content: /*
   * Copyright (C) 2017 The Android Open Source Project
   *
   * 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.android.internal.widget;
  
  import android.annotation.Nullable;
  import android.content.Context;
  import android.graphics.PointF;
  import android.util.DisplayMetrics;
  import android.util.Log;
  import android.view.View;
  import android.view.animation.DecelerateInterpolator;
  import android.view.animation.LinearInterpolator;
  
  /**
   * {@link RecyclerView.SmoothScroller} implementation which uses a {@link LinearInterpolator} until
   * the target position becomes a child of the RecyclerView and then uses a
   * {@link DecelerateInterpolator} to slowly approach to target position.
   * <p>
   * If the {@link RecyclerView.LayoutManager} you are using does not implement the
   * {@link RecyclerView.SmoothScroller.ScrollVectorProvider} interface, then you must override the
   * {@link #computeScrollVectorForPosition(int)} method. All the LayoutManagers bundled with
   * the support library implement this interface.
   */
  public class LinearSmoothScroller extends RecyclerView.SmoothScroller {
  
      private static final String TAG = "LinearSmoothScroller";
  
      private static final boolean DEBUG = false;
  
      private static final float MILLISECONDS_PER_INCH = 25f;
  
      private static final int TARGET_SEEK_SCROLL_DISTANCE_PX = 10000;
  
      /**
       * Align child view's left or top with parent view's left or top
       *
       * @see #calculateDtToFit(int, int, int, int, int)
       * @see #calculateDxToMakeVisible(android.view.View, int)
       * @see #calculateDyToMakeVisible(android.view.View, int)
       */
      public static final int SNAP_TO_START = -1;
  
      /**
       * Align child view's right or bottom with parent view's right or bottom
       *
       * @see #calculateDtToFit(int, int, int, int, int)
       * @see #calculateDxToMakeVisible(android.view.View, int)
       * @see #calculateDyToMakeVisible(android.view.View, int)
       */
      public static final int SNAP_TO_END = 1;
  
      /**
       * <p>Decides if the child should be snapped from start or end, depending on where it
       * currently is in relation to its parent.</p>
       * <p>For instance, if the view is virtually on the left of RecyclerView, using
       * {@code SNAP_TO_ANY} is the same as using {@code SNAP_TO_START}</p>
       *
       * @see #calculateDtToFit(int, int, int, int, int)
       * @see #calculateDxToMakeVisible(android.view.View, int)
       * @see #calculateDyToMakeVisible(android.view.View, int)
       */
      public static final int SNAP_TO_ANY = 0;
  
      // Trigger a scroll to a further distance than TARGET_SEEK_SCROLL_DISTANCE_PX so that if target
      // view is not laid out until interim target position is reached, we can detect the case before
      // scrolling slows down and reschedule another interim target scroll
      private static final float TARGET_SEEK_EXTRA_SCROLL_RATIO = 1.2f;
  
      protected final LinearInterpolator mLinearInterpolator = new LinearInterpolator();
  
      protected final DecelerateInterpolator mDecelerateInterpolator = new DecelerateInterpolator();
  
      protected PointF mTargetVector;
  
      private final float MILLISECONDS_PER_PX;
  
      // Temporary variables to keep track of the interim scroll target. These values do not
      // point to a real item position, rather point to an estimated location pixels.
      protected int mInterimTargetDx = 0, mInterimTargetDy = 0;
  
      public LinearSmoothScroller(Context context) {
          MILLISECONDS_PER_PX = calculateSpeedPerPixel(context.getResources().getDisplayMetrics());
      }
  
      /**
       * {@inheritDoc}
       */
      @Override
      protected void onStart() {
  
      }
  
      /**
       * {@inheritDoc}
       */
      @Override
      protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
          final int dx = calculateDxToMakeVisible(targetView, getHorizontalSnapPreference());
          final int dy = calculateDyToMakeVisible(targetView, getVerticalSnapPreference());
          final int distance = (int) Math.sqrt(dx * dx + dy * dy);
          final int time = calculateTimeForDeceleration(distance);
          if (time > 0) {
              action.update(-dx, -dy, time, mDecelerateInterpolator);
          }
      }
  
      /**
       * {@inheritDoc}
       */
      @Override
      protected void onSeekTargetStep(int dx, int dy, RecyclerView.State state, Action action) {
          if (getChildCount() == 0) {
              stop();
              return;
          }
          //noinspection PointlessBooleanExpression
          if (DEBUG && mTargetVector != null
                  && ((mTargetVector.x * dx < 0 || mTargetVector.y * dy < 0))) {
              throw new IllegalStateException("Scroll happened in the opposite direction"
                      + " of the target. Some calculations are wrong");
          }
          mInterimTargetDx = clampApplyScroll(mInterimTargetDx, dx);
          mInterimTargetDy = clampApplyScroll(mInterimTargetDy, dy);
  
          if (mInterimTargetDx == 0 && mInterimTargetDy == 0) {
              updateActionForInterimTarget(action);
          } // everything is valid, keep going
  
      }
  
      /**
       * {@inheritDoc}
       */
      @Override
      protected void onStop() {
          mInterimTargetDx = mInterimTargetDy = 0;
          mTargetVector = null;
      }
  
      /**
       * Calculates the scroll speed.
       *
       * @param displayMetrics DisplayMetrics to be used for real dimension calculations
       * @return The time (in ms) it should take for each pixel. For instance, if returned value is
       * 2 ms, it means scrolling 1000 pixels with LinearInterpolation should take 2 seconds.
       */
      protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
          return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
      }
  
      /**
       * <p>Calculates the time for deceleration so that transition from LinearInterpolator to
       * DecelerateInterpolator looks smooth.</p>
       *
       * @param dx Distance to scroll
       * @return Time for DecelerateInterpolator to smoothly traverse the distance when transitioning
       * from LinearInterpolation
       */
      protected int calculateTimeForDeceleration(int dx) {
          // we want to cover same area with the linear interpolator for the first 10% of the
          // interpolation. After that, deceleration will take control.
          // area under curve (1-(1-x)^2) can be calculated as (1 - x/3) * x * x
          // which gives 0.100028 when x = .3356
          // this is why we divide linear scrolling time with .3356
          return  (int) Math.ceil(calculateTimeForScrolling(dx) / .3356);
      }
  
      /**
       * Calculates the time it should take to scroll the given distance (in pixels)
       *
       * @param dx Distance in pixels that we want to scroll
       * @return Time in milliseconds
       * @see #calculateSpeedPerPixel(android.util.DisplayMetrics)
       */
      protected int calculateTimeForScrolling(int dx) {
          // In a case where dx is very small, rounding may return 0 although dx > 0.
          // To avoid that issue, ceil the result so that if dx > 0, we'll always return positive
          // time.
          return (int) Math.ceil(Math.abs(dx) * MILLISECONDS_PER_PX);
      }
  
      /**
       * When scrolling towards a child view, this method defines whether we should align the left
       * or the right edge of the child with the parent RecyclerView.
       *
       * @return SNAP_TO_START, SNAP_TO_END or SNAP_TO_ANY; depending on the current target vector
       * @see #SNAP_TO_START
       * @see #SNAP_TO_END
       * @see #SNAP_TO_ANY
       */
      protected int getHorizontalSnapPreference() {
          return mTargetVector == null || mTargetVector.x == 0 ? SNAP_TO_ANY :
                  mTargetVector.x > 0 ? SNAP_TO_END : SNAP_TO_START;
      }
  
      /**
       * When scrolling towards a child view, this method defines whether we should align the top
       * or the bottom edge of the child with the parent RecyclerView.
       *
       * @return SNAP_TO_START, SNAP_TO_END or SNAP_TO_ANY; depending on the current target vector
       * @see #SNAP_TO_START
       * @see #SNAP_TO_END
       * @see #SNAP_TO_ANY
       */
      protected int getVerticalSnapPreference() {
          return mTargetVector == null || mTargetVector.y == 0 ? SNAP_TO_ANY :
                  mTargetVector.y > 0 ? SNAP_TO_END : SNAP_TO_START;
      }
  
      /**
       * When the target scroll position is not a child of the RecyclerView, this method calculates
       * a direction vector towards that child and triggers a smooth scroll.
       *
       * @see #computeScrollVectorForPosition(int)
       */
      protected void updateActionForInterimTarget(Action action) {
          // find an interim target position
          PointF scrollVector = computeScrollVectorForPosition(getTargetPosition());
          if (scrollVector == null || (scrollVector.x == 0 && scrollVector.y == 0)) {
              final int target = getTargetPosition();
              action.jumpTo(target);
              stop();
              return;
          }
          normalize(scrollVector);
          mTargetVector = scrollVector;
  
          mInterimTargetDx = (int) (TARGET_SEEK_SCROLL_DISTANCE_PX * scrollVector.x);
          mInterimTargetDy = (int) (TARGET_SEEK_SCROLL_DISTANCE_PX * scrollVector.y);
          final int time = calculateTimeForScrolling(TARGET_SEEK_SCROLL_DISTANCE_PX);
          // To avoid UI hiccups, trigger a smooth scroll to a distance little further than the
          // interim target. Since we track the distance travelled in onSeekTargetStep callback, it
          // won't actually scroll more than what we need.
          action.update((int) (mInterimTargetDx * TARGET_SEEK_EXTRA_SCROLL_RATIO),
                  (int) (mInterimTargetDy * TARGET_SEEK_EXTRA_SCROLL_RATIO),
                  (int) (time * TARGET_SEEK_EXTRA_SCROLL_RATIO), mLinearInterpolator);
      }
  
      private int clampApplyScroll(int tmpDt, int dt) {
          final int before = tmpDt;
          tmpDt -= dt;
          if (before * tmpDt <= 0) { // changed sign, reached 0 or was 0, reset
              return 0;
          }
          return tmpDt;
      }
  
      /**
       * Helper method for {@link #calculateDxToMakeVisible(android.view.View, int)} and
       * {@link #calculateDyToMakeVisible(android.view.View, int)}
       */
      public int calculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd, int
              snapPreference) {
          switch (snapPreference) {
              case SNAP_TO_START:
                  return boxStart - viewStart;
              case SNAP_TO_END:
                  return boxEnd - viewEnd;
              case SNAP_TO_ANY:
                  final int dtStart = boxStart - viewStart;
                  if (dtStart > 0) {
                      return dtStart;
                  }
                  final int dtEnd = boxEnd - viewEnd;
                  if (dtEnd < 0) {
                      return dtEnd;
                  }
                  break;
              default:
                  throw new IllegalArgumentException("snap preference should be one of the"
                          + " constants defined in SmoothScroller, starting with SNAP_");
          }
          return 0;
      }
  
      /**
       * Calculates the vertical scroll amount necessary to make the given view fully visible
       * inside the RecyclerView.
       *
       * @param view           The view which we want to make fully visible
       * @param snapPreference The edge which the view should snap to when entering the visible
       *                       area. One of {@link #SNAP_TO_START}, {@link #SNAP_TO_END} or
       *                       {@link #SNAP_TO_ANY}.
       * @return The vertical scroll amount necessary to make the view visible with the given
       * snap preference.
       */
      public int calculateDyToMakeVisible(View view, int snapPreference) {
          final RecyclerView.LayoutManager layoutManager = getLayoutManager();
          if (layoutManager == null || !layoutManager.canScrollVertically()) {
              return 0;
          }
          final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)
                  view.getLayoutParams();
          final int top = layoutManager.getDecoratedTop(view) - params.topMargin;
          final int bottom = layoutManager.getDecoratedBottom(view) + params.bottomMargin;
          final int start = layoutManager.getPaddingTop();
          final int end = layoutManager.getHeight() - layoutManager.getPaddingBottom();
          return calculateDtToFit(top, bottom, start, end, snapPreference);
      }
  
      /**
       * Calculates the horizontal scroll amount necessary to make the given view fully visible
       * inside the RecyclerView.
       *
       * @param view           The view which we want to make fully visible
       * @param snapPreference The edge which the view should snap to when entering the visible
       *                       area. One of {@link #SNAP_TO_START}, {@link #SNAP_TO_END} or
       *                       {@link #SNAP_TO_END}
       * @return The vertical scroll amount necessary to make the view visible with the given
       * snap preference.
       */
      public int calculateDxToMakeVisible(View view, int snapPreference) {
          final RecyclerView.LayoutManager layoutManager = getLayoutManager();
          if (layoutManager == null || !layoutManager.canScrollHorizontally()) {
              return 0;
          }
          final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)
                  view.getLayoutParams();
          final int left = layoutManager.getDecoratedLeft(view) - params.leftMargin;
          final int right = layoutManager.getDecoratedRight(view) + params.rightMargin;
          final int start = layoutManager.getPaddingLeft();
          final int end = layoutManager.getWidth() - layoutManager.getPaddingRight();
          return calculateDtToFit(left, right, start, end, snapPreference);
      }
  
      /**
       * Compute the scroll vector for a given target position.
       * <p>
       * This method can return null if the layout manager cannot calculate a scroll vector
       * for the given position (e.g. it has no current scroll position).
       *
       * @param targetPosition the position to which the scroller is scrolling
       *
       * @return the scroll vector for a given target position
       */
      @Nullable
      public PointF computeScrollVectorForPosition(int targetPosition) {
          RecyclerView.LayoutManager layoutManager = getLayoutManager();
          if (layoutManager instanceof ScrollVectorProvider) {
              return ((ScrollVectorProvider) layoutManager)
                      .computeScrollVectorForPosition(targetPosition);
          }
          Log.w(TAG, "You should override computeScrollVectorForPosition when the LayoutManager"
                  + " does not implement " + ScrollVectorProvider.class.getCanonicalName());
          return null;
      }
  }
  