PagerLayoutManager.java

public class PagerLayoutManager extends LinearLayoutManager {

    private static final int SETTLE_DURATION = 600;
    private static final float DEFAULT_FLING_THRESHOLD_DP = 10f;

    private final int flingThreshold;

    private int scrollState;

    public PagerLayoutManager(Context context) {
        super(context, HORIZONTAL, false);
        flingThreshold = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, DEFAULT_FLING_THRESHOLD_DP, context.getResources().getDisplayMetrics());
    }

    @Override
    public void onAttachedToWindow(RecyclerView view) {
        super.onAttachedToWindow(view);
        view.setScrollingTouchSlop(RecyclerView.TOUCH_SLOP_PAGING);
    }

    @Override
    public void onDetachedFromWindow(RecyclerView view, RecyclerView.Recycler recycler) {
        super.onDetachedFromWindow(view, recycler);
        view.setScrollingTouchSlop(RecyclerView.TOUCH_SLOP_DEFAULT);
    }

    @Override
    public int scrollHorizontallyBy(int dx, RecyclerView.Recycler recycler, RecyclerView.State state) {
        if (scrollState == RecyclerView.SCROLL_STATE_SETTLING && !isSmoothScrolling()) {
            if (dx > 0) { //forward
                if (dx > flingThreshold) {
                    startSmoothScroll(new PagerScroller(findFirstVisibleItemPosition() + 1));
                }
            } else if (dx < 0) { //backward
                if (dx < -flingThreshold) {
                    startSmoothScroll(new PagerScroller(findFirstVisibleItemPosition()));
                }
            } else { //possible impossible case :)
                scrollToMostVisibleView(findFirstVisibleItemPosition());
            }
        }
        return super.scrollHorizontallyBy(dx, recycler, state);
    }

    private void scrollToMostVisibleView(int firstVisibleItemPosition) {
        View firstVisibleView = findViewByPosition(firstVisibleItemPosition);
        int offscreen = Math.abs(firstVisibleView.getLeft());
        int onscreen = Math.abs(firstVisibleView.getRight());
        int posToScroll = offscreen <= onscreen ? firstVisibleItemPosition : firstVisibleItemPosition + 1;
        startSmoothScroll(new PagerScroller(posToScroll));
    }

    @Override
    public void onScrollStateChanged(int state) {
        this.scrollState = state;
        if (isSmoothScrolling()) return;

        if (state == RecyclerView.SCROLL_STATE_IDLE) {
            int firstVisibleItemPosition = findFirstVisibleItemPosition();
            View firstVisibleView = findViewByPosition(firstVisibleItemPosition);
            if (firstVisibleView.getLeft() != 0) {
                scrollToMostVisibleView(firstVisibleItemPosition);
            }
        }
    }

    private final static class PagerScroller extends RecyclerView.SmoothScroller {

        private final Interpolator interpolator = new Interpolator() {
            @Override
            public float getInterpolation(float t) {
                t -= 1.0f;
                return t * t * t * t * t + 1.0f;
            }
        };

        public PagerScroller(int targetPosition) {
            setTargetPosition(targetPosition);
        }

        @Override
        protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
            if (targetView.getLeft() != 0) {
                action.update(
                        targetView.getLeft(),
                        0,
                        SETTLE_DURATION,
                        interpolator
                );
            }
        }

        @Override protected void onStart() {}
        @Override protected void onStop() {}
        @Override protected void onSeekTargetStep(int dx, int dy, RecyclerView.State state, Action action) {}
    }

    @Override
    public RecyclerView.LayoutParams generateDefaultLayoutParams() {
        return patchLayoutParams(super.generateDefaultLayoutParams());
    }

    @Override
    public RecyclerView.LayoutParams generateLayoutParams(ViewGroup.LayoutParams lp) {
        return patchLayoutParams(super.generateLayoutParams(lp));
    }

    @Override
    public RecyclerView.LayoutParams generateLayoutParams(Context c, AttributeSet attrs) {
        return patchLayoutParams(super.generateLayoutParams(c, attrs));
    }

    @Override
    public boolean checkLayoutParams(RecyclerView.LayoutParams lp) {
        return super.checkLayoutParams(lp) && lp.width == getWidth();
    }

    private RecyclerView.LayoutParams patchLayoutParams(RecyclerView.LayoutParams layoutParams) {
        layoutParams.width = getWidth();
        return layoutParams;
    }
}