Vertical ViewPager and Android Pie Inconsistent Behavior with Swipe Gesture

Question

My problem is closely related to two other questions that haven\'t been answered yet.

ViewPager not responding to touch in layout area created dynamically in Fragmen

Answer 1

After spending hell amount of time on SO, trying many GitHub libraries and waiting for someone to respond on the bounty, then I came up with below solutions and hope that it helps whoever needs it.

At first, this question got my attention and I think most of the answers over there are helpful, hence I upvoted them. The main answer which helped me are link-1 and link-2.

Even though I have to make a few minor changes to ignore horizontal swipe fling events.

Please do try this code and provide your feedback for any further improvements, thanks in advance. Happy Coding :)

import android.content.Context;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.ViewPager;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;

public class VerticalViewPager extends ViewPager {
    float x = 0;
    float mStartDragX = 0;
    private static final float SWIPE_X_MIN_THRESHOLD = 50; // Decide this magical nuber as per your requirement

    public VerticalViewPager(Context context) {
        super(context);
        init();
    }

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

    private void init() {
        // The majority of the magic happens here
        setPageTransformer(true, new VerticalPageTransformer());
        // The easiest way to get rid of the overscroll drawing that happens on the left and right
        setOverScrollMode(OVER_SCROLL_NEVER);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        if (getAdapter() != null) {
            if (getCurrentItem() >= 0 || getCurrentItem() < getAdapter().getCount()) {
                swapXY(event);
                final int action = event.getAction();
                switch (action & MotionEventCompat.ACTION_MASK) {
                    case MotionEvent.ACTION_MOVE:
                        break;
                    case MotionEvent.ACTION_UP:
                        mStartDragX = event.getX();
                        if (x < mStartDragX
                                && (mStartDragX - x > SWIPE_X_MIN_THRESHOLD)
                                && getCurrentItem() > 0) {
                            Log.i("VerticalViewPager", "down " + x + " : " + mStartDragX + " : " + (mStartDragX - x));
                            setCurrentItem(getCurrentItem() - 1, true);
                            return true;
                        } else if (x > mStartDragX
                                && (x - mStartDragX > SWIPE_X_MIN_THRESHOLD)
                                && getCurrentItem() < getAdapter().getCount() - 1) {
                            Log.i("VerticalViewPager", "up " + x + " : " + mStartDragX + " : " + (x - mStartDragX));
                            mStartDragX = 0;
                            setCurrentItem(getCurrentItem() + 1, true);
                            return true;
                        }
                        break;
                }
            } else {
                mStartDragX = 0;
            }
            swapXY(event);
            return super.onTouchEvent(swapXY(event));
        }
        return false;
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        boolean intercepted = super.onInterceptTouchEvent(swapXY(event));
        switch (event.getAction() & MotionEventCompat.ACTION_MASK) {
            case MotionEvent.ACTION_DOWN:
                x = event.getX();
                break;
        }
        swapXY(event); // return touch coordinates to original reference frame for any child views
        return intercepted;
    }

    /**
     * Swaps the X and Y coordinates of your touch event.
     */
    private MotionEvent swapXY(MotionEvent ev) {
        float width = getWidth();
        float height = getHeight();

        float newX = (ev.getY() / height) * width;
        float newY = (ev.getX() / width) * height;

        ev.setLocation(newX, newY);

        return ev;
    }

    private class VerticalPageTransformer implements PageTransformer {
        @Override
        public void transformPage(View view, float position) {

            if (position < -1) { // [-Infinity,-1)
                // This page is way off-screen to the left.
                view.setAlpha(0);

            } else if (position <= 1) { // [-1,1]
                view.setAlpha(1);

                // Counteract the default slide transition
                view.setTranslationX(view.getWidth() * -position);

                //set Y position to swipe in from top
                float yPosition = position * view.getHeight();
                view.setTranslationY(yPosition);

            } else { // (1,+Infinity]
                // This page is way off-screen to the right.
                view.setAlpha(0);
            }
        }
    }
}