src/com/android/deskclock/VerticalViewPager.java

/*
 * Copyright (C) 2014 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.deskclock;

import android.content.Context;
import androidx.viewpager.widget.ViewPager;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class VerticalViewPager extends ViewPager {

    public VerticalViewPager(Context context) {
        this(context, null);
    }

    public VerticalViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    /**
     * @return {@code false} since a vertical view pager can never be scrolled horizontally
     */
    @Override
    public boolean canScrollHorizontally(int direction) {
        return false;
    }

    /**
     * @return {@code true} iff a normal view pager would support horizontal scrolling at this time
     */
    @Override
    public boolean canScrollVertically(int direction) {
        return super.canScrollHorizontally(direction);
    }

    private void init() {
        // Make page transit vertical
        setPageTransformer(true, new VerticalPageTransformer());
        // Get rid of the overscroll drawing that happens on the left and right (the ripple)
        setOverScrollMode(View.OVER_SCROLL_NEVER);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        final boolean toIntercept = super.onInterceptTouchEvent(flipXY(ev));
        // Return MotionEvent to normal
        flipXY(ev);
        return toIntercept;
    }

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        final boolean toHandle = super.onTouchEvent(flipXY(ev));
        // Return MotionEvent to normal
        flipXY(ev);
        return toHandle;
    }

    private MotionEvent flipXY(MotionEvent ev) {
        final float width = getWidth();
        final float height = getHeight();

        final float x = (ev.getY() / height) * width;
        final float y = (ev.getX() / width) * height;

        ev.setLocation(x, y);

        return ev;
    }

    private static final class VerticalPageTransformer implements ViewPager.PageTransformer {
        @Override
        public void transformPage(View view, float position) {
            final int pageWidth = view.getWidth();
            final int pageHeight = view.getHeight();
            if (position < -1) {
                // This page is way off-screen to the left.
                view.setAlpha(0);
            } else if (position <= 1) {
                view.setAlpha(1);
                // Counteract the default slide transition
                view.setTranslationX(pageWidth * -position);
                // set Y position to swipe in from top
                float yPosition = position * pageHeight;
                view.setTranslationY(yPosition);
            } else {
                // This page is way off-screen to the right.
                view.setAlpha(0);
            }
        }
    }
}