FirstAvailableDataSourceSupplier.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.ThreadSafe;

import java.util.List;

import com.facebook.common.executors.CallerThreadExecutor;
import com.facebook.common.internal.Objects;
import com.facebook.common.internal.Preconditions;
import com.facebook.common.internal.Supplier;

/**
 * {@link DataSource} supplier that provides a data source which will forward results of the first
 * available data source.
 *
 * <p>Data sources are obtained in order. Only if the current data source fails, or if it finishes
 * without result, the next one will be tried.
 */
@ThreadSafe
public class FirstAvailableDataSourceSupplier<T> implements Supplier<DataSource<T>> {

  private final List<Supplier<DataSource<T>>> mDataSourceSuppliers;

  private FirstAvailableDataSourceSupplier(List<Supplier<DataSource<T>>> dataSourceSuppliers) {
    Preconditions.checkArgument(!dataSourceSuppliers.isEmpty(), "List of suppliers is empty!");
    mDataSourceSuppliers = dataSourceSuppliers;
  }

  public static <T> FirstAvailableDataSourceSupplier<T> create(
      List<Supplier<DataSource<T>>> dataSourceSuppliers) {
    return new FirstAvailableDataSourceSupplier<T>(dataSourceSuppliers);
  }

  @Override
  public DataSource<T> get() {
    return new FirstAvailableDataSource();
  }

  @Override
  public int hashCode() {
    return mDataSourceSuppliers.hashCode();
  }

  @Override
  public boolean equals(Object other) {
    if (other == this) {
      return true;
    }
    if (!(other instanceof FirstAvailableDataSourceSupplier)) {
      return false;
    }
    FirstAvailableDataSourceSupplier that = (FirstAvailableDataSourceSupplier) other;
    return Objects.equal(this.mDataSourceSuppliers, that.mDataSourceSuppliers);
  }

  @Override
  public String toString() {
    return Objects.toStringHelper(this)
        .add("list", mDataSourceSuppliers)
        .toString();
  }

  @ThreadSafe
  private class FirstAvailableDataSource extends AbstractDataSource<T> {

    private int mIndex = 0;
    private DataSource<T> mCurrentDataSource = null;
    private DataSource<T> mDataSourceWithResult = null;

    public FirstAvailableDataSource() {
      if (!startNextDataSource()) {
        setFailure(new RuntimeException("No data source supplier or supplier returned null."));
      }
    }

    @Override
    @Nullable
    public synchronized T getResult() {
      DataSource<T> dataSourceWithResult = getDataSourceWithResult();
      return (dataSourceWithResult != null) ? dataSourceWithResult.getResult() : null;
    }

    @Override
    public synchronized boolean hasResult() {
      DataSource<T> dataSourceWithResult = getDataSourceWithResult();
      return (dataSourceWithResult != null) && dataSourceWithResult.hasResult();
    }

    @Override
    public boolean close() {
      DataSource<T> currentDataSource;
      DataSource<T> dataSourceWithResult;
      synchronized (FirstAvailableDataSource.this) {
        // it's fine to call {@code super.close()} within a synchronized block because we don't
        // implement {@link #closeResult()}, but perform result closing ourselves.
        if (!super.close()) {
          return false;
        }
        currentDataSource = mCurrentDataSource;
        mCurrentDataSource = null;
        dataSourceWithResult = mDataSourceWithResult;
        mDataSourceWithResult = null;
      }
      closeSafely(dataSourceWithResult);
      closeSafely(currentDataSource);
      return true;
    }

    private boolean startNextDataSource() {
      Supplier<DataSource<T>> dataSourceSupplier = getNextSupplier();
      DataSource<T> dataSource = (dataSourceSupplier != null) ? dataSourceSupplier.get() : null;
      if (setCurrentDataSource(dataSource) && dataSource != null) {
        dataSource.subscribe(new InternalDataSubscriber(), CallerThreadExecutor.getInstance());
        return true;
      } else {
        closeSafely(dataSource);
        return false;
      }
    }

    @Nullable
    private synchronized Supplier<DataSource<T>> getNextSupplier() {
      if (!isClosed() && mIndex < mDataSourceSuppliers.size()) {
        return mDataSourceSuppliers.get(mIndex++);
      }
      return null;
    }

    private synchronized boolean setCurrentDataSource(DataSource<T> dataSource) {
      if (isClosed()) {
        return false;
      }
      mCurrentDataSource = dataSource;
      return true;
    }

    private synchronized boolean clearCurrentDataSource(DataSource<T> dataSource) {
      if (isClosed() || dataSource != mCurrentDataSource) {
        return false;
      }
      mCurrentDataSource = null;
      return true;
    }

    @Nullable
    private synchronized DataSource<T> getDataSourceWithResult() {
      return mDataSourceWithResult;
    }

    private void maybeSetDataSourceWithResult(
        DataSource<T> dataSource,
        boolean isFinished) {
      DataSource<T> oldDataSource = null;
      synchronized (FirstAvailableDataSource.this) {
        if (dataSource != mCurrentDataSource || dataSource == mDataSourceWithResult) {
          return;
        }
        // If we didn't have any result so far, we got one now, so we'll set
        // {@code mDataSourceWithResult} to point to the current data source.
        // If we did have a result which came from another data source,
        // we'll only set {@code mDataSourceWithResult} to point to the current
        // data source if it has finished (i.e. the new result is final).
        if (mDataSourceWithResult == null || isFinished) {
          oldDataSource = mDataSourceWithResult;
          mDataSourceWithResult = dataSource;
        }
      }
      closeSafely(oldDataSource);
    }

    private void onDataSourceFailed(DataSource<T> dataSource) {
      if (!clearCurrentDataSource(dataSource)) {
        return;
      }
      if (dataSource != getDataSourceWithResult()) {
        closeSafely(dataSource);
      }
      if (!startNextDataSource()) {
        setFailure(dataSource.getFailureCause());
      }
    }

    private void onDataSourceNewResult(DataSource<T> dataSource) {
      maybeSetDataSourceWithResult(dataSource, dataSource.isFinished());
      // If the data source with the new result is our {@code mDataSourceWithResult},
      // we have to notify our subscribers about the new result.
      if (dataSource == getDataSourceWithResult()) {
        setResult(null, dataSource.isFinished());
      }
    }

    private void closeSafely(DataSource<T> dataSource) {
      if (dataSource != null) {
        dataSource.close();
      }
    }

    private class InternalDataSubscriber implements DataSubscriber<T> {

      @Override
      public void onFailure(DataSource<T> dataSource) {
        FirstAvailableDataSource.this.onDataSourceFailed(dataSource);
      }

      @Override
      public void onCancellation(DataSource<T> dataSource) {
      }

      @Override
      public void onNewResult(DataSource<T> dataSource) {
        if (dataSource.hasResult()) {
          FirstAvailableDataSource.this.onDataSourceNewResult(dataSource);
        } else if (dataSource.isFinished()) {
          FirstAvailableDataSource.this.onDataSourceFailed(dataSource);
        }
      }
    }
  }
}