This decorator allows you to easily create section headers for RecyclerViews using a LinearLayoutManager in either vertical or horizontal orientation.
Credit to Emil Sjölander for creating StickyListHeaders, a library that many of us relied on for sticky headers in our listviews.
Here is a quick video of it in action (click to see the full video):
Current version: 
compile 'com.timehop.stickyheadersrecyclerview:library:[latest.version.number]@aar'There are three main classes, StickyRecyclerHeadersAdapter, StickyRecyclerHeadersDecoration,
and StickyRecyclerHeadersTouchListener.
StickyRecyclerHeadersAdapter has a very similar interface to the RecyclerView.Adapter, and it
is recommended that you make your RecyclerView.Adapter implement StickyRecyclerHeadersAdapter.
There interface looks like this:
public interface StickyRecyclerHeadersAdapter<VH extends RecyclerView.ViewHolder> {
public long getHeaderId(int position);
public VH onCreateHeaderViewHolder(ViewGroup parent);
public void onBindHeaderViewHolder(VH holder, int position);
public int getItemCount();
}The second class, StickyRecyclerHeadersDecoration, is where most of the magic happens, and does
not require any configuration on your end. Here's an example from onCreate() in an activity:
mRecyclerView = (RecyclerView) findViewById(R.id.recyclerview);
mAdapter = new MyStickyRecyclerHeadersAdapter();
mRecyclerView.setAdapter(mAdapter);
mRecyclerView.setLayoutManager(new LinearLayoutManager(context));
mRecyclerView.addItemDecoration(new StickyRecyclerHeadersDecoration(mAdapter));StickyRecyclerHeadersTouchListener allows you to listen for clicks on header views.
Simply create an instance of StickyRecyclerHeadersTouchListener, set the OnHeaderClickListener,
and add the StickyRecyclerHeadersTouchListener as a touch listener to your RecyclerView.
StickyRecyclerHeadersTouchListener touchListener =
new StickyRecyclerHeadersTouchListener(recyclerView, headersDecor);
touchListener.setOnHeaderClickListener(
new StickyRecyclerHeadersTouchListener.OnHeaderClickListener() {
@Override
public void onHeaderClick(View header, int position, long headerId) {
Toast.makeText(MainActivity.this, "Header position: " + position + ", id: " + headerId,
Toast.LENGTH_SHORT).show();
}
});
mRecyclerView.addOnItemTouchListener(touchListener);The StickyHeaders aren't aware of your adapter so if you must notify them when your data set changes.
mAdapter.registerAdapterDataObserver(new RecyclerView.AdapterDataObserver() {
@Override public void onChanged() {
headersDecor.invalidateHeaders();
}
});If the Recyclerview's layout manager implements getExtraLayoutSpace (to preload more content then is visible for performance reasons), you must implement ItemVisibilityAdapter and pass an instance as a second argument to StickyRecyclerHeadersDecoration's constructor.
@Override
public boolean isPositionVisible(final int position) {
return layoutManager.findFirstVisibleItemPosition() <= position
&& layoutManager.findLastVisibleItemPosition() >= position;
}Item animators don't play nicely with RecyclerView decorations, so your mileage with that may vary.
API 11+
The header views aren't recycled at this time. Contributions are most welcome.
I haven't tested this with ItemAnimators yet.
The header views are drawn to a canvas, and are not actually a part of the view hierarchy. As such, they can't have touch states, and you may run into issues if you try to load images into them asynchronously.
0.4.3 (12/24/2015) - Change minSDK to 11, fix issue with header bounds caching
0.4.2 (8/21/2015) - Add support for reverse ReverseLayout in LinearLayoutManager by AntonPukhonin
0.4.1 (6/24/2015) - Fix "dancing headers" by DarkJaguar91
0.4.0 (4/16/2015) - Code reorganization by danoz73, fixes for different sized headers, performance improvements
0.3.6 (1/30/2015) - Prevent header clicks from passing on the touch event
0.3.5 (12/12/2014) - Add StickyRecyclerHeadersDecoration.invalidateHeaders() method
0.3.4 (12/3/2014) - Fix issues with rendering of header views with header ID = 0
0.3.3 (11/13/2014) - Fixes for padding, support views without headers
0.3.2 (11/1/2014) - Bug fixes for list items with margins and deleting items
0.2 (10/3/2014) - Add StickyRecyclerHeadersTouchListener
0.1 (10/2/2014) - Initial Release