# FirebaseUI for Realtime Database FirebaseUI makes it simple to bind data from the Firebase Realtime Database to your app's UI. Before using this library, you should be familiar with the following topics: * [Working with lists of data in Firebase Realtime Database][firebase-lists]. * [Displaying lists of data using a RecyclerView][recyclerview]. ## Table of contents 1. [Data model](#data-model) 1. [Querying](#querying) 1. [Populating a RecyclerView](#using-firebaseui-to-populate-a-recyclerview) 1. [Using the FirebaseRecyclerAdapter](#using-the-firebaserecycleradapter) 1. [Using the FirebaseRecyclerPagingAdapter](#using-the-firebaserecyclerpagingadapter) 1. [Populating a ListView](#using-firebaseui-to-populate-a-listview) 1. [Handling indexed data](#using-firebaseui-with-indexed-data) 1. [Warnings](#a-note-on-ordering) ## Data model 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: ```java 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: ```java Query query = FirebaseDatabase.getInstance() .getReference() .child("chats") .limitToLast(50); ``` To retrieve this data without FirebaseUI, you might use `addChildEventListener` to listen for live updates: ```java 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); ``` ## Using FirebaseUI to populate a `RecyclerView` 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: ```java FirebaseRecyclerOptions options = new FirebaseRecyclerOptions.Builder() .setQuery(query, Chat.class) .build(); ``` If you need to customize how your model class is parsed, you can use a custom `SnapshotParser`: ```java ...setQuery(..., new SnapshotParser() { @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: ```java FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter(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. ```java @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: ```java @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][arch-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: ```java FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter(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][paging-support]. Before using the adapter in your application, you must add a dependency on that library: ```groovy 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`: ```java // 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 options = new DatabasePagingOptions.Builder() .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`: ```java ...setQuery(..., new SnapshotParser() { @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: ```java FirebaseRecyclerPagingAdapter adapter = new FirebaseRecyclerPagingAdapter(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. ```java @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: ```java @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][arch-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: ```java adapter.addLoadStateListener(new Function1() { @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: ```kotlin // 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 // ... } } } } } ``` ## Using FirebaseUI to populate a `ListView` 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`: ```java FirebaseListOptions options = new FirebaseListOptions.Builder() .setQuery(query, Chat.class) .build(); FirebaseListAdapter adapter = new FirebaseListAdapter(options) { @Override protected void populateView(View v, Chat model, int position) { // Bind the Chat to the view // ... } }; ``` ## Using FirebaseUI with indexed data If your data is [properly indexed][indexed-data], change your adapter initialization to use `setIndexedQuery()`: ```java // 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 options = new FirebaseRecyclerOptions.Builder() .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`: ```javascript { "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 } } } ``` [firebase-lists]: https://firebase.google.com/docs/database/android/lists-of-data [indexed-data]: https://firebase.google.com/docs/database/android/structure-data#best_practices_for_data_structure [recyclerview]: https://developer.android.com/reference/androidx/recyclerview/widget/RecyclerView [arch-components]: https://developer.android.com/topic/libraries/architecture/index.html [paging-support]: https://developer.android.com/topic/libraries/architecture/paging/v3-overview