How to snap RecyclerView items so that every X ite

Question

Background

It's possible to snap a RecyclerView to its center using :

LinearSnapHelper().attachToRecyclerView(recyclerView)

Example:

MainActivity.kt

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val inflater = LayoutInflater.from(this)

        recyclerView.adapter = object : RecyclerView.Adapter<RecyclerView.ViewHolder>() {
            override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
                val textView = holder.itemView as TextView
                textView.setBackgroundColor(if (position % 2 == 0) 0xffff0000.toInt() else 0xff00ff00.toInt())
                textView.text = position.toString()
            }

            override fun getItemCount(): Int {
                return 100
            }

            override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): RecyclerView.ViewHolder {
                val view = inflater.inflate(android.R.layout.simple_list_item_1, parent, false) as TextView
                val cellSize = recyclerView.width / 3
                view.layoutParams.height = cellSize
                view.layoutParams.width = cellSize
                view.gravity = Gravity.CENTER
                return object : RecyclerView.ViewHolder(view) {}
            }
        }
        LinearSnapHelper().attachToRecyclerView(recyclerView)
    }
}

activity_main.xml

<android.support.v7.widget.RecyclerView
    android:id="@+id/recyclerView" xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="horizontal"
    app:layoutManager="android.support.v7.widget.LinearLayoutManager"/>

It's also possible to snap it to other sides, as was done in some libraries, such as here.

There are also libraries that allow to have a RecyclerView that can work like a ViewPager, such as here.

The problem

Supposed I have a RecyclerView (horizontal in my case) with many items, and I want that it will treat every X items (X is constant) as a single unit, and snap to each of those units.

For example, if I scroll a bit, it could snap to either the 0-item, or the X-item, but not to something in between them.

In a way, it's similar in its behavior to a case of a normal ViewPager, just that each page would have X items in it.

For example, if we continue from the sample code I wrote above,suppose X==3 , the snapping would be from this idle state:

to this idle state (in case we scrolled enough, otherwise would stay in previous state) :

Flinging or scrolling more should be handled like on ViewPager, just like the library I've mentioned above.

Scrolling more (in the same direction) to the next snapping point would be to reach item "6" , "9", and so on...

What I tried

I tried to search for alternative libraries, and I also tried to read the docs regarding this, but I didn't find anything that might be useful.

It might also be possible by using a ViewPager, but I think that's not the best way, because ViewPager doesn't recycle its items well, and I think it's less flexible than RecyclerView in terms of how to snap.

The questions

1. Is it possible to set RecyclerView to snap every X items, to treat each X items as a single page to snap to?

   Of course, the items will take enough space for the whole RecyclerView, evenly.

2. Supposed it is possible, how would I get a callback when the RecyclerView is about to snap to a certain item, including having this item, before it got snapped? I ask this because it's related to the same question I asked here.

---

Kotlin solution

A working Kotlin solution based on "Cheticamp" answer (here), without the need to verify that you have the RecyclerView size, and with the choice of having a grid instead of a list, in the sample:

MainActivity.kt

class MainActivity : AppCompatActivity() {
    val USE_GRID = false
    //        val USE_GRID = true
    val ITEMS_PER_PAGE = 4
    var selectedItemPos = 0

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        val inflater = LayoutInflater.from(this)

        recyclerView.adapter = object : RecyclerView.Adapter<RecyclerView.ViewHolder>() {
            override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
                val textView = holder.itemView as TextView
                textView.setBackgroundColor(if (position % 2 == 0) 0xffff0000.toInt() else 0xff00ff00.toInt())
                textView.text = if (selectedItemPos == position) "selected: $position" else position.toString()
            }

            override fun getItemCount(): Int {
                return 100
            }

            override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): RecyclerView.ViewHolder {
                val view = inflater.inflate(android.R.layout.simple_list_item_1, parent, false) as TextView
                view.layoutParams.width = if (USE_GRID)
                    recyclerView.width / (ITEMS_PER_PAGE / 2)
                else
                    recyclerView.width / 4
                view.layoutParams.height = recyclerView.height / (ITEMS_PER_PAGE / 2)
                view.gravity = Gravity.CENTER
                return object : RecyclerView.ViewHolder(view) {
                }
            }
        }
        recyclerView.layoutManager = if (USE_GRID)
            GridLayoutManager(this, ITEMS_PER_PAGE / 2, GridLayoutManager.HORIZONTAL, false)
        else
            LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false)
        val snapToBlock = SnapToBlock(recyclerView, ITEMS_PER_PAGE)
        snapToBlock.attachToRecyclerView(recyclerView)
        snapToBlock.setSnapBlockCallback(object : SnapToBlock.SnapBlockCallback {
            override fun onBlockSnap(snapPosition: Int) {
                if (selectedItemPos == snapPosition)
                    return
                selectedItemPos = snapPosition
                recyclerView.adapter.notifyDataSetChanged()
            }

            override fun onBlockSnapped(snapPosition: Int) {
                if (selectedItemPos == snapPosition)
                    return
                selectedItemPos = snapPosition
                recyclerView.adapter.notifyDataSetChanged()
            }

        })
    }

}

SnapToBlock.kt

/**@param maxFlingBlocks Maxim blocks to move during most vigorous fling*/
class SnapToBlock constructor(private val maxFlingBlocks: Int) : SnapHelper() {
    private var recyclerView: RecyclerView? = null
    // Total number of items in a block of view in the RecyclerView
    private var blocksize: Int = 0
    // Maximum number of positions to move on a fling.
    private var maxPositionsToMove: Int = 0
    // Width of a RecyclerView item if orientation is horizonal; height of the item if vertical
    private var itemDimension: Int = 0
    // Callback interface when blocks are snapped.
    private var snapBlockCallback: SnapBlockCallback? = null
    // When snapping, used to determine direction of snap.
    private var priorFirstPosition = RecyclerView.NO_POSITION
    // Our private scroller
    private var scroller: Scroller? = null
    // Horizontal/vertical layout helper
    private var orientationHelper: OrientationHelper? = null
    // LTR/RTL helper
    private var layoutDirectionHelper: LayoutDirectionHelper? = null

    @Throws(IllegalStateException::class)
    override fun attachToRecyclerView(recyclerView: RecyclerView?) {
        if (recyclerView != null) {
            this.recyclerView = recyclerView
            val layoutManager = recyclerView.layoutManager as LinearLayoutManager
            orientationHelper = when {
                layoutManager.canScrollHorizontally() -> OrientationHelper.createHorizontalHelper(layoutManager)
                layoutManager.canScrollVertically() -> OrientationHelper.createVerticalHelper(layoutManager)
                else -> throw IllegalStateException("RecyclerView must be scrollable")
            }
            scroller = Scroller(this.recyclerView!!.context, sInterpolator)
            initItemDimensionIfNeeded(layoutManager)
        }
        super.attachToRecyclerView(recyclerView)
    }

    // Called when the target view is available and we need to know how much more
    // to scroll to get it lined up with the side of the RecyclerView.
    override fun calculateDistanceToFinalSnap(layoutManager: RecyclerView.LayoutManager, targetView: View): IntArray {
        val out = IntArray(2)
        initLayoutDirectionHelperIfNeeded(layoutManager)
        if (layoutManager.canScrollHorizontally())
            out[0] = layoutDirectionHelper!!.getScrollToAlignView(targetView)
        if (layoutManager.canScrollVertically())
            out[1] = layoutDirectionHelper!!.getScrollToAlignView(targetView)
        if (snapBlockCallback != null)
            if (out[0] == 0 && out[1] == 0)
                snapBlockCallback!!.onBlockSnapped(layoutManager.getPosition(targetView))
            else
                snapBlockCallback!!.onBlockSnap(layoutManager.getPosition(targetView))
        return out
    }

    private fun initLayoutDirectionHelperIfNeeded(layoutManager: RecyclerView.LayoutManager) {
        if (layoutDirectionHelper == null)
            if (layoutManager.canScrollHorizontally())
                layoutDirectionHelper = LayoutDirectionHelper()
            else if (layoutManager.canScrollVertically())
            // RTL doesn't matter for vertical scrolling for this class.
                layoutDirectionHelper = LayoutDirectionHelper(false)
    }

    // We are flinging and need to know where we are heading.
    override fun findTargetSnapPosition(layoutManager: RecyclerView.LayoutManager, velocityX: Int, velocityY: Int): Int {
        initLayoutDirectionHelperIfNeeded(layoutManager)
        val lm = layoutManager as LinearLayoutManager
        initItemDimensionIfNeeded(layoutManager)
        scroller!!.fling(0, 0, velocityX, velocityY, Integer.MIN_VALUE, Integer.MAX_VALUE, Integer.MIN_VALUE, Integer.MAX_VALUE)
        return when {
            velocityX != 0 -> layoutDirectionHelper!!.getPositionsToMove(lm, scroller!!.finalX, itemDimension)
            else -> if (velocityY != 0)
                layoutDirectionHelper!!.getPositionsToMove(lm, scroller!!.finalY, itemDimension)
            else RecyclerView.NO_POSITION
        }
    }

    // We have scrolled to the neighborhood where we will snap. Determine the snap position.
    override fun findSnapView(layoutManager: RecyclerView.LayoutManager): View? {
        // Snap to a view that is either 1) toward the bottom of the data and therefore on screen,
        // or, 2) toward the top of the data and may be off-screen.
        val snapPos = calcTargetPosition(layoutManager as LinearLayoutManager)
        val snapView = if (snapPos == RecyclerView.NO_POSITION)
            null
        else
            layoutManager.findViewByPosition(snapPos)
        if (snapView == null)
            Log.d(TAG, "<<<<findSnapView is returning null!")
        Log.d(TAG, "<<<<findSnapView snapos=" + snapPos)
        return snapView
    }

    // Does the heavy lifting for findSnapView.
    private fun calcTargetPosition(layoutManager: LinearLayoutManager): Int {
        val snapPos: Int
        initLayoutDirectionHelperIfNeeded(layoutManager)
        val firstVisiblePos = layoutManager.findFirstVisibleItemPosition()
        if (firstVisiblePos == RecyclerView.NO_POSITION)
            return RecyclerView.NO_POSITION
        initItemDimensionIfNeeded(layoutManager)
        if (firstVisiblePos >= priorFirstPosition) {
            // Scrolling toward bottom of data
            val firstCompletePosition = layoutManager.findFirstCompletelyVisibleItemPosition()
            snapPos = if (firstCompletePosition != RecyclerView.NO_POSITION && firstCompletePosition % blocksize == 0)
                firstCompletePosition
            else
                roundDownToBlockSize(firstVisiblePos + blocksize)
        } else {
            // Scrolling toward top of data
            snapPos = roundDownToBlockSize(firstVisiblePos)
            // Check to see if target view exists. If it doesn't, force a smooth scroll.
            // SnapHelper only snaps to existing views and will not scroll to a non-existant one.
            // If limiting fling to single block, then the following is not needed since the
            // views are likely to be in the RecyclerView pool.
            if (layoutManager.findViewByPosition(snapPos) == null) {
                val toScroll = layoutDirectionHelper!!.calculateDistanceToScroll(layoutManager, snapPos)
                recyclerView!!.smoothScrollBy(toScroll[0], toScroll[1], sInterpolator)
            }
        }
        priorFirstPosition = firstVisiblePos
        return snapPos
    }

    private fun initItemDimensionIfNeeded(layoutManager: RecyclerView.LayoutManager) {
        if (itemDimension != 0)
            return
        val child = layoutManager.getChildAt(0) ?: return
        if (layoutManager.canScrollHorizontally()) {
            itemDimension = child.width
            blocksize = getSpanCount(layoutManager) * (recyclerView!!.width / itemDimension)
        } else if (layoutManager.canScrollVertically()) {
            itemDimension = child.height
            blocksize = getSpanCount(layoutManager) * (recyclerView!!.height / itemDimension)
        }
        maxPositionsToMove = blocksize * maxFlingBlocks
    }

    private fun getSpanCount(layoutManager: RecyclerView.LayoutManager): Int = (layoutManager as? GridLayoutManager)?.spanCount ?: 1

    private fun roundDownToBlockSize(trialPosition: Int): Int = trialPosition - trialPosition % blocksize

    private fun roundUpToBlockSize(trialPosition: Int): Int = roundDownToBlockSize(trialPosition + blocksize - 1)

    override fun createScroller(layoutManager: RecyclerView.LayoutManager): LinearSmoothScroller? {
        return if (layoutManager !is RecyclerView.SmoothScroller.ScrollVectorProvider)
            null
        else object : LinearSmoothScroller(recyclerView!!.context) {
            override fun onTargetFound(targetView: View, state: RecyclerView.State?, action: RecyclerView.SmoothScroller.Action) {
                val snapDistances = calculateDistanceToFinalSnap(recyclerView!!.layoutManager, targetView)
                val dx = snapDistances[0]
                val dy = snapDistances[1]
                val time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy)))
                if (time > 0)
                    action.update(dx, dy, time, sInterpolator)
            }

            override fun calculateSpeedPerPixel(displayMetrics: DisplayMetrics): Float = MILLISECONDS_PER_INCH / displayMetrics.densityDpi
        }
    }

    fun setSnapBlockCallback(callback: SnapBlockCallback?) {
        snapBlockCallback = callback
    }

    /*
        Helper class that handles calculations for LTR and RTL layouts.
     */
    private inner class LayoutDirectionHelper {
        // Is the layout an RTL one?
        private val mIsRTL: Boolean

        constructor() {
            mIsRTL = ViewCompat.getLayoutDirection(recyclerView) == ViewCompat.LAYOUT_DIRECTION_RTL
        }

        constructor(isRTL: Boolean) {
            mIsRTL = isRTL
        }

        /*
            Calculate the amount of scroll needed to align the target view with the layout edge.
         */
        fun getScrollToAlignView(targetView: View): Int = if (mIsRTL)
            orientationHelper!!.getDecoratedEnd(targetView) - recyclerView!!.width
        else
            orientationHelper!!.getDecoratedStart(targetView)

        /**
         * Calculate the distance to final snap position when the view corresponding to the snap
         * position is not currently available.
         *
         * @param layoutManager LinearLayoutManager or descendent class
         * @param targetPos     - Adapter position to snap to
         * @return int[2] {x-distance in pixels, y-distance in pixels}
         */
        fun calculateDistanceToScroll(layoutManager: LinearLayoutManager, targetPos: Int): IntArray {
            val out = IntArray(2)
            val firstVisiblePos = layoutManager.findFirstVisibleItemPosition()
            if (layoutManager.canScrollHorizontally()) {
                if (targetPos <= firstVisiblePos)  // scrolling toward top of data
                    if (mIsRTL) {
                        val lastView = layoutManager.findViewByPosition(layoutManager.findLastVisibleItemPosition())
                        out[0] = orientationHelper!!.getDecoratedEnd(lastView) + (firstVisiblePos - targetPos) * itemDimension
                    } else {
                        val firstView = layoutManager.findViewByPosition(firstVisiblePos)
                        out[0] = orientationHelper!!.getDecoratedStart(firstView) - (firstVisiblePos - targetPos) * itemDimension
                    }
            }
            if (layoutManager.canScrollVertically() && targetPos <= firstVisiblePos) { // scrolling toward top of data
                val firstView = layoutManager.findViewByPosition(firstVisiblePos)
                out[1] = firstView.top - (firstVisiblePos - targetPos) * itemDimension
            }
            return out
        }

        /*
            Calculate the number of positions to move in the RecyclerView given a scroll amount
            and the size of the items to be scrolled. Return integral multiple of mBlockSize not
            equal to zero.
         */
        fun getPositionsToMove(llm: LinearLayoutManager, scroll: Int, itemSize: Int): Int {
            var positionsToMove: Int
            positionsToMove = roundUpToBlockSize(Math.abs(scroll) / itemSize)
            if (positionsToMove < blocksize)
            // Must move at least one block
                positionsToMove = blocksize
            else if (positionsToMove > maxPositionsToMove)
            // Clamp number of positions to move so we don't get wild flinging.
                positionsToMove = maxPositionsToMove
            if (scroll < 0)
                positionsToMove *= -1
            if (mIsRTL)
                positionsToMove *= -1
            return if (layoutDirectionHelper!!.isDirectionToBottom(scroll < 0)) {
                // Scrolling toward the bottom of data.
                roundDownToBlockSize(llm.findFirstVisibleItemPosition()) + positionsToMove
            } else roundDownToBlockSize(llm.findLastVisibleItemPosition()) + positionsToMove
            // Scrolling toward the top of the data.
        }

        fun isDirectionToBottom(velocityNegative: Boolean): Boolean = if (mIsRTL) velocityNegative else !velocityNegative
    }

    interface SnapBlockCallback {
        fun onBlockSnap(snapPosition: Int)
        fun onBlockSnapped(snapPosition: Int)
    }

    companion object {
        // Borrowed from ViewPager.java
        private val sInterpolator = Interpolator { input ->
            var t = input
            // _o(t) = t * t * ((tension + 1) * t + tension)
            // o(t) = _o(t - 1) + 1
            t -= 1.0f
            t * t * t + 1.0f
        }

        private val MILLISECONDS_PER_INCH = 100f
        private val TAG = "SnapToBlock"
    }
}

---

Update

Even though I've marked an answer as accepted, as it works fine, I've noticed it has serious issues:

1. Smooth scrolling doesn't seem to work fine (doesn't scroll to correct place). Only scrolling that work is as such (but with the "smearing" effect) :

   (recyclerView.layoutManager as LinearLayoutManager).scrollToPositionWithOffset(targetPos,0)
   

2. When switching to RTL (Right to left) locale such as Hebrew ("עברית"), it doesn't let me scroll at all.

3. I've noticed that onCreateViewHolder is called a lot. In fact it is called every time I scroll, even for times it should have recycled the ViewHolders. This means there is an excessive creation of views, and it might also mean there is a memory leak.

I've tried to fix those myself, but failed so far.

If anyone here knows how to fix it, I will grant the extra, new bounty

---

Update: as we got a fix for RTL/LTR, I've updated the Kotlin solution within this post.

---

Update: about point #3 , this seems to be because there is a pool of views for the recyclerView, which gets filled too soon. To handle this, we can simply enlarge the pool size, by using recyclerView.getRecycledViewPool() .setMaxRecycledViews(viewType, Integer.MAX_VALUE) for each view type we have in it. Weird thing that this is really needed. I've posted about it to Google (here and here) but was rejected that the pool should be unlimited by default. In the end, I decided to at least request to have a more convinient function to do it for all view types (here).

Answer 1

I would do something like that

1. Block scrolling inside RecyclerView (e.g How to disable RecyclerView scrolling?)

2. Create Gesture Fling Detecor and attach it to RecyclerView

3. Inside Gesture Detector detect fling events events
4. On Fling event, detect side (left right)
5. Scroll RecyclerView to position (first Visible item + your const * (left?-1:1))

should work :)

Answer 2

SnapHelper supplies the necessary framework for what you are attempting, but it needs to be extended to handle blocks of views. The class SnapToBlock below extends SnapHelper to snap to blocks of views. In the example, I have used four views to a block but it can be more or less.

Update: The code has been change to accommodate GridLayoutManager as well as LinearLayoutManager. Flinging is now inhibited so the snapping works more list a ViewPager. Horizontal and vertical scrolling is now supported as well as LTR and RTL layouts.

Update: Changed smooth scroll interpolator to be more like ViewPager.

Update: Adding callbacks for pre/post snapping.

Update: Adding support for RTL layouts.

Here is a quick video of the sample app:

Set up the layout manager as follows:

// For LinearLayoutManager horizontal orientation
recyclerView.setLayoutManager(new LinearLayoutManager(this, RecyclerView.HORIZONTAL, false));

// For GridLayoutManager vertical orientation
recyclerView.setLayoutManager(new GridLayoutManager(this, SPAN_COUNT, RecyclerView.VERTICAL, false));

Add the following to attach the SnapToBlock to the RecyclerView.

SnapToBlock snapToBlock = new SnapToBlock(mMaxFlingPages);
snapToBlock.attachToRecyclerView(recyclerView);

mMaxFlingPages is the maximum number of blocks (rowsCols * spans) to allow to be flung at one time.

For call backs when a snap is about to be made and has been completed, add the following:

snapToBlock.setSnapBlockCallback(new SnapToBlock.SnapBlockCallback() {
    @Override
    public void onBlockSnap(int snapPosition) {
        ...
    }

    @Override
    public void onBlockSnapped(int snapPosition) {
        ...
    }
});

SnapToBlock.java

/*  The number of items in the RecyclerView should be a multiple of block size; otherwise, the
    extra item views will not be positioned on a block boundary when the end of the data is reached.
    Pad out with empty item views if needed.

    Updated to accommodate RTL layouts.
 */

public class SnapToBlock extends SnapHelper {
    private RecyclerView mRecyclerView;

    // Total number of items in a block of view in the RecyclerView
    private int mBlocksize;

    // Maximum number of positions to move on a fling.
    private int mMaxPositionsToMove;

    // Width of a RecyclerView item if orientation is horizonal; height of the item if vertical
    private int mItemDimension;

    // Maxim blocks to move during most vigorous fling.
    private final int mMaxFlingBlocks;

    // Callback interface when blocks are snapped.
    private SnapBlockCallback mSnapBlockCallback;

    // When snapping, used to determine direction of snap.
    private int mPriorFirstPosition = RecyclerView.NO_POSITION;

    // Our private scroller
    private Scroller mScroller;

    // Horizontal/vertical layout helper
    private OrientationHelper mOrientationHelper;

    // LTR/RTL helper
    private LayoutDirectionHelper mLayoutDirectionHelper;

    // Borrowed from ViewPager.java
    private static final Interpolator sInterpolator = new Interpolator() {
        public float getInterpolation(float t) {
            // _o(t) = t * t * ((tension + 1) * t + tension)
            // o(t) = _o(t - 1) + 1
            t -= 1.0f;
            return t * t * t + 1.0f;
        }
    };

    SnapToBlock(int maxFlingBlocks) {
        super();
        mMaxFlingBlocks = maxFlingBlocks;
    }

    @Override
    public void attachToRecyclerView(@Nullable final RecyclerView recyclerView)
        throws IllegalStateException {

        if (recyclerView != null) {
            mRecyclerView = recyclerView;
            final LinearLayoutManager layoutManager =
                (LinearLayoutManager) recyclerView.getLayoutManager();
            if (layoutManager.canScrollHorizontally()) {
                mOrientationHelper = OrientationHelper.createHorizontalHelper(layoutManager);
                mLayoutDirectionHelper =
                    new LayoutDirectionHelper(ViewCompat.getLayoutDirection(mRecyclerView));
            } else if (layoutManager.canScrollVertically()) {
                mOrientationHelper = OrientationHelper.createVerticalHelper(layoutManager);
                // RTL doesn't matter for vertical scrolling for this class.
                mLayoutDirectionHelper = new LayoutDirectionHelper(RecyclerView.LAYOUT_DIRECTION_LTR);
            } else {
                throw new IllegalStateException("RecyclerView must be scrollable");
            }
            mScroller = new Scroller(mRecyclerView.getContext(), sInterpolator);
            initItemDimensionIfNeeded(layoutManager);
        }
        super.attachToRecyclerView(recyclerView);
    }

    // Called when the target view is available and we need to know how much more
    // to scroll to get it lined up with the side of the RecyclerView.
    @NonNull
    @Override
    public int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager,
                                              @NonNull View targetView) {
        int[] out = new int[2];

        if (layoutManager.canScrollHorizontally()) {
            out[0] = mLayoutDirectionHelper.getScrollToAlignView(targetView);
        }
        if (layoutManager.canScrollVertically()) {
            out[1] = mLayoutDirectionHelper.getScrollToAlignView(targetView);
        }
        if (mSnapBlockCallback != null) {
            if (out[0] == 0 && out[1] == 0) {
                mSnapBlockCallback.onBlockSnapped(layoutManager.getPosition(targetView));
            } else {
                mSnapBlockCallback.onBlockSnap(layoutManager.getPosition(targetView));
            }
        }
        return out;
    }

    // We are flinging and need to know where we are heading.
    @Override
    public int findTargetSnapPosition(RecyclerView.LayoutManager layoutManager,
                                      int velocityX, int velocityY) {
        LinearLayoutManager lm = (LinearLayoutManager) layoutManager;

        initItemDimensionIfNeeded(layoutManager);
        mScroller.fling(0, 0, velocityX, velocityY, Integer.MIN_VALUE, Integer.MAX_VALUE,
                        Integer.MIN_VALUE, Integer.MAX_VALUE);

        if (velocityX != 0) {
            return mLayoutDirectionHelper
                .getPositionsToMove(lm, mScroller.getFinalX(), mItemDimension);
        }

        if (velocityY != 0) {
            return mLayoutDirectionHelper
                .getPositionsToMove(lm, mScroller.getFinalY(), mItemDimension);
        }

        return RecyclerView.NO_POSITION;
    }

    // We have scrolled to the neighborhood where we will snap. Determine the snap position.
    @Override
    public View findSnapView(RecyclerView.LayoutManager layoutManager) {
        // Snap to a view that is either 1) toward the bottom of the data and therefore on screen,
        // or, 2) toward the top of the data and may be off-screen.
        int snapPos = calcTargetPosition((LinearLayoutManager) layoutManager);
        View snapView = (snapPos == RecyclerView.NO_POSITION)
            ? null : layoutManager.findViewByPosition(snapPos);

        if (snapView == null) {
            Log.d(TAG, "<<<<findSnapView is returning null!");
        }
        Log.d(TAG, "<<<<findSnapView snapos=" + snapPos);
        return snapView;
    }

    // Does the heavy lifting for findSnapView.
    private int calcTargetPosition(LinearLayoutManager layoutManager) {
        int snapPos;
        int firstVisiblePos = layoutManager.findFirstVisibleItemPosition();

        if (firstVisiblePos == RecyclerView.NO_POSITION) {
            return RecyclerView.NO_POSITION;
        }
        initItemDimensionIfNeeded(layoutManager);
        if (firstVisiblePos >= mPriorFirstPosition) {
            // Scrolling toward bottom of data
            int firstCompletePosition = layoutManager.findFirstCompletelyVisibleItemPosition();
            if (firstCompletePosition != RecyclerView.NO_POSITION
                && firstCompletePosition % mBlocksize == 0) {
                snapPos = firstCompletePosition;
            } else {
                snapPos = roundDownToBlockSize(firstVisiblePos + mBlocksize);
            }
        } else {
            // Scrolling toward top of data
            snapPos = roundDownToBlockSize(firstVisiblePos);
            // Check to see if target view exists. If it doesn't, force a smooth scroll.
            // SnapHelper only snaps to existing views and will not scroll to a non-existant one.
            // If limiting fling to single block, then the following is not needed since the
            // views are likely to be in the RecyclerView pool.
            if (layoutManager.findViewByPosition(snapPos) == null) {
                int[] toScroll = mLayoutDirectionHelper.calculateDistanceToScroll(layoutManager, snapPos);
                mRecyclerView.smoothScrollBy(toScroll[0], toScroll[1], sInterpolator);
            }
        }
        mPriorFirstPosition = firstVisiblePos;

        return snapPos;
    }

    private void initItemDimensionIfNeeded(final RecyclerView.LayoutManager layoutManager) {
        if (mItemDimension != 0) {
            return;
        }

        View child;
        if ((child = layoutManager.getChildAt(0)) == null) {
            return;
        }

        if (layoutManager.canScrollHorizontally()) {
            mItemDimension = child.getWidth();
            mBlocksize = getSpanCount(layoutManager) * (mRecyclerView.getWidth() / mItemDimension);
        } else if (layoutManager.canScrollVertically()) {
            mItemDimension = child.getHeight();
            mBlocksize = getSpanCount(layoutManager) * (mRecyclerView.getHeight() / mItemDimension);
        }
        mMaxPositionsToMove = mBlocksize * mMaxFlingBlocks;
    }

    private int getSpanCount(RecyclerView.LayoutManager layoutManager) {
        return (layoutManager instanceof GridLayoutManager)
            ? ((GridLayoutManager) layoutManager).getSpanCount()
            : 1;
    }

    private int roundDownToBlockSize(int trialPosition) {
        return trialPosition - trialPosition % mBlocksize;
    }

    private int roundUpToBlockSize(int trialPosition) {
        return roundDownToBlockSize(trialPosition + mBlocksize - 1);
    }

    @Nullable
    protected LinearSmoothScroller createScroller(RecyclerView.LayoutManager layoutManager) {
        if (!(layoutManager instanceof RecyclerView.SmoothScroller.ScrollVectorProvider)) {
            return null;
        }
        return new LinearSmoothScroller(mRecyclerView.getContext()) {
            @Override
            protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
                int[] snapDistances = calculateDistanceToFinalSnap(mRecyclerView.getLayoutManager(),
                                                                   targetView);
                final int dx = snapDistances[0];
                final int dy = snapDistances[1];
                final int time = calculateTimeForDeceleration(Math.max(Math.abs(dx), Math.abs(dy)));
                if (time > 0) {
                    action.update(dx, dy, time, sInterpolator);
                }
            }

            @Override
            protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
                return MILLISECONDS_PER_INCH / displayMetrics.densityDpi;
            }
        };
    }

    public void setSnapBlockCallback(@Nullable SnapBlockCallback callback) {
        mSnapBlockCallback = callback;
    }

    /*
        Helper class that handles calculations for LTR and RTL layouts.
     */
    private class LayoutDirectionHelper {

        // Is the layout an RTL one?
        private final boolean mIsRTL;

        @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
        LayoutDirectionHelper(int direction) {
            mIsRTL = direction == View.LAYOUT_DIRECTION_RTL;
        }

        /*
            Calculate the amount of scroll needed to align the target view with the layout edge.
         */
        int getScrollToAlignView(View targetView) {
            return (mIsRTL)
                ? mOrientationHelper.getDecoratedEnd(targetView) - mRecyclerView.getWidth()
                : mOrientationHelper.getDecoratedStart(targetView);
        }

        /**
         * Calculate the distance to final snap position when the view corresponding to the snap
         * position is not currently available.
         *
         * @param layoutManager LinearLayoutManager or descendent class
         * @param targetPos     - Adapter position to snap to
         * @return int[2] {x-distance in pixels, y-distance in pixels}
         */
        int[] calculateDistanceToScroll(LinearLayoutManager layoutManager, int targetPos) {
            int[] out = new int[2];

            int firstVisiblePos;

            firstVisiblePos = layoutManager.findFirstVisibleItemPosition();
            if (layoutManager.canScrollHorizontally()) {
                if (targetPos <= firstVisiblePos) { // scrolling toward top of data
                    if (mIsRTL) {
                        View lastView = layoutManager.findViewByPosition(layoutManager.findLastVisibleItemPosition());
                        out[0] = mOrientationHelper.getDecoratedEnd(lastView)
                            + (firstVisiblePos - targetPos) * mItemDimension;
                    } else {
                        View firstView = layoutManager.findViewByPosition(firstVisiblePos);
                        out[0] = mOrientationHelper.getDecoratedStart(firstView)
                            - (firstVisiblePos - targetPos) * mItemDimension;
                    }
                }
            }
            if (layoutManager.canScrollVertically()) {
                if (targetPos <= firstVisiblePos) { // scrolling toward top of data
                    View firstView = layoutManager.findViewByPosition(firstVisiblePos);
                    out[1] = firstView.getTop() - (firstVisiblePos - targetPos) * mItemDimension;
                }
            }

            return out;
        }

        /*
            Calculate the number of positions to move in the RecyclerView given a scroll amount
            and the size of the items to be scrolled. Return integral multiple of mBlockSize not
            equal to zero.
         */
        int getPositionsToMove(LinearLayoutManager llm, int scroll, int itemSize) {
            int positionsToMove;

            positionsToMove = roundUpToBlockSize(Math.abs(scroll) / itemSize);

            if (positionsToMove < mBlocksize) {
                // Must move at least one block
                positionsToMove = mBlocksize;
            } else if (positionsToMove > mMaxPositionsToMove) {
                // Clamp number of positions to move so we don't get wild flinging.
                positionsToMove = mMaxPositionsToMove;
            }

            if (scroll < 0) {
                positionsToMove *= -1;
            }
            if (mIsRTL) {
                positionsToMove *= -1;
            }

            if (mLayoutDirectionHelper.isDirectionToBottom(scroll < 0)) {
                // Scrolling toward the bottom of data.
                return roundDownToBlockSize(llm.findFirstVisibleItemPosition()) + positionsToMove;
            }
            // Scrolling toward the top of the data.
            return roundDownToBlockSize(llm.findLastVisibleItemPosition()) + positionsToMove;
        }

        boolean isDirectionToBottom(boolean velocityNegative) {
            //noinspection SimplifiableConditionalExpression
            return mIsRTL ? velocityNegative : !velocityNegative;
        }
    }

    public interface SnapBlockCallback {
        void onBlockSnap(int snapPosition);

        void onBlockSnapped(int snapPosition);

    }

    private static final float MILLISECONDS_PER_INCH = 100f;
    @SuppressWarnings("unused")
    private static final String TAG = "SnapToBlock";
}

The SnapBlockCallback interface defined above can be used to report the adapter position of the view at the start of the block to be snapped. The view associated with that position may not be instantiated when the call is made if the view is off screen.