car/src/main/java/androidx/car/widget/PagedSmoothScroller.java

/*
 * 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 androidx.car.widget;

import android.content.Context;
import androidx.recyclerview.widget.LinearSmoothScroller;
import androidx.recyclerview.widget.RecyclerView;
import android.util.DisplayMetrics;
import android.view.View;
import android.view.animation.DecelerateInterpolator;
import android.view.animation.Interpolator;

/**
 * Custom {@link LinearSmoothScroller} that has:
 *
 * <ul>
 * <li>Custom control over the speed of scrolls.
 * <li>Scrolling that snaps to start of a child view.
 * </ul>
 */
public final class PagedSmoothScroller extends LinearSmoothScroller {
    private static final float MILLISECONDS_PER_INCH = 150f;
    private static final float DECELERATION_TIME_DIVISOR = 0.45f;

    private final Interpolator mInterpolator = new DecelerateInterpolator(1.8f);

    public PagedSmoothScroller(Context context) {
        super(context);
    }

    @Override
    protected int getVerticalSnapPreference() {
        // This is key for most of the scrolling logic that guarantees that scrolling
        // will settle with a view aligned to the top.
        return SNAP_TO_START;
    }

    @Override
    protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
        int dy = calculateDyToMakeVisible(targetView, SNAP_TO_START);
        if (dy == 0) {
            return;
        }

        final int time = calculateTimeForDeceleration(dy);
        if (time > 0) {
            action.update(0, -dy, time, mInterpolator);
        }
    }

    @Override
    protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
        return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
    }

    @Override
    protected int calculateTimeForDeceleration(int dx) {
        return (int) Math.ceil(calculateTimeForScrolling(dx) / DECELERATION_TIME_DIVISOR);
    }
}