A Recycler View Item Decorator that adds sticky headers.

StickyHeaderItemDecorator.java

import android.graphics.Canvas;
import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.view.View;

public abstract class StickHeaderItemDecorator extends RecyclerView.ItemDecoration {
  
    /* Make your view holder implement this interface */
    public interface HeaderRelationship {
        /* Return true if this object is located directly after a header */
        public boolean isDirectlyAfterHeader();
    }
  
    private int headerHeight;

    public StickHeaderItemDecorator(int headerHeight) {
        this.headerHeight = headerHeight;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        outRect.set(0, 0, 0, 0);

        RecyclerView.ViewHolder viewHolder = parent.getChildViewHolder(view);

        if (isDirectlyAfterHeader(viewHolder)) {
            outRect.top = headerHeight;
        }
    }

    private boolean isDirectlyAfterHeader(RecyclerView.ViewHolder viewHolder) {
        return viewHolder != null
                && viewHolder instanceof HeaderRelationship
                && ((HeaderRelationship) viewHolder).isDirectlyAfterHeader();
    }

    /* Draw your sticky header inside of "headerBounds" using the "viewHolder." */
    protected abstract void drawHeader(Canvas c, RecyclerView parent, RecyclerView.State state, Rect headerBounds, RecyclerView.ViewHolder viewHolder);

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);

        Rect headerBounds = new Rect();

        int highestHeaderTop = Integer.MAX_VALUE;
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();

        int childCount = parent.getChildCount();

        if(childCount == 0) return;

        for (int i = 0; i < childCount; ++i) {

            View view = parent.getChildAt(i);
            RecyclerView.ViewHolder viewHolder = parent.getChildViewHolder(view);

            if (!isDirectlyAfterHeader(viewHolder)) continue;

            int bottom = view.getTop();
            int top = bottom - headerHeight;

            if (top <= 0) continue;

            highestHeaderTop = Math.min(top, highestHeaderTop);
            headerBounds.set(left, top, right, bottom);
            drawHeader(c, parent, state, headerBounds, viewHolder);
        }

        int headerTop = highestHeaderTop > headerHeight ? 0 : highestHeaderTop - headerHeight;
        int headerBottom = headerTop + headerHeight;

        RecyclerView.ViewHolder viewHolder = parent.getChildViewHolder(parent.getChildAt(0));

        headerBounds.set(left, headerTop, right, headerBottom);
        drawHeader(c, parent, state, headerBounds, viewHolder);
    }
}