Synchronizing two ViewPagers using OnPageChangeListener

Question

I\'m trying to synchronize two ViewPagers, as apparently have quite a lot of people before me, and I\'ve got as far as this:

private ViewPager m         


        

Answer 1

I had a similar issue, also needed to combine my syncing with animations in PageTransformer.

Originally posted this answer here: https://stackoverflow.com/a/43638796/2867886

But I will paste it here for convenience.

The solution that worked best for me was to pass MotionEvent in OnTouchListener between ViewPager instances. Tried fake dragging but it was always laggy and buggy - I needed a smooth, parallax-like effect.

So, my advice is to implement a View.OnTouchListener. The MotionEvent has to be scaled to compensate for the difference in width.

public class SyncScrollOnTouchListener implements View.OnTouchListener {

private final View syncedView;

public SyncScrollOnTouchListener(@NonNull View syncedView) {
    this.syncedView = syncedView;
}

@Override
public boolean onTouch(View view, MotionEvent motionEvent) {
    MotionEvent syncEvent = MotionEvent.obtain(motionEvent);
    float width1 = view.getWidth();
    float width2 = syncedView.getWidth();

    //sync motion of two view pagers by simulating a touch event
    //offset by its X position, and scaled by width ratio
    syncEvent.setLocation(syncedView.getX() + motionEvent.getX() * width2 / width1,
            motionEvent.getY());
    syncedView.onTouchEvent(syncEvent);
    return false;
}
}

Then set it to your ViewPager

    sourcePager.setOnTouchListener(new SyncScrollOnTouchListener(targetPager));

Note that this solution will only work if both pagers have the same orientation. If you need it to work for different orientations - adjust syncEvent Y coordinate instead of X.

There is one more issue that we need to take into account - minimum fling speed and distance that can cause just one pager to change page.

It can be easily fixed by adding an OnPageChangeListener to our pager

sourcePager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
        @Override
        public void onPageScrolled(int position, float positionOffset,
                                   int positionOffsetPixels) {
            //no-op
        }

        @Override
        public void onPageSelected(int position) {
            targetPager.setCurrentItem(position, true);
        }

        @Override
        public void onPageScrollStateChanged(int state) {
            //no-op
        }
    });