ViewPagerIndicatorAnimatorImpl.java

private class Animator extends DataSetObserver implements OnPageChangeListener, OnAdapterChangeListener, OnGlobalLayoutListener {

        private float lastPositionOffset;

        private Animator() {
            PagerAdapter adapter = viewPager.getAdapter();
            if (adapter != null) adapter.registerDataSetObserver(this);

            viewPager.addOnPageChangeListener(this);
            viewPager.addOnAdapterChangeListener(this);
        }

        public void onAdapterChanged(@NonNull ViewPager viewPager, @Nullable PagerAdapter oldAdapter, @Nullable PagerAdapter newAdapter) {
            if (newAdapter == null) return;
            newAdapter.registerDataSetObserver(this);
            buildIndicators(newAdapter);
        }

        public void onPageScrolled(int position, float fraction, int pixelOffset) {
            float currentPositionOffset = ((float) position) + fraction;
            boolean toTheRight = currentPositionOffset > lastPositionOffset;

            onMoved(toTheRight, toTheRight ? position : position + 1, fraction);
            lastPositionOffset = currentPositionOffset;
        }

        public void onGlobalLayout() {
            ViewTreeObserver observer = guide.getViewTreeObserver();
            if (observer.isAlive()) observer.removeOnGlobalLayoutListener(this);

            guideLineWidth = guide.getWidth();
            onMoved(false, viewPager.getCurrentItem() + 1, 0.0f);
        }

        private float getTranslation(boolean toTheRight, int originalPosition, float fraction) {
            if (!toTheRight) fraction = 1.0f - fraction;

            float chunkWidth = ((float) guideLineWidth) / requireNonNull(viewPager.getAdapter()).getCount();
            float currentChunk = ((float) originalPosition) * chunkWidth;
            float diff = chunkWidth * fraction;

            return toTheRight ? currentChunk + diff : currentChunk - diff;
        }

        private void onMoved(boolean toTheRight, int position, float fraction) {
            float translation = getTranslation(toTheRight, position, fraction);
            indicator.setTranslationX(translation);
            for (int i = watchers.size() - 1; i >= 0; i--) {
                watchers.get(i).onIndicatorMoved(indicator, position, fraction, translation);
            }
        }

        public void onPageSelected(int position) { }

        public void onPageScrollStateChanged(int state) { }
    }