FirebaseUI for Realtime Database

Imagine you have a chat app where each chat message is an item in the chats node of your database. In your app, you may represent a chat message like this:

public class Chat {
    private String mName;
    private String mMessage;
    private String mUid;

    public Chat() {}  // Needed for Firebase

    public Chat(String name, String message, String uid) {
        mName = name;
        mMessage = message;
        mUid = uid;
    }

    public String getName() { return mName; }

    public void setName(String name) { mName = name; }

    public String getMessage() { return mMessage; }

    public void setMessage(String message) { mMessage = message; }

    public String getUid() { return mUid; }

    public void setUid(String uid) { mUid = uid; }
}

A few things to note about this model class:

• The getters and setters follow the JavaBean naming pattern which allows Firebase to map the data to field names (ex: getName() provides the name field).
• The class has an empty constructor, which is required for Firebase's automatic data mapping.

For a properly constructed model class like the Chat class above, Firebase can perform automatic serialization in DatabaseReference#setValue() and automatic deserialization in DataSnapshot#getValue().

Querying

On the main screen of your app, you may want to show the 50 most recent chat messages. With Firebase you would use the following query:

Query query = FirebaseDatabase.getInstance()
        .getReference()
        .child("chats")
        .limitToLast(50);

To retrieve this data without FirebaseUI, you might use addChildEventListener to listen for live updates:

ChildEventListener childEventListener = new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String previousChildName) {
        // ...
    }

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String previousChildName) {
        // ...
    }

    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {
        // ...
    }

    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String previousChildName) {
        // ...
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        // ...
    }
};
query.addChildEventListener(childEventListener);

If you're displaying a list of data, you likely want to bind the Chat objects to a RecyclerView. This means implementing a custom RecyclerView.Adapter and coordinating updates with the ChildEventListener.

Fear not, FirebaseUI does all of this for you automatically!

Choosing an adapter

FirebaseUI offers two types of RecyclerView adapters for the Realtime Database:

• FirebaseRecyclerAdapter — binds a Query to a RecyclerView and responds to all real-time events included items being added, removed, moved, or changed. Best used with small result sets since all results are loaded at once.
• FirebaseRecyclerPagingAdapter — binds a Query to a RecyclerView by loading data in pages. Best used with large, static data sets. Real-time events are not respected by this adapter, so it will not detect new/removed items or changes to items already loaded.

Using the FirebaseRecyclerAdapter

The FirebaseRecyclerAdapter binds a Query to a RecyclerView. When data is added, removed, or changed these updates are automatically applied to your UI in real time.

First, configure the adapter by building FirebaseRecyclerOptions. In this case we will continue with our chat example:

 FirebaseRecyclerOptions<Chat> options =
                new FirebaseRecyclerOptions.Builder<Chat>()
                        .setQuery(query, Chat.class)
                        .build();

If you need to customize how your model class is parsed, you can use a custom SnapshotParser:

...setQuery(..., new SnapshotParser<Chat>() {
    @NonNull
    @Override
    public Chat parseSnapshot(@NonNull DataSnapshot snapshot) {
        return ...;
    }
});

Next create the FirebaseRecyclerAdapter object. You should already have a ViewHolder subclass for displaying each item. In this case we will use a custom ChatHolder class:

FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter<Chat, ChatHolder>(options) {
    @Override
    public ChatHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        // Create a new instance of the ViewHolder, in this case we are using a custom
        // layout called R.layout.message for each item
        View view = LayoutInflater.from(parent.getContext())
                .inflate(R.layout.message, parent, false);

        return new ChatHolder(view);
    }

    @Override
    protected void onBindViewHolder(ChatHolder holder, int position, Chat model) {
        // Bind the Chat object to the ChatHolder
        // ...
    }
};

Finally attach the adapter to your RecyclerView with the RecyclerView#setAdapter() method. Don't forget to also set a LayoutManager!

FirebaseRecyclerAdapter lifecycle

Start/stop listening

The FirebaseRecyclerAdapter uses an event listener to monitor changes to the Firebase query. To begin listening for data, call the startListening() method. You may want to call this in your onStart() method. Make sure you have finished any authentication necessary to read the data before calling startListening() or your query will fail.

@Override
protected void onStart() {
    super.onStart();
    adapter.startListening();
}

Similarly, the stopListening() call removes the event listener and all data in the adapter. Call this method when the containing Activity or Fragment stops:

@Override
protected void onStop() {
    super.onStop();
    adapter.stopListening();
}

Automatic listening

If you don't want to manually start/stop listening you can use Android Architecture Components to automatically manage the lifecycle of the FirebaseRecyclerAdapter. Pass a LifecycleOwner to FirebaseRecyclerOptions.Builder#setLifecycleOwner(...) and FirebaseUI will automatically start and stop listening in onStart() and onStop().

Data and error events

When using the FirebaseRecyclerAdapter you may want to perform some action every time data changes or when there is an error. To do this, override the onDataChanged() and onError() methods of the adapter:

FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter<Chat, ChatHolder>(options) {
    // ...

    @Override
    public void onDataChanged() {
        // Called each time there is a new data snapshot. You may want to use this method
        // to hide a loading spinner or check for the "no documents" state and update your UI.
        // ...
    }

    @Override
    public void onError(DatabaseError e) {
        // Called when there is an error getting data. You may want to update
        // your UI to display an error message to the user.
        // ...
    }
};

Using the FirebaseRecyclerPagingAdapter

The FirebaseRecyclerPagingAdapter binds a Query to a RecyclerView by loading documents in pages. This results in a time and memory efficient binding, however it gives up the real-time events afforded by the FirebaseRecyclerAdapter.

The FirebaseRecyclerPagingAdapter is built on top of the Android Paging 3 Library. Before using the adapter in your application, you must add a dependency on that library:

implementation 'androidx.paging:paging-runtime:3.x.x'

First, configure the adapter by building DatabasePagingOptions. Since the paging adapter is not appropriate for a chat application (it would not detect new messages), we will consider an adapter that loads a generic Item:

// The "base query" is a query with no startAt/endAt/limit clauses that the adapter can use
// to form smaller queries for each page.
Query baseQuery = mDatabase.getReference().child("items");

// This configuration comes from the Paging 3 Library
// https://developer.android.com/reference/kotlin/androidx/paging/PagingConfig
PagingConfig config = new PagingConfig(/* page size */ 20, /* prefetchDistance */ 10,
    /* enablePlaceHolders */ false);

// The options for the adapter combine the paging configuration with query information
// and application-specific options for lifecycle, etc.
DatabasePagingOptions<Item> options = new DatabasePagingOptions.Builder<Item>()
        .setLifecycleOwner(this)
        .setQuery(baseQuery, config, Item.class)
        .build();

If you need to customize how your model class is parsed, you can use a custom SnapshotParser:

...setQuery(..., new SnapshotParser<Item>() {
    @NonNull
    @Override
    public Item parseSnapshot(@NonNull DocumentSnapshot snapshot) {
        return ...;
    }
});

Next, create the FirebaseRecyclerPagingAdapter object. You should already have a ViewHolder subclass for displaying each item. In this case we will use a custom ItemViewHolder class:

FirebaseRecyclerPagingAdapter<Item, ItemViewHolder> adapter =
        new FirebaseRecyclerPagingAdapter<Item, ItemViewHolder>(options) {
            @NonNull
            @Override
            public ItemViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
                // Create the ItemViewHolder
                // ...
            }

            @Override
            protected void onBindViewHolder(@NonNull ItemViewHolder holder,
                                            int position,
                                            @NonNull Item model) {
                // Bind the item to the view holder
                // ...
            }
        };

Finally attach the adapter to your RecyclerView with the RecyclerView#setAdapter() method. Don't forget to also set a LayoutManager!

FirebaseRecyclerPagingAdapter lifecycle

Start/stop listening

The FirebaseRecyclerPagingAdapter listens for scrolling events and loads additional pages from the database only when needed.

To begin populating data, call the startListening() method. You may want to call this in your onStart() method. Make sure you have finished any authentication necessary to read the data before calling startListening() or your query will fail.

@Override
protected void onStart() {
    super.onStart();
    adapter.startListening();
}

Similarly, the stopListening() call freezes the data in the RecyclerView and prevents any future loading of data pages.

Call this method when the containing Activity or Fragment stops:

@Override
protected void onStop() {
    super.onStop();
    adapter.stopListening();
}

Automatic listening

If you don't want to manually start/stop listening you can use Android Architecture Components to automatically manage the lifecycle of the FirebaseRecyclerPagingAdapter. Pass a LifecycleOwner to DatabasePagingOptions.Builder#setLifecycleOwner(...) and FirebaseUI will automatically start and stop listening in onStart() and onStop().

Paging events

When using the FirebaseRecyclerPagingAdapter, you may want to perform some action every time data changes or when there is an error. To do this:

In Java

Use the addLoadStateListener method from the adapter:

adapter.addLoadStateListener(new Function1<CombinedLoadStates, Unit>() {
            @Override
            public Unit invoke(CombinedLoadStates states) {
                LoadState refresh = states.getRefresh();
                LoadState append = states.getAppend();

                if (refresh instanceof LoadState.Error || append instanceof LoadState.Error) {
                    // The previous load (either initial or additional) failed. Call
                    // the retry() method in order to retry the load operation.
                    // ...
                }

                if (refresh instanceof LoadState.Loading) {
                    // The initial Load has begun
                    // ...
                }

                if (append instanceof LoadState.Loading) {
                    // The adapter has started to load an additional page
                    // ...
                }

                if (append instanceof LoadState.NotLoading) {
                    LoadState.NotLoading notLoading = (LoadState.NotLoading) append;
                    if (notLoading.getEndOfPaginationReached()) {
                        // The adapter has finished loading all of the data set
                        // ...
                        return null;
                    }

                    if (refresh instanceof LoadState.NotLoading) {
                        // The previous load (either initial or additional) completed
                        // ...
                        return null;
                    }
                }
                return null;
            }
        });

In Kotlin

Use the loadStateFlow exposed by the adapter, in a Coroutine Scope:

// Activities can use lifecycleScope directly, but Fragments should instead use
// viewLifecycleOwner.lifecycleScope.
lifecycleScope.launch {
    pagingAdapter.loadStateFlow.collectLatest { loadStates ->
        when (loadStates.refresh) {
            is LoadState.Error -> {
                // The initial load failed. Call the retry() method
                // in order to retry the load operation.
                // ...
            }
            is LoadState.Loading -> {
                // The initial Load has begun
                // ...
            }
        }

        when (loadStates.append) {
            is LoadState.Error -> {
                // The additional load failed. Call the retry() method
                // in order to retry the load operation.
                // ...
            }
            is LoadState.Loading -> {
                // The adapter has started to load an additional page
                // ...
            }
            is LoadState.NotLoading -> {
                if (loadStates.append.endOfPaginationReached) {
                    // The adapter has finished loading all of the data set
                    // ...
                }
                if (loadStates.refresh is LoadState.NotLoading) {
                    // The previous load (either initial or additional) completed
                    // ...
                }
            }
        }
  }
}

ListView is the older, yet simpler way to handle lists of items. Using it is analogous to using a FirebaseRecyclerAdapter, but with FirebaseListAdapter instead and no ViewHolder:

FirebaseListOptions<Chat> options = new FirebaseListOptions.Builder<Chat>()
        .setQuery(query, Chat.class)
        .build();

FirebaseListAdapter<Chat> adapter = new FirebaseListAdapter<Chat>(options) {
    @Override
    protected void populateView(View v, Chat model, int position) {
        // Bind the Chat to the view
        // ...
    }
};

If your data is properly indexed, change your adapter initialization to use setIndexedQuery():

// keyQuery - the Firebase location containing the list of keys to be found in dataRef
// dataRef - the Firebase location to watch for data changes. Each key found at
//           keyRef's location represents a list item.
FirebaseRecyclerOptions<Chat> options = new FirebaseRecyclerOptions.Builder<Chat>()
        .setIndexedQuery(keyQuery, dataRef, Chat.class)
        .build();

Where keyQuery is the location of your keys, and dataRef is the location of your data.

A note on ordering

The order in which you receive your data depends on the order from keyRef, not dataRef:

{
  "data": {
    // This order doesn't matter, the order is taken from keys/(user1 or user2).
    "3": true,
    "1": "some data",
    "2": 5
  },
  "keys": {
    // These two users have different orders for their data thanks to key side ordering.
    "user1": {
      "1": true,
      "2": true,
      "3": true
    },
    "user2": {
      "3": true,
      "2": true,
      "1": true
    }
  }
}