).
  private BType builder;

  // If builder is non-null, this represents a cached message from the builder.
  // If builder is null, this is the authoritative message for the field.
  private MType message;

  // Indicates that we've built a message and so we are now obligated
  // to dispatch dirty invalidations. See GeneratedMessage.BuilderListener.
  private boolean isClean;

  public SingleFieldBuilder(
      MType message,
      GeneratedMessage.BuilderParent parent,
      boolean isClean) {
    if (message == null) {
      throw new NullPointerException();
    }
    this.message = message;
    this.parent = parent;
    this.isClean = isClean;
  }

  public void dispose() {
    // Null out parent so we stop sending it invalidations.
    parent = null;
  }

  /**
   * Get the message for the field. If the message is currently stored
   * as a {@code Builder}, it is converted to a {@code Message} by
   * calling {@link Message.Builder#buildPartial} on it. If no message has
   * been set, returns the default instance of the message.
   *
   * @return the message for the field
   */
  @SuppressWarnings("unchecked")
  public MType getMessage() {
    if (message == null) {
      // If message is null, the invariant is that we must be have a builder.
      message = (MType) builder.buildPartial();
    }
    return message;
  }

  /**
   * Builds the message and returns it.
   *
   * @return the message
   */
  public MType build() {
    // Now that build has been called, we are required to dispatch
    // invalidations.
    isClean = true;
    return getMessage();
  }

  /**
   * Gets a builder for the field. If no builder has been created yet, a
   * builder is created on demand by calling {@link Message#toBuilder}.
   *
   * @return The builder for the field
   */
  @SuppressWarnings("unchecked")
  public BType getBuilder() {
    if (builder == null) {
      // builder.mergeFrom() on a fresh builder
      // does not create any sub-objects with independent clean/dirty states,
      // therefore setting the builder itself to clean without actually calling
      // build() cannot break any invariants.
      builder = (BType) message.newBuilderForType(this);
      builder.mergeFrom(message); // no-op if message is the default message
      builder.markClean();
    }
    return builder;
  }

  /**
   * Gets the base class interface for the field. This may either be a builder
   * or a message. It will return whatever is more efficient.
   *
   * @return the message or builder for the field as the base class interface
   */
  @SuppressWarnings("unchecked")
  public IType getMessageOrBuilder() {
    if (builder != null) {
      return  (IType) builder;
    } else {
      return (IType) message;
    }
  }

  /**
   * Sets a  message for the field replacing any existing value.
   *
   * @param message the message to set
   * @return the builder
   */
  public SingleFieldBuilder<MType, BType, IType> setMessage(
      MType message) {
    if (message == null) {
      throw new NullPointerException();
    }
    this.message = message;
    if (builder != null) {
      builder.dispose();
      builder = null;
    }
    onChanged();
    return this;
  }

  /**
   * Merges the field from another field.
   *
   * @param value the value to merge from
   * @return the builder
   */
  public SingleFieldBuilder<MType, BType, IType> mergeFrom(
      MType value) {
    if (builder == null && message == message.getDefaultInstanceForType()) {
      message = value;
    } else {
      getBuilder().mergeFrom(value);
    }
    onChanged();
    return this;
  }

  /**
   * Clears the value of the field.
   *
   * @return the builder
   */
  @SuppressWarnings("unchecked")
  public SingleFieldBuilder<MType, BType, IType> clear() {
    message = (MType) (message != null ?
        message.getDefaultInstanceForType() :
        builder.getDefaultInstanceForType());
    if (builder != null) {
      builder.dispose();
      builder = null;
    }
    onChanged();
    return this;
  }

  /**
   * Called when a the builder or one of its nested children has changed
   * and any parent should be notified of its invalidation.
   */
  private void onChanged() {
    // If builder is null, this is the case where onChanged is being called
    // from setMessage or clear.
    if (builder != null) {
      message = null;
    }
    if (isClean && parent != null) {
      parent.markDirty();

      // Don't keep dispatching invalidations until build is called again.
      isClean = false;
    }
  }

  @Override
  public void markDirty() {
    onChanged();
  }
}