Sample App (Using Firebase Recycler Pagination Library)

Maven Setup

    repositories {
        jcenter()
    }

Gradle Setup

dependencies {

    //RecyclerView
    implementation 'com.android.support:recyclerview-v7:28.0.0'

    //Firebase Database
    implementation 'com.google.firebase:firebase-database:16.1.0'
    implementation 'com.google.firebase:firebase-core:16.0.7'

    //Firebase-UI Library
    implementation 'com.firebaseui:firebase-ui-database:4.3.1'

    //Android Paging Libray
    implementation "android.arch.paging:runtime:1.0.1"

    //Firebase Pagination Library
    implementation 'com.shreyaspatil:FirebaseRecyclerPagination:1.0.1'
}

App Setup

In this app, you are showing paginated list of Posts. Posts will load in RecyclerView

Data Model Class (Post.java)

public class Post {
    public String title;
    public String body;

    public Post(){}

    public Post(String title, String body) {
        this.title = title;
        this.body = body;
    }
}

MainActivity.java

Declarations

public class MainActivity extends AppCompatActivity {

    private RecyclerView mRecyclerView;
    private DatabaseReference mDatabase;

    private SwipeRefreshLayout mSwipeRefreshLayout;

    FirebaseRecyclerPagingAdapter<Post, PostViewHolder> mAdapter;

Initialization

   @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_main);

           mSwipeRefreshLayout = findViewById(R.id.swipe_refresh_layout);

           //Initialize RecyclerView
           mRecyclerView = findViewById(R.id.recycler_view);
           mRecyclerView.setHasFixedSize(true);

           LinearLayoutManager mManager = new LinearLayoutManager(this);
           mRecyclerView.setLayoutManager(mManager);

           //Initialize Database
           mDatabase = FirebaseDatabase.getInstance().getReference().child("posts");

Don't forget to set LayoutManager to the RecyclerView.
Set it using RecyclerView#setLayoutManager()

Setup Configuration for PagedList

First of all configure a PagedList
Remember that, the size you will pass to setPageSize() method will load x3 items of that size.

        PagedList.Config config = new PagedList.Config.Builder()
                .setEnablePlaceholders(false)
                .setPrefetchDistance(5)
                .setPageSize(10)
                .build();

Then Configure Adapter by building FirebasePagingOptions. It will generic.
Remember one thing, don't pass Query with orderByKey(), limitToFirst() or limitToLast(). This will cause an error.

 DatabasePagingOptions<Post> options = new DatabasePagingOptions.Builder<Post>()
                .setLifecycleOwner(this)
                .setQuery(mDatabase, config, Post.class)
                .build();

Init Adapter

FirebaseRecyclerPagingAdapter is built on the top of Android Architecture Components - Paging Support Library. To implement, you should already have RecyclerView.ViewHolder subclass. Here We used PostViewHolder class.

        mAdapter = new FirebaseRecyclerPagingAdapter<Post, PostViewHolder>(options) {
            @NonNull
            @Override
            public PostViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
                return new PostViewHolder(LayoutInflater.from(parent.getContext()).inflate(R.layout.item_list, parent, false));
            }

            @Override
            protected void onBindViewHolder(@NonNull PostViewHolder holder,
                                         int position,
                                         @NonNull Post model) {

                holder.setItem(model);
            }

            @Override
            protected void onLoadingStateChanged(@NonNull LoadingState state) {
                switch (state) {
                    case LOADING_INITIAL:
                    case LOADING_MORE:
                        // Do your loading animation
                        mSwipeRefreshLayout.setRefreshing(true);
                        break;

                    case LOADED:
                        // Stop Animation
                        mSwipeRefreshLayout.setRefreshing(false);
                        break;

                    case FINISHED:
                        //Reached end of Data set
                        mSwipeRefreshLayout.setRefreshing(false);
                        break;

                    case ERROR:
                        retry();
                        break;
                }
        };

Get Child Reference

To get the reference of child from list, FirebaseRecyclerPagingAdapter has method called getRef(). You can obtain DatabaseReference of the child using it.
Get it using FirebaseRecyclerPagingAdapter#getRef() For e.g.

            @Override
            protected void onBindViewHolder(@NonNull PostViewHolder holder,
                                         int position,
                                         @NonNull Post model) {

                DatabaseReference reference = getRef(position);
            }

Error Handling

To get to know about DatabaseError caught during Paging, Override onError() method in adapter.

            @Override
            protected void onError(@NonNull DatabaseError databaseError) {
                mSwipeRefreshLayout.setRefreshing(false);
                databaseError.toException().printStackTrace();
                // Handle Error

            }

Retrying List (After Error / Failure)

To retry items loading in RecyclerView, retry() method from Adapter class is used.
Use it as FirebaseRecyclerPagingAdapter#retry().
This method should be used only after caught in Error. retry() should not be invoked anytime other than ERROR state.
Whenever LoadingState becomes LoadingState.ERROR we can use retry() to load items in RecyclerView which were unable to load due to recent failure/error and to maintain Paging List stable.
See demo for method.

        @Override
        protected void onError(@NonNull DatabaseError databaseError) {
            retry();          
        }

Or outside FirebaseRecyclerPagingAdapter

        mAdapter.retry();

Refreshing List

To refresh items in RecyclerView, refresh() method from Adapter class is used.
Use it as FirebaseRecyclerPagingAdapter#refresh().
This method clears all the items in RecyclerView and reloads the data again from beginning.
See demo for method.

        mSwipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
            @Override
            public void onRefresh() {
                mAdapter.refresh();
            }
        });

Set Adapter

Finally, Set adapter to RecyclerView.

        mRecyclerView.setAdapter(mAdapter);

Lifecycle

At last, To begin populating data, call startListening() method. stopListening() stops the data being loaded.

//Start Listening Adapter
    @Override
    protected void onStart() {
        super.onStart();
        mAdapter.startListening();
    }

    //Stop Listening Adapter
    @Override
    protected void onStop() {
        super.onStop();
        mAdapter.stopListening();
    }

Thus, we have implemented Firebase Recycler Pagination. Thank You !