/**
 * A ViewHolder describes an item view and metadata about its place within the RecyclerView.
 *
 * <p>{@link Adapter} implementations should subclass ViewHolder and add fields for caching
 * potentially expensive {@link View#findViewById(int)} results.</p>
 *
 * <p>While {@link LayoutParams} belong to the {@link LayoutManager},
 * {@link ViewHolder ViewHolders} belong to the adapter. Adapters should feel free to use
 * their own custom ViewHolder implementations to store data that makes binding view contents
 * easier. Implementations should assume that individual item views will hold strong references
 * to <code>ViewHolder</code> objects and that <code>RecyclerView</code> instances may hold
 * strong references to extra off-screen item views for caching purposes</p>
 */
public static abstract class ViewHolder {
 public final View itemView;

 .......

 public ViewHolder(View itemView) {
  if (itemView == null) {
   throw new IllegalArgumentException("itemView may not be null");
  }
  this.itemView = itemView;
 }

 .......
}

public class XRViewHolder extends RecyclerView.ViewHolder {
 public XRViewHolder(View view) {
  super(view);
  ButterKnife.bind(this, view);
 }
 .......
 public void onBindViewHolder(VH holder, final int position) {
  if (position >= 0) {
   holder.itemView.setOnClickListener(new OnClickListener() {
    @Override
    public void onClick(View v) {
     itemClick(getItem(position), position);
    }
   });
  }
 }
}

public class XRecyclerView extends RecyclerView {

 private WrapAdapter mWrapAdapter;
 ......

 public void addHeaderView(View view) {
 sHeaderTypes.add(HEADER_INIT_INDEX + mHeaderViews.size());
 mHeaderViews.add(view);
 if (mWrapAdapter != null) {
  mWrapAdapter.notifyDataSetChanged();
 }
 }

 ......
 private class WrapAdapter extends RecyclerView.Adapter<ViewHolder> {

 @Override
 public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
  if (viewType == TYPE_REFRESH_HEADER) {
   return new SimpleViewHolder(mRefreshHeader);
  } else if (isHeaderType(viewType)) {
   return new SimpleViewHolder(getHeaderViewByType(viewType));
  } else if (viewType == TYPE_FOOTER) {
   return new SimpleViewHolder(mFootView);
  }
  return adapter.onCreateViewHolder(parent, viewType);
 }
}