Some Recyclerview Stuff

InterpolatorSmoothScroller.java

package com.lacronicus;

import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.View;
import android.view.animation.Interpolator;

import java.util.Random;

/**
 * Created by fdoyle on 3/8/16.
 */
public class InterpolatorSmoothScroller extends RecyclerView.SmoothScroller {
    Interpolator interpolator;
    boolean shouldStart = true;
    RecyclerView recyclerView;


    public InterpolatorSmoothScroller(Interpolator interpolator, RecyclerView recyclerView) {
        this.interpolator = interpolator;
        this.recyclerView = recyclerView;
    }

    @Override
    protected void onStart() {
        shouldStart = true;
    }

    @Override
    protected void onStop() {
    }

    @Override
    protected void onSeekTargetStep(int dx, int dy, RecyclerView.State state, Action action) {
        scroll(state, action, 3000);
    }

    @Override
    protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
        scroll(state, action, 1000);
    }

    private void scroll(RecyclerView.State state, Action action, int duration) {
        if(state.didStructureChange() || shouldStart) {
            int targetIndex = state.getTargetScrollPosition();
            View currentView = recyclerView.getLayoutManager().getChildAt(0);
            int currentIndex = recyclerView.getLayoutManager().getPosition(recyclerView.getChildAt(0));
            int yDelta = currentView.getHeight() * (targetIndex - currentIndex) + (int) currentView.getY();
            int xDelta = currentView.getWidth() * (targetIndex - currentIndex) + (int) currentView.getX();

            action.setDy(yDelta);
            action.setDx(xDelta);
            if(Math.abs(xDelta) < 10 && Math.abs(yDelta) < 10) {
                action.setDuration(1);
            } else {
                action.setDuration(duration);
                action.setInterpolator(interpolator);
            }
            shouldStart = false;
            stop();
        }
    }

    public int getAbsYDelta(int target) {
        View currentView = recyclerView.getLayoutManager().getChildAt(0);
        int currentIndex = recyclerView.getLayoutManager().getPosition(recyclerView.getChildAt(0));
        return Math.abs(currentView.getHeight() * (target - currentIndex) + (int) currentView.getY());
    }

    public int getAbsXDelta(int target) {
        View currentView = recyclerView.getLayoutManager().getChildAt(0);
        int currentIndex = recyclerView.getLayoutManager().getPosition(recyclerView.getChildAt(0));
        return Math.abs(currentView.getWidth() * (target - currentIndex) + (int) currentView.getX());

    }

    public boolean wouldMoveSignificantly() {
        return getAbsXDelta(getTargetPosition()) > 3 || getAbsYDelta(getTargetPosition()) > 3;
    }
}

SnapScrollListener.java

package com.lacronicus;

import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.animation.DecelerateInterpolator;
import android.view.animation.Interpolator;

/**
 * Created by fdoyle on 3/8/16.
 */
public class SnapScrollListener extends RecyclerView.OnScrollListener {
    RecyclerView listView;

    public SnapScrollListener(RecyclerView listView) {
        this.listView = listView;
    }

    @Override
    public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
        if (recyclerView.getChildCount() > 2)
            return;
        if (newState == RecyclerView.SCROLL_STATE_IDLE) {
            Interpolator interpolator = new DecelerateInterpolator();
            InterpolatorSmoothScroller scroller = new InterpolatorSmoothScroller(interpolator, listView);
            if (listView.getLayoutManager() instanceof LinearLayoutManager) {
                LinearLayoutManager layoutManager = (LinearLayoutManager) listView.getLayoutManager();
                if (layoutManager.getOrientation() == LinearLayoutManager.VERTICAL) {
                    boolean pastHalfWayVertical = Math.abs(listView.getChildAt(0).getY()) < listView.getChildAt(0).getHeight() / 2;
                    scroller.setTargetPosition(listView.getLayoutManager().getPosition(listView.getChildAt(pastHalfWayVertical ? 0 : 1)));
                } else {
                    boolean pastHalfWayHorizontal = Math.abs(listView.getChildAt(0).getX()) < listView.getChildAt(0).getWidth() / 2;
                    scroller.setTargetPosition(listView.getLayoutManager().getPosition(listView.getChildAt(pastHalfWayHorizontal ? 0 : 1)));
                }
                if (scroller.wouldMoveSignificantly()) { // don't animate if you're already on your target (this counts as "settling" to the recyclerview, so if you animate when you don't need to, you'll start another animation when it goes back to idle, looping forever. Android has OCD)
                    layoutManager.startSmoothScroll(scroller);
                }
            } else {
                throw new IllegalStateException("This only works with LinearLayoutManagers");
            }
        }
    }

}

The first class is a SmoothScroller that takes a LinearLayoutManager based Recyclerview where all items are equal height and allows you to smooth scroll using any interpolator. if your interpolator tries to scroll past bounds, the whole thing breaks, so don't use any of those.

The second class uses the first to provide a "snapping" effect, where the recyclerview aligns the top edge of the first item to the top of the recycler. Also works horizontally. If your items are full height or full width, it'll work kinda like a viewpager.