A SimpleSectionedRecyclerViewAdapter: use this class to realize a simple sectioned `RecyclerView.Adapter`.

Readme.md

You can use this class to realize a simple sectioned RecyclerView.Adapter without changing your code.

The RecyclerView should use a LinearLayoutManager. You can use this code also with the TwoWayView with the ListLayoutManager (https://github.com/lucasr/twoway-view)

This is a porting of the class SimpleSectionedListAdapter provided by Google

Example:

        //Your RecyclerView
        mRecyclerView = (RecyclerView) findViewById(R.id.list);
        mRecyclerView.setHasFixedSize(true);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
        mRecyclerView.addItemDecoration(new DividerItemDecoration(this,LinearLayoutManager.VERTICAL));
        
        //Your RecyclerView.Adapter
        mAdapter = new SimpleAdapter(this,sCheeseStrings);


        //This is the code to provide a sectioned list
        List<SimpleSectionedRecyclerViewAdapter.Section> sections =
                new ArrayList<SimpleSectionedRecyclerViewAdapter.Section>();
        
        //Sections
        sections.add(new SimpleSectionedRecyclerViewAdapter.Section(0,"Section 1"));
        sections.add(new SimpleSectionedRecyclerViewAdapter.Section(5,"Section 2"));
        sections.add(new SimpleSectionedRecyclerViewAdapter.Section(12,"Section 3"));
        sections.add(new SimpleSectionedRecyclerViewAdapter.Section(14,"Section 4"));
        sections.add(new SimpleSectionedRecyclerViewAdapter.Section(20,"Section 5"));

        //Add your adapter to the sectionAdapter
        SimpleSectionedRecyclerViewAdapter.Section[] dummy = new SimpleSectionedRecyclerViewAdapter.Section[sections.size()];
        SimpleSectionedRecyclerViewAdapter mSectionedAdapter = new
                  SimpleSectionedRecyclerViewAdapter(this,R.layout.section,R.id.section_text,mAdapter);
        mSectionedAdapter.setSections(sections.toArray(dummy));

        //Apply this adapter to the RecyclerView
        mRecyclerView.setAdapter(mSectionedAdapter);

You can customize the section layout, changing the layout section.xml and changing the code in the SimpleSectionedRecyclerViewAdapter.SectionViewHolder class and SimpleSectionedRecyclerViewAdapter#onBindViewHolder method.

section.xml

<?xml version="1.0" encoding="utf-8"?>

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
          android:layout_width="match_parent"
          android:layout_height="48dp"
          android:gravity="center_vertical"
          android:paddingLeft="16dp"
          android:singleLine="true"
          android:textAllCaps="true"
          android:textColor="@color/demo_theme_status_bar_color"
          android:background="@android:color/transparent"
          android:textSize="16sp"
          android:id="@+id/section_text"
          android:textStyle="bold" />

SimpleAdapter.java

public class SimpleAdapter extends RecyclerView.Adapter<SimpleAdapter.SimpleViewHolder> {

    private final Context mContext;
    private List<String> mData;

    public void add(String s,int position) {
        position = position == -1 ? getItemCount()  : position;
        mData.add(position,s);
        notifyItemInserted(position);
    }

    public void remove(int position){
        if (position < getItemCount()  ) {
            mData.remove(position);
            notifyItemRemoved(position);
        }
    }

    public static class SimpleViewHolder extends RecyclerView.ViewHolder {
        public final TextView title;

        public SimpleViewHolder(View view) {
            super(view);
            title = (TextView) view.findViewById(R.id.simple_text);
        }
    }

    public SimpleAdapter(Context context, String[] data) {
        mContext = context;
        if (data != null)
            mData = new ArrayList<String>(Arrays.asList(data));
        else mData = new ArrayList<String>();
    }

    public SimpleViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        final View view = LayoutInflater.from(mContext).inflate(R.layout.simple_item, parent, false);
        return new SimpleViewHolder(view);
    }

    @Override
    public void onBindViewHolder(SimpleViewHolder holder, final int position) {
        holder.title.setText(mData.get(position));
        holder.title.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(mContext,"Position ="+position,Toast.LENGTH_SHORT).show();
            }
        });
    }

    @Override
    public int getItemCount() {
        return mData.size();
    }
}

SimpleSectionedRecyclerViewAdapter.java

public class SimpleSectionedRecyclerViewAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> {

    private final Context mContext;
    private static final int SECTION_TYPE = 0;

    private boolean mValid = true;
    private int mSectionResourceId;
    private int mTextResourceId;
    private LayoutInflater mLayoutInflater;
    private RecyclerView.Adapter mBaseAdapter;
    private SparseArray<Section> mSections = new SparseArray<Section>();


    public SimpleSectionedRecyclerViewAdapter(Context context, int sectionResourceId, int textResourceId,
                                              RecyclerView.Adapter baseAdapter) {

        mLayoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        mSectionResourceId = sectionResourceId;
        mTextResourceId = textResourceId;
        mBaseAdapter = baseAdapter;
        mContext = context;

        mBaseAdapter.registerAdapterDataObserver(new RecyclerView.AdapterDataObserver() {
            @Override
            public void onChanged() {
                mValid = mBaseAdapter.getItemCount()>0;
                notifyDataSetChanged();
            }

            @Override
            public void onItemRangeChanged(int positionStart, int itemCount) {
                mValid = mBaseAdapter.getItemCount()>0;
                notifyItemRangeChanged(positionStart, itemCount);
            }

            @Override
            public void onItemRangeInserted(int positionStart, int itemCount) {
                mValid = mBaseAdapter.getItemCount()>0;
                notifyItemRangeInserted(positionStart, itemCount);
            }

            @Override
            public void onItemRangeRemoved(int positionStart, int itemCount) {
                mValid = mBaseAdapter.getItemCount()>0;
                notifyItemRangeRemoved(positionStart, itemCount);
            }
        });
    }


    public static class SectionViewHolder extends RecyclerView.ViewHolder {

        public TextView title;

        public SectionViewHolder(View view,int mTextResourceid) {
            super(view);
            title = (TextView) view.findViewById(mTextResourceid);
        }
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int typeView) {
        if (typeView == SECTION_TYPE) {
            final View view = LayoutInflater.from(mContext).inflate(mSectionResourceId, parent, false);
            return new SectionViewHolder(view,mTextResourceId);
        }else{
            return mBaseAdapter.onCreateViewHolder(parent, typeView -1);
        }
    }

    @Override
    public void onBindViewHolder(RecyclerView.ViewHolder sectionViewHolder, int position) {
        if (isSectionHeaderPosition(position)) {
            ((SectionViewHolder)sectionViewHolder).title.setText(mSections.get(position).title);
        }else{
            mBaseAdapter.onBindViewHolder(sectionViewHolder,sectionedPositionToPosition(position));
        }

    }

    @Override
    public int getItemViewType(int position) {
        return isSectionHeaderPosition(position)
                ? SECTION_TYPE
                : mBaseAdapter.getItemViewType(sectionedPositionToPosition(position)) +1 ;
    }


    public static class Section {
        int firstPosition;
        int sectionedPosition;
        CharSequence title;

        public Section(int firstPosition, CharSequence title) {
            this.firstPosition = firstPosition;
            this.title = title;
        }

        public CharSequence getTitle() {
            return title;
        }
    }


    public void setSections(Section[] sections) {
        mSections.clear();

        Arrays.sort(sections, new Comparator<Section>() {
            @Override
            public int compare(Section o, Section o1) {
                return (o.firstPosition == o1.firstPosition)
                        ? 0
                        : ((o.firstPosition < o1.firstPosition) ? -1 : 1);
            }
        });

        int offset = 0; // offset positions for the headers we're adding
        for (Section section : sections) {
            section.sectionedPosition = section.firstPosition + offset;
            mSections.append(section.sectionedPosition, section);
            ++offset;
        }

        notifyDataSetChanged();
    }

    public int positionToSectionedPosition(int position) {
        int offset = 0;
        for (int i = 0; i < mSections.size(); i++) {
            if (mSections.valueAt(i).firstPosition > position) {
                break;
            }
            ++offset;
        }
        return position + offset;
    }

    public int sectionedPositionToPosition(int sectionedPosition) {
        if (isSectionHeaderPosition(sectionedPosition)) {
            return RecyclerView.NO_POSITION;
        }

        int offset = 0;
        for (int i = 0; i < mSections.size(); i++) {
            if (mSections.valueAt(i).sectionedPosition > sectionedPosition) {
                break;
            }
            --offset;
        }
        return sectionedPosition + offset;
    }

    public boolean isSectionHeaderPosition(int position) {
        return mSections.get(position) != null;
    }


    @Override
    public long getItemId(int position) {
        return isSectionHeaderPosition(position)
                ? Integer.MAX_VALUE - mSections.indexOfKey(position)
                : mBaseAdapter.getItemId(sectionedPositionToPosition(position));
    }

    @Override
    public int getItemCount() {
        return (mValid ? mBaseAdapter.getItemCount() + mSections.size() : 0);
    }

}

Great idea, but maybe you could add a license so that I can be copy it without the risk of a copyright infringement?

what it's the mean that code "sCheeseStrings" ?

How could I expand the sections? There is a way : I click in the section and remove all their items and viceversa

Is there a better way? Thanks!

Will this work for HORIZONTAL LinearLayoutManager?

how can we put a search feature into this? it is going to iterate over headers too?