src/main/java/com/google/android/libraries/feed/basicstream/internal/scroll/ScrollRestorer.java

// Copyright 2018 The Feed Authors.
//
// Licensed 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 com.google.android.libraries.feed.basicstream.internal.scroll;

import static com.google.android.libraries.feed.common.Validators.checkNotNull;
import static com.google.android.libraries.feed.common.Validators.checkState;

import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import com.google.android.libraries.feed.api.host.config.Configuration;
import com.google.android.libraries.feed.common.logging.Logger;
import com.google.android.libraries.feed.sharedstream.proto.ScrollStateProto.ScrollState;
import com.google.android.libraries.feed.sharedstream.scroll.ScrollListenerNotifier;
import com.google.android.libraries.feed.sharedstream.scroll.ScrollRestoreHelper;

/**
 * Class which is able to save a scroll position for a RecyclerView and restore to that scroll
 * position at a later time.
 */
public class ScrollRestorer {

  private static final String TAG = "ScrollRestorer";

  private final Configuration configuration;
  private final RecyclerView recyclerView;
  private final ScrollListenerNotifier scrollListenerNotifier;
  private final int scrollPosition;
  private final int scrollOffset;

  private boolean canRestore;

  public ScrollRestorer(
      Configuration configuration,
      RecyclerView recyclerView,
      ScrollListenerNotifier scrollListenerNotifier,
      /*@Nullable*/ ScrollState scrollState) {
    this.configuration = configuration;
    this.recyclerView = recyclerView;
    this.scrollListenerNotifier = scrollListenerNotifier;

    if (scrollState != null) {
      canRestore = true;
      scrollPosition = scrollState.getPosition();
      scrollOffset = scrollState.getOffset();
    } else {
      this.scrollPosition = 0;
      this.scrollOffset = 0;
    }
  }

  private ScrollRestorer(
      Configuration configuration,
      RecyclerView recyclerView,
      ScrollListenerNotifier scrollListenerNotifier) {
    canRestore = false;
    this.configuration = configuration;
    this.recyclerView = recyclerView;
    this.scrollListenerNotifier = scrollListenerNotifier;
    scrollPosition = 0;
    scrollOffset = 0;
  }

  /**
   * Creates a {@code StreamRestorer} which will never restore state. This can be used to fulfill
   * {@code StreamRestorer} requests when no restore state is present.
   */
  public static ScrollRestorer nonRestoringRestorer(
      Configuration configuration,
      RecyclerView recyclerView,
      ScrollListenerNotifier scrollListenerNotifier) {
    return new ScrollRestorer(configuration, recyclerView, scrollListenerNotifier);
  }

  /**
   * Disables the ability of the {@code ScrollRestorer} from restoring the scroll. This should be
   * call if the previous scroll position is no longer valid. An example use case of this would be
   * if the restoring session is no longer valid.
   */
  public void abandonRestoringScroll() {
    canRestore = false;
  }

  /**
   * Attempts to restore scroll position if possible. If the scroll position has already been
   * restored, then this method will no-op.
   */
  public void maybeRestoreScroll() {
    if (!canRestore) {
      return;
    }
    Logger.d(TAG, "Restoring scroll");
    getLayoutManager().scrollToPositionWithOffset(scrollPosition, scrollOffset);
    scrollListenerNotifier.onProgrammaticScroll(recyclerView);
    canRestore = false;
  }

  /**
   * Returns a bundle which can be used for restoring scroll state on an activity restart.
   *
   * @param currentHeaderCount The amount of headers which appear before Stream content.
   */
  /*@Nullable*/
  public ScrollState getScrollStateForScrollRestore(int currentHeaderCount) {
    return ScrollRestoreHelper.getScrollStateForScrollRestore(
        getLayoutManager(), configuration, currentHeaderCount);
  }

  private LinearLayoutManager getLayoutManager() {
    checkState(
        recyclerView.getLayoutManager() instanceof LinearLayoutManager,
        "Scroll state can only be restored when using a LinearLayoutManager.");
    return checkNotNull((LinearLayoutManager) recyclerView.getLayoutManager());
  }
}