Android viewpager synchronized scrolling

I have two ViewPagers -- Pager1 and Pager2. I added an OnPageChangeListener to Pager1 and in the onPageScrolled callback, I call Pager2.scrollTo(x, y) to move it. Both of the ViewPagers开发者_Python百科 do scroll smoothly and are synchronized, but the problem is that the contents of Pager2 do not change. I checked it with LogCat -- instantiateItem() for Pager2 doesn't get called at all.

As a workaround, I added Pager2.setCurrentItem() to the onPageSelected() callback for Pager1. While this does scroll both Views, it's not synchronized to the pixel. I'm wondering if there's a way to achieve this effect without having to override the actual ViewPager class.

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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 solution was 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
        }
    }); 

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Have you tried beginFakeDrag()?