RecyclerView is an important graphical UI component in any android application. Android provides RecyclerView.Adapter class which manages all the functionality of RecyclerView. I don’t know why but android people have kept this class in a very abstract form with only basic functionalities implemented by default. On the plus side it opens many doors for custom adapters with new functionalities for example, sticky headers, scroll indicator, drag and drop actions on items, multiview types items etc. A developer should be able to make an adapter of his need by extending RecyclerView.Adapter. There are many custom adapters developers have shared which comes with built in functionalities. FastAdapter is one of them which comes with all the good functionalities built in and also it is very easy to use. I just got to use this in the Open Event Organizer Android App of which the core feature is Attendees Check In. We have used FastAdapter library to show attendees list which needs many features which are absent in plane RecyclerView.Adapter. FastAdapter is built in such way that there are many different ways of using it on developer’s need. I have found a simplest way which I will be sharing here. The first part is extending the item model to inherit AbstractItem.

public class Attendee extends AbstractItem<Attendee, AttendeeViewHolder> {
  @PrimaryKey
  private long id;
  ...
  ...

  @Override
  public long getIdentifier() {
      return id;
  }

  @Override
  public int getType() {
      return 0;
  }

  @Override
  public int getLayoutRes() {
      return R.layout.attendee_layout;
  }

  @Override
  public AttendeeViewHolder getViewHolder(View view) {
      return new AttendeeViewHolder(DataBindingUtil.bind(view));
  }

  @Override
  public void bindView(AttendeeViewHolder holder, List<Object> list) {
      super.bindView(holder, list);
      holder.bindAttendee(this);
  }

  @Override
  public void unbindView(AttendeeViewHolder holder) {
      super.unbindView(holder);
      holder.unbindAttendee();
  }
}

The methods are pretty obvious by name. Implement these methods accordingly. You may notice that we have used Databinding here to bind data to views but it is not necessary. Also you will have to create your ViewHolder for adapter. You can either use RecyclerView.ViewHolder or you can just create a custom one by inheriting it as per your need. Once this part is over you are half done as most of the things are been taken care in model itself. Now we will be writing code for adapter and setting it to your RecyclerView.

FastItemAdapter<Attendee> fastItemAdapter = new FastItemAdapter<>();
fastItemAdapter.setHasStableIds(true);
...
// functionalities related code
...
recyclerView.setAdapter(fastItemAdapter);

Initialize FastItemAdapter which will be our main adapter handling all the direct functions related to the RecyclerView. Set up some boolean constants according to the project need. In our project we have Attendee model which has id as a primary field. FastItemAdapter can take advantage of distinct field of the model called as identifier . Hence it is set true as Attendee model has id field. But you should be careful about setting it to True as then you must have implemented getIdentifier in the model to return correct field which will be used as an identifier by our adapter. And the adapter is good to set to the RecyclerView.

Now we got to decide which functionalities we will be implementing to our RecyclerView. In our case we needed: 1. Search filter for attendees, 2. Sticky header for attendees groups arranged alphabetically and 3. On click listener for attendee item.

FastItemAdapter has ItemFilter adapter wrapped inside which manages all the filtering stuff. Filtering logic can be set using it.

fastItemAdapter.getItemFilter().withFilterPredicate(this::shallFilter);

Where shallFilter is method which takes attendee object and returns boolean whether to filter the item or not. And after this you can use FastItemAdapter’s filter method to filter the items. For sticky headers you need to implement StickyRecyclerHeadersAdapter extending AbstractAdapter. In this class you will have to implement your filter logic in getHeaderId method. This must return an unique id for items of the same group.

@Override
public long getHeaderId(int position) {
   IItem item = getItem(position);
   if(item instanceof Attendee && ((Attendee)item).getFirstName() != null)
       return ((Attendee) item).getFirstName().toUpperCase().charAt(0);
   return -1;
}

Like in this case we have grouped attendees alphabetically hence just returning initial character’s ASCII value will do good. You can modify this method according to your need. For other unimplemented methods just keep their default return values. With this you will also have to implement onCreateHeaderViewHolder and onBindHeaderViewHolder methods to bind view and data to the header layout. Once this is done you are ready to set sticky headers to your RecyclerView with following code:

stickyHeaderAdapter = new StickyHeaderAdapter();
final HeaderAdapter headerAdapter = new HeaderAdapter();
recyclerView.setAdapter(stickyHeaderAdapter.wrap((headerAdapter.wrap(fastItemAdapter))));

final StickyRecyclerHeadersDecoration decoration = new StickyRecyclerHeadersDecoration(stickyHeaderAdapter);
recyclerView.addItemDecoration(decoration);
adapterDataObserver = new RecyclerView.AdapterDataObserver() {
   @Override
   public void onChanged() {
       decoration.invalidateHeaders();
   }
};
stickyHeaderAdapter.registerAdapterDataObserver(adapterDataObserver);

For click listener, the code is similar to the RecyclerView.Adapter’s one.

fastItemAdapter.withOnClickListener(new FastAdapter.OnClickListener<Item>() {
  @Override
  public boolean onClick(View v, IAdapter<Item> adapter, Item item, int position) {
     // your on click logic
   return true;
  }
});

With this now you have successfully implemented FastItemAdapter to your RecyclerView. Although there are some important points to be taken care of. If you are using filter in your application then you will have to modify your updateItem logic. As when filter is applied to the adapter its items list is filtered. And if you are updating the item using its position from original list it then it will result in exception or updating the wrong item. So you will have to change the position to the one in filtered list. For example the updateAttendee method from Organizer App code looks like this:

public void updateAttendee(int position, Attendee attendee) {
   position = fastItemAdapter.getAdapterPosition(attendee);
   fastItemAdapter.getItemFilter().set(position, attendee);
}