AbstractDataSource.java

/*
 * Copyright (c) 2015-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 */

package com.facebook.datasource;

import javax.annotation.Nullable;
import javax.annotation.concurrent.GuardedBy;

import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.Executor;

import android.util.Pair;

import com.facebook.common.internal.Preconditions;

/**
 * An abstract implementation of {@link DataSource} interface.
 *
 * <p> It is highly recommended that other data sources extend this class as it takes care of the
 * state, as well as of notifying listeners when the state changes.
 *
 * <p> Subclasses should override {@link #closeResult(T result)} if results need clean up
 *
 * @param <T>
 */
public abstract class AbstractDataSource<T> implements DataSource<T> {
  /**
   * Describes state of data source
   */
  private enum DataSourceStatus {
    // data source has not finished yet
    IN_PROGRESS,

    // data source has finished with success
    SUCCESS,

    // data source has finished with failure
    FAILURE,
  }

  @GuardedBy("this")
  private DataSourceStatus mDataSourceStatus;
  @GuardedBy("this")
  private boolean mIsClosed;
  @GuardedBy("this")
  private @Nullable T mResult = null;
  @GuardedBy("this")
  private Throwable mFailureThrowable = null;
  private final ConcurrentLinkedQueue<Pair<DataSubscriber<T>, Executor>> mSubscribers;

  protected AbstractDataSource() {
    mIsClosed = false;
    mDataSourceStatus = DataSourceStatus.IN_PROGRESS;
    mSubscribers = new ConcurrentLinkedQueue<Pair<DataSubscriber<T>, Executor>>();
  }

  @Override
  public synchronized boolean isClosed() {
    return mIsClosed;
  }

  @Override
  public synchronized boolean isFinished() {
    return mDataSourceStatus != DataSourceStatus.IN_PROGRESS;
  }

  @Override
  public synchronized boolean hasResult() {
    return mResult != null;
  }

  @Override
  @Nullable
  public synchronized T getResult() {
    return mResult;
  }

  @Override
  public synchronized boolean hasFailed() {
    return mDataSourceStatus == DataSourceStatus.FAILURE;
  }

  @Override
  @Nullable
  public synchronized Throwable getFailureCause() {
    return mFailureThrowable;
  }

  @Override
  public boolean close() {
    T resultToClose;
    synchronized (this) {
      if (mIsClosed) {
        return false;
      }
      mIsClosed = true;
      resultToClose = mResult;
      mResult = null;
    }
    if (resultToClose != null) {
      closeResult(resultToClose);
    }
    if (!isFinished()) {
      notifyDataSubscribers();
    }
    synchronized (this) {
      mSubscribers.clear();
    }
    return true;
  }

  /**
   * Subclasses should override this method to close the result that is not needed anymore.
   *
   * <p> This method is called in two cases:
   * 1. to clear the result when data source gets closed
   * 2. to clear the previous result when a new result is set
   */
  protected void closeResult(@Nullable T result) {
    // default implementation does nothing
  }

  @Override
  public void subscribe(final DataSubscriber<T> dataSubscriber, final Executor executor) {
    Preconditions.checkNotNull(dataSubscriber);
    Preconditions.checkNotNull(executor);
    boolean shouldNotify;

    synchronized(this) {
      if (mIsClosed) {
        return;
      }

      if (mDataSourceStatus == DataSourceStatus.IN_PROGRESS) {
        mSubscribers.add(Pair.create(dataSubscriber, executor));
      }

      shouldNotify = hasResult() || isFinished() || wasCancelled();
    }

    if (shouldNotify) {
      notifyDataSubscriber(dataSubscriber, executor, hasFailed(), wasCancelled());
    }
  }

  private void notifyDataSubscribers() {
    final boolean isFailure = hasFailed();
    final boolean isCancellation = wasCancelled();
    for (Pair<DataSubscriber<T>, Executor> pair : mSubscribers) {
      notifyDataSubscriber(pair.first, pair.second, isFailure, isCancellation);
    }
  }

  private void notifyDataSubscriber(
      final DataSubscriber<T> dataSubscriber,
      final Executor executor,
      final boolean isFailure,
      final boolean isCancellation) {
    executor.execute(
        new Runnable() {
          @Override
          public void run() {
            if (isFailure) {
              dataSubscriber.onFailure(AbstractDataSource.this);
            } else if (isCancellation) {
              dataSubscriber.onCancellation(AbstractDataSource.this);
            } else {
              dataSubscriber.onNewResult(AbstractDataSource.this);
            }
          }
        });
  }

  private synchronized boolean wasCancelled() {
    return isClosed() && !isFinished();
  }

  /**
   * Subclasses should invoke this method to set the result to {@code value}.
   *
   * <p> This method will return {@code true} if the value was successfully set, or
   * {@code false} if the data source has already been set, failed or closed.
   *
   * <p> If the value was successfully set and {@code isLast} is {@code true}, state of the
   * data source will be set to {@link AbstractDataSource.DataSourceStatus#SUCCESS}.
   *
   * <p> {@link #closeResult} will be called for the previous result if the new value was
   * successfully set, OR for the new result otherwise.
   *
   * <p> This will also notify the subscribers if the value was successfully set.
   *
   * <p> Do NOT call this method from a synchronized block as it invokes external code of the
   * subscribers.
   *
   * @param value the value that was the result of the task.
   * @param isLast whether or not the value is last.
   * @return true if the value was successfully set.
   */
  protected boolean setResult(@Nullable T value, boolean isLast) {
    boolean result = setResultInternal(value, isLast);
    if (result) {
      notifyDataSubscribers();
    }
    return result;
  }

  /**
   * Subclasses should invoke this method to set the failure.

   *
   * <p> This method will return {@code true} if the failure was successfully set, or
   * {@code false} if the data source has already been set, failed or closed.
   *
   * <p> If the failure was successfully set, state of the data source will be set to
   * {@link AbstractDataSource.DataSourceStatus#FAILURE}.
   *
   * <p> This will also notify the subscribers if the failure was successfully set.
   *
   * <p> Do NOT call this method from a synchronized block as it invokes external code of the
   * subscribers.
   *
   * @param throwable the failure cause to be set.
   * @return true if the failure was successfully set.
   */
  protected boolean setFailure(Throwable throwable) {
    boolean result = setFailureInternal(throwable);
    if (result) {
      notifyDataSubscribers();
    }
    return result;
  }

  private boolean setResultInternal(@Nullable T value, boolean isLast) {
    T resultToClose = null;
    try {
      synchronized (this) {
        if (mIsClosed || mDataSourceStatus != DataSourceStatus.IN_PROGRESS) {
          resultToClose = value;
          return false;
        } else {
          if (isLast) {
            mDataSourceStatus = DataSourceStatus.SUCCESS;
          }
          if (mResult != value) {
            resultToClose = mResult;
            mResult = value;
          }
          return true;
        }
      }
    } finally {
      if (resultToClose != null) {
        closeResult(resultToClose);
      }
    }
  }

  private synchronized boolean setFailureInternal(Throwable throwable) {
    if (mIsClosed || mDataSourceStatus != DataSourceStatus.IN_PROGRESS) {
      return false;
    } else {
      mDataSourceStatus = DataSourceStatus.FAILURE;
      mFailureThrowable = throwable;
      return true;
    }
  }
}