Snappy Scrolling in Recyclerview

Recyclerview as Snappy Gallery

I had a similar use case and I got good results using the suggestions in this SO thread:

Snappy scrolling in RecyclerView

The upshot is: use a custom RecyclerView.OnScrollListener to detect when the scroll state changes to SCROLL_STATE_IDLE, at which point use a subclass of LayoutManager which implements getFixScrollPos() to compute and then scroll to the correct position, snapping in the process.

I was also able to use a styleable attribute to set the number of items in the row at design time. You may want to do this as well.

Android Recyclerview Horizontal - Snappy Scroll Effect

I used this class:

SnappyRecyclerView

package icn.premierandroid.misc;

import android.content.Context;
import android.content.res.Resources;
import android.support.annotation.Nullable;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.AttributeSet;
import android.view.View;

public class SnappyRecyclerView extends RecyclerView {

// Use it with a horizontal LinearLayoutManager
// Based on http://stackoverflow.com/a/29171652/4034572

public SnappyRecyclerView(Context context) {
    super(context);
}

public SnappyRecyclerView(Context context, @Nullable AttributeSet attrs) {
    super(context, attrs);
}

public SnappyRecyclerView(Context context, @Nullable AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
}

@Override
public boolean fling(int velocityX, int velocityY) {

    LinearLayoutManager linearLayoutManager = (LinearLayoutManager) getLayoutManager();

    int screenWidth = Resources.getSystem().getDisplayMetrics().widthPixels;

    // views on the screen
    int lastVisibleItemPosition = linearLayoutManager.findLastVisibleItemPosition();
    View lastView = linearLayoutManager.findViewByPosition(lastVisibleItemPosition);
    int firstVisibleItemPosition = linearLayoutManager.findFirstVisibleItemPosition();
    View firstView = linearLayoutManager.findViewByPosition(firstVisibleItemPosition);

    // distance we need to scroll
    int leftMargin = (screenWidth - lastView.getWidth()) / 2;
    int rightMargin = (screenWidth - firstView.getWidth()) / 2 + firstView.getWidth();
    int leftEdge = lastView.getLeft();
    int rightEdge = firstView.getRight();
    int scrollDistanceLeft = leftEdge - leftMargin;
    int scrollDistanceRight = rightMargin - rightEdge;

    if (Math.abs(velocityX) < 1000) {
        // The fling is slow -> stay at the current page if we are less than half through,
        // or go to the next page if more than half through

        if (leftEdge > screenWidth / 2) {
            // go to next page
            smoothScrollBy(-scrollDistanceRight, 0);
        } else if (rightEdge < screenWidth / 2) {
            // go to next page
            smoothScrollBy(scrollDistanceLeft, 0);
        } else {
            // stay at current page
            if (velocityX > 0) {
                smoothScrollBy(-scrollDistanceRight, 0);
            } else {
                smoothScrollBy(scrollDistanceLeft, 0);
            }
        }
        return true;

    } else {
        // The fling is fast -> go to next page

        if (velocityX > 0) {
            smoothScrollBy(scrollDistanceLeft, 0);
        } else {
            smoothScrollBy(-scrollDistanceRight, 0);
        }
        return true;

    }

}

@Override
public void onScrollStateChanged(int state) {
    super.onScrollStateChanged(state);

    // If you tap on the phone while the RecyclerView is scrolling it will stop in the middle.
    // This code fixes this. This code is not strictly necessary but it improves the behaviour.

    if (state == SCROLL_STATE_IDLE) {
        LinearLayoutManager linearLayoutManager = (LinearLayoutManager) getLayoutManager();

        int screenWidth = Resources.getSystem().getDisplayMetrics().widthPixels;

        // views on the screen
        int lastVisibleItemPosition = linearLayoutManager.findLastVisibleItemPosition();
        View lastView = linearLayoutManager.findViewByPosition(lastVisibleItemPosition);
        int firstVisibleItemPosition = linearLayoutManager.findFirstVisibleItemPosition();
        View firstView = linearLayoutManager.findViewByPosition(firstVisibleItemPosition);

        // distance we need to scroll
        int leftMargin = (screenWidth - lastView.getWidth()) / 2;
        int rightMargin = (screenWidth - firstView.getWidth()) / 2 + firstView.getWidth();
        int leftEdge = lastView.getLeft();
        int rightEdge = firstView.getRight();
        int scrollDistanceLeft = leftEdge - leftMargin;
        int scrollDistanceRight = rightMargin - rightEdge;

        if (leftEdge > screenWidth / 2) {
            smoothScrollBy(-scrollDistanceRight, 0);
        } else if (rightEdge < screenWidth / 2) {
            smoothScrollBy(scrollDistanceLeft, 0);
        }
    }
}

}

in XML (put your package route to the class e.g. mine is icn.premierandroid.misc.SnappyRecyclerView:

 <packagename.SnappyRecyclerView
            xmlns:android="http://schemas.android.com/apk/res/android"
            android:id="@+id/recycler_view"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:scrollbars="none"
            android:layout_weight="0.34" />

You shouldn't need to change anything if you have a RecyclerView initialized in your class already.

Like so:

recyclerView = (RecyclerView) rootView.findViewById(R.id.recycler_view);
recyclerView.setHasFixedSize(true);
                // LinearLayoutManager is used here, this will layout the elements in a similar fashion
                // to the way ListView would layout elements. The RecyclerView.LayoutManager defines how
                // elements are laid out.
RecyclerView.LayoutManager mLayoutManager = new LinearLayoutManager(getActivity(), LinearLayoutManager.HORIZONTAL, false);
recyclerView.setLayoutManager(mLayoutManager);

This should satisfy full width of screen elements only, as you've asked for.

How to snap RecyclerView items so that every X items would be considered like a single unit to snap to?

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.

Snapping in horizontal recycler view

Finally figured it out with some help from other codes:

public class FlingRecyclerView extends RecyclerView {

    int screenWidth;

    public FlingRecyclerView(Context context) {
        super(context);
        WindowManager windowManager = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
        Display display = windowManager.getDefaultDisplay();
        Point size = new Point();
        display.getSize(size);
        screenWidth = size.x;
    }

    public FlingRecyclerView(Context context, AttributeSet attrs) {
        super(context, attrs);
        WindowManager windowManager = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
        Display display = windowManager.getDefaultDisplay();
        Point size = new Point();
        display.getSize(size);
        screenWidth = size.x;
    }

    public FlingRecyclerView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        WindowManager windowManager = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
        Display display = windowManager.getDefaultDisplay();
        Point size = new Point();
        display.getSize(size);
        screenWidth = size.x;
    }

    @Override
    public boolean fling(int velocityX, int velocityY) {
        LinearLayoutManager linearLayoutManager = (LinearLayoutManager) getLayoutManager();

//these four variables identify the views you see on screen.
        int lastVisibleView = linearLayoutManager.findLastVisibleItemPosition();
        int firstVisibleView = linearLayoutManager.findFirstVisibleItemPosition();
        View firstView = linearLayoutManager.findViewByPosition(firstVisibleView);
        View lastView = linearLayoutManager.findViewByPosition(lastVisibleView);

//these variables get the distance you need to scroll in order to center your views.
//my views have variable sizes, so I need to calculate side margins separately.
//note the subtle difference in how right and left margins are calculated, as well as
//the resulting scroll distances.

        int leftMargin = (screenWidth - lastView.getWidth()) / 2;
        int rightMargin = (screenWidth - firstView.getWidth()) / 2 + firstView.getWidth();
        int leftEdge = lastView.getLeft();
        int rightEdge = firstView.getRight();
        int scrollDistanceLeft = leftEdge - leftMargin;
        int scrollDistanceRight = rightMargin - rightEdge;

//if(user swipes to the left)
        if (velocityX > 0) smoothScrollBy(scrollDistanceLeft, 0);
        else smoothScrollBy(-scrollDistanceRight, 0);

        return true;
    }
}

Works awesome for my requirements. Hope it helps someone else too.

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