Light weight Sticky Headers RecyclerView Decoration

StickyHeaderInterface.java

import android.view.View;

public interface StickyHeaderInterface {

    /**
     * This method gets called by {@link StickyHeaderItemDecoration} to fetch the position of the header item in the adapter
     * that is used for (represents) item at specified position.
     *
     * @param itemPosition int. Adapter's position of the item for which to do the search of the position of the header item.
     * @return int. Position of the header item in the adapter.
     */
    int getHeaderPositionForItem(int itemPosition);

    /**
     * This method gets called by {@link StickyHeaderItemDecoration} to get layout resource id for the header item at specified adapter's position.
     *
     * @param headerPosition int. Position of the header item in the adapter.
     * @return int. Layout resource id.
     */
    int getHeaderLayout(int headerPosition);

    /**
     * This method gets called by {@link StickyHeaderItemDecoration} to setup the header View.
     *
     * @param header         View. Header to set the data on.
     * @param headerPosition int. Position of the header item in the adapter.
     */
    void bindHeaderData(View header, int headerPosition);

    /**
     * This method gets called by {@link StickyHeaderItemDecoration} to verify whether the item represents a header.
     *
     * @param itemPosition int.
     * @return true, if item at the specified adapter's position represents a header.
     */
    boolean isHeader(int itemPosition);
}

StickyHeaderItemDecoration.java

import android.graphics.Canvas;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class StickyHeaderItemDecoration extends RecyclerView.ItemDecoration {
    private StickyHeaderInterface adapter;
    private int stickyHeaderHeight;

    public StickyHeaderItemDecoration(RecyclerView recyclerView, @NonNull StickyHeaderInterface adapter, StickyHeaderTouchListener.OnHeaderClickListener listener) {
        this.adapter = adapter;

        StickyHeaderTouchListener touchListener = new StickyHeaderTouchListener(recyclerView, this);
        touchListener.setOnHeaderClickListener(listener);
        recyclerView.addOnItemTouchListener(touchListener);
    }

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

        View topChild = parent.getChildAt(0);
        if (topChild == null) {
            return;
        }

        int topChildPosition = parent.getChildAdapterPosition(topChild);
        if (topChildPosition == RecyclerView.NO_POSITION) {
            return;
        }

        View currentHeader = getHeaderViewForItem(topChildPosition, parent);
        fixLayoutSize(parent, currentHeader);
        int contactPoint = currentHeader.getBottom();
        View childInContact = getChildInContact(parent, contactPoint);
        if (childInContact == null) {
            return;
        }

        if (adapter.isHeader(parent.getChildAdapterPosition(childInContact))) {
            moveHeader(c, currentHeader, childInContact);
            return;
        }

        drawHeader(c, currentHeader);
    }

    public boolean isInStickyHeaderArea(int x, int y) {
        return y <= stickyHeaderHeight;
    }

    private View getHeaderViewForItem(int itemPosition, RecyclerView parent) {
        int headerPosition = adapter.getHeaderPositionForItem(itemPosition);
        int layoutResId = adapter.getHeaderLayout(headerPosition);
        View header = LayoutInflater.from(parent.getContext()).inflate(layoutResId, parent, false);
        adapter.bindHeaderData(header, headerPosition);
        return header;
    }

    private void drawHeader(Canvas c, View header) {
        c.save();
        c.translate(0, 0);
        header.draw(c);
        c.restore();
    }

    private void moveHeader(Canvas c, View currentHeader, View nextHeader) {
        c.save();
        c.translate(0, nextHeader.getTop() - currentHeader.getHeight());
        currentHeader.draw(c);
        c.restore();
    }

    private View getChildInContact(RecyclerView parent, int contactPoint) {
        View childInContact = null;
        for (int i = 0; i < parent.getChildCount(); i++) {
            View child = parent.getChildAt(i);
            if (child.getBottom() > contactPoint) {
                if (child.getTop() <= contactPoint) {
                    // This child overlaps the contactPoint
                    childInContact = child;
                    break;
                }
            }
        }
        return childInContact;
    }

    /**
     * Properly measures and layouts the top sticky header.
     *
     * @param parent ViewGroup: RecyclerView in this case.
     */
    private void fixLayoutSize(ViewGroup parent, View view) {

        // Specs for parent (RecyclerView)
        int widthSpec = View.MeasureSpec.makeMeasureSpec(parent.getWidth(), View.MeasureSpec.EXACTLY);
        int heightSpec = View.MeasureSpec.makeMeasureSpec(parent.getHeight(), View.MeasureSpec.UNSPECIFIED);

        // Specs for children (headers)
        int childWidthSpec = ViewGroup.getChildMeasureSpec(widthSpec, parent.getPaddingLeft() + parent.getPaddingRight(), view.getLayoutParams().width);
        int childHeightSpec = ViewGroup.getChildMeasureSpec(heightSpec, parent.getPaddingTop() + parent.getPaddingBottom(), view.getLayoutParams().height);

        view.measure(childWidthSpec, childHeightSpec);

        view.layout(0, 0, view.getMeasuredWidth(), stickyHeaderHeight = view.getMeasuredHeight());
    }
}

StickyHeaderTouchListener.java

import android.support.v7.widget.RecyclerView;
import android.view.GestureDetector;
import android.view.MotionEvent;

public class StickyHeaderTouchListener implements RecyclerView.OnItemTouchListener {
    private final GestureDetector gestureDetector;
    private final StickyHeaderItemDecoration decoration;
    private OnHeaderClickListener onHeaderClickListener;

    public interface OnHeaderClickListener {
        void onStickyHeaderClick(int touchX, int touchY);
    }

    public StickyHeaderTouchListener(final RecyclerView recyclerView,
                                     final StickyHeaderItemDecoration decor) {
        gestureDetector = new GestureDetector(recyclerView.getContext(), new SingleTapDetector());
        decoration = decor;
    }

    public void setOnHeaderClickListener(OnHeaderClickListener listener) {
        onHeaderClickListener = listener;
    }

    @Override
    public boolean onInterceptTouchEvent(RecyclerView view, MotionEvent e) {
        if (this.onHeaderClickListener != null) {
            boolean tapDetectorResponse = this.gestureDetector.onTouchEvent(e);
            if (tapDetectorResponse) {
                // Don't return false if a single tap is detected
                return true;
            }
            if (e.getAction() == MotionEvent.ACTION_DOWN) {
                return decoration.isInStickyHeaderArea((int) e.getX(), (int) e.getY());
            }
        }
        return false;
    }

    @Override
    public void onTouchEvent(RecyclerView view, MotionEvent e) {
    }

    @Override
    public void onRequestDisallowInterceptTouchEvent(boolean disallowIntercept) {
    }

    private class SingleTapDetector extends GestureDetector.SimpleOnGestureListener {
        @Override
        public boolean onSingleTapUp(MotionEvent e) {
            boolean inTouchArea = decoration.isInStickyHeaderArea((int) e.getX(), (int) e.getY());
            if (inTouchArea) {
                onHeaderClickListener.onStickyHeaderClick((int) e.getX(), (int) e.getY());
                return true;
            }
            return false;
        }

        @Override
        public boolean onDoubleTap(MotionEvent e) {
            return true;
        }
    }
}

Mainly inspired by answer at: https://stackoverflow.com/questions/32949971/how-can-i-make-sticky-headers-in-recyclerview-without-external-lib

And the touch event handling part is take reference from: https://github.com/timehop/sticky-headers-recyclerview

Note: do not use margin in the header layout, use padding instead.
Otherwise the decoration will not take margin, and the push up animation between two headers will look junky.