Firebase for Android

Setup — always use BoM for version management

[versions]
firebaseBom = "33.5.1"
googleServices = "4.4.2"

// build.gradle.kts
plugins { alias(libs.plugins.google.services) }
dependencies {
    implementation(platform(libs.firebase.bom))
    implementation(libs.firebase.auth.ktx)
    implementation(libs.firebase.firestore.ktx)
    implementation(libs.firebase.storage.ktx)
    implementation(libs.firebase.analytics.ktx)
    implementation(libs.firebase.crashlytics.ktx)
    implementation(libs.firebase.messaging.ktx)
}

Firebase Auth

// ✅ Auth repository
class AuthRepositoryImpl @Inject constructor(
    private val auth: FirebaseAuth
) : AuthRepository {

    override val currentUser: Flow<User?> = callbackFlow {
        val listener = FirebaseAuth.AuthStateListener { auth ->
            trySend(auth.currentUser?.toDomain())
        }
        auth.addAuthStateListener(listener)
        awaitClose { auth.removeAuthStateListener(listener) }
    }

    override suspend fun signInWithEmail(email: String, password: String): Result<User> =
        runCatching {
            auth.signInWithEmailAndPassword(email, password).await()
                .user?.toDomain() ?: throw IllegalStateException("User is null after sign in")
        }

    override suspend fun createAccount(email: String, password: String): Result<User> =
        runCatching {
            auth.createUserWithEmailAndPassword(email, password).await()
                .user?.toDomain() ?: throw IllegalStateException("User is null after creation")
        }

    override suspend fun signInWithGoogle(idToken: String): Result<User> = runCatching {
        val credential = GoogleAuthProvider.getCredential(idToken, null)
        auth.signInWithCredential(credential).await()
            .user?.toDomain() ?: throw IllegalStateException("User is null")
    }

    override fun signOut() = auth.signOut()

    override fun isSignedIn(): Boolean = auth.currentUser != null
}

fun FirebaseUser.toDomain() = User(uid, email ?: "", displayName ?: "", photoUrl?.toString())

Firestore — CRUD + real-time

// ✅ Firestore repository
class ItemFirestoreDataSource @Inject constructor(
    private val firestore: FirebaseFirestore,
    private val auth: FirebaseAuth
) {
    private val itemsCollection
        get() = firestore.collection("users")
            .document(auth.currentUser?.uid ?: throw UnauthorizedException())
            .collection("items")

    // Real-time stream
    fun getItemsStream(): Flow<List<ItemDto>> = callbackFlow {
        val listener = itemsCollection
            .orderBy("created_at", Query.Direction.DESCENDING)
            .addSnapshotListener { snapshot, error ->
                if (error != null) { close(error); return@addSnapshotListener }
                val items = snapshot?.documents?.mapNotNull { doc ->
                    doc.toObject(ItemDto::class.java)?.copy(id = doc.id)
                } ?: emptyList()
                trySend(items)
            }
        awaitClose { listener.remove() }
    }

    // One-shot get
    suspend fun getItem(id: String): ItemDto? =
        itemsCollection.document(id).get().await()
            .toObject(ItemDto::class.java)?.copy(id = id)

    // Write
    suspend fun upsertItem(item: ItemDto): String {
        return if (item.id.isEmpty()) {
            itemsCollection.add(item).await().id
        } else {
            itemsCollection.document(item.id).set(item).await()
            item.id
        }
    }

    // Update specific fields
    suspend fun updateFavorite(id: String, isFavorite: Boolean) {
        itemsCollection.document(id).update(
            "is_favorite", isFavorite,
            "updated_at", FieldValue.serverTimestamp()
        ).await()
    }

    // Delete
    suspend fun deleteItem(id: String) {
        itemsCollection.document(id).delete().await()
    }

    // Batch write — atomic multi-document update
    suspend fun batchUpdate(updates: List<Pair<String, Map<String, Any>>>) {
        firestore.runBatch { batch ->
            updates.forEach { (id, fields) ->
                batch.update(itemsCollection.document(id), fields)
            }
        }.await()
    }

    // Transaction
    suspend fun transferItem(fromId: String, toUserId: String): Result<Unit> = runCatching {
        firestore.runTransaction { transaction ->
            val fromRef = itemsCollection.document(fromId)
            val item = transaction.get(fromRef).toObject(ItemDto::class.java)
                ?: throw NotFoundException()
            val toRef = firestore.collection("users").document(toUserId)
                .collection("items").document(fromId)
            transaction.set(toRef, item)
            transaction.delete(fromRef)
        }.await()
    }
}

Hilt Firebase module

@Module
@InstallIn(SingletonComponent::class)
object FirebaseModule {
    @Provides @Singleton
    fun provideFirebaseAuth(): FirebaseAuth = Firebase.auth

    @Provides @Singleton
    fun provideFirestore(): FirebaseFirestore = Firebase.firestore.also {
        it.firestoreSettings = firestoreSettings { isPersistenceEnabled = true }
    }

    @Provides @Singleton
    fun provideStorage(): FirebaseStorage = Firebase.storage

    @Provides @Singleton
    fun provideAnalytics(@ApplicationContext context: Context): FirebaseAnalytics =
        Firebase.analytics
}

FCM — Firebase Cloud Messaging

@AndroidEntryPoint
class MyFirebaseMessagingService : FirebaseMessagingService() {
    @Inject lateinit var notificationHandler: NotificationHandler

    override fun onMessageReceived(message: RemoteMessage) {
        message.notification?.let { notification ->
            notificationHandler.showNotification(
                title = notification.title ?: "",
                body = notification.body ?: "",
                data = message.data
            )
        }
    }

    override fun onNewToken(token: String) {
        // Send token to your server
    }
}

Common Mistakes

❌ Missing google-services.json in app/ directory ❌ Not using BoM — version conflicts between Firebase libraries ❌ Calling Firebase on Main thread without .await() — use suspend + .await() ❌ No offline persistence for Firestore — enable isPersistenceEnabled = true ❌ Missing Firestore security rules — always configure before production ❌ Storing sensitive data in Firestore without encryption