Kotlin-first design

Unlike the Kotlin Extensions for the Firebase Android SDK this project does not extend a Java based SDK so we get the full power of Kotlin including coroutines and serialization!

Suspending functions

Asynchronous operations that return a single or no value are represented by suspending functions in the SDK instead of callbacks, listeners or OS specific types such as Task, for example:

suspend fun signInWithCustomToken(token: String): AuthResult

It is important to remember that unlike a callback based API, wating for suspending functions to complete is implicit and so if you don't want to wait for the result you can launch a new coroutine:

//TODO don't use GlobalScope
GlobalScope.launch {


Asynchronous streams of values are represented by Flows in the SDK instead of repeatedly invoked callbacks or listeners, for example:

val snapshots: Flow<DocumentSnapshot>

The flows are cold, which means a new listener is added every time a terminal operator is applied to the resulting flow. A buffer with the default size is used to buffer values received from the listener, use the buffer operator on the flow to specify a user-defined value and to control what happens when data is produced faster than consumed, i.e. to control the back-pressure behavior. Often you are only interested in the latest value received, in this case you can use the conflate operator to disable buffering.

The listener is removed once the flow completes or is cancelled.


The official Firebase SDKs use different platform-specific ways to support writing data with and without custom classes in Cloud Firestore, Realtime Database and Functions.

The Firebase Kotlin SDK uses Kotlin serialization to read and write custom classes to Firebase. To use Kotlin serialization in your project add the plugin to your gradle file:

plugins {
    kotlin("multiplatform") version "1.9.20" // or kotlin("jvm") or any other kotlin plugin
    kotlin("plugin.serialization") version "1.9.20"

Then mark you custom classes @Serializable:

data class City(val name: String)

Instances of these classes can now be passed along with their serializer to the SDK:

db.collection("cities").document("LA").set(City.serializer(), city) { encodeDefaults = true }

The buildSettings closure is optional and allows for configuring serialization behaviour.

Setting the encodeDefaults parameter is optional and defaults to true, set this to false to omit writing optional properties if they are equal to theirs default values. Using @EncodeDefault on properties is a recommended way to locally override the behavior set with encodeDefaults.

You can also omit the serializer if it can be inferred using serializer<KType>(). To support contextual serialization or open polymorphism the serializersModule can be overridden in the buildSettings closure:

abstract class AbstractCity {
    abstract val name: String

data class Capital(override val name: String, val isSeatOfGovernment: Boolean) : AbstractCity()

val module = SerializersModule {
    polymorphic(AbstractCity::class, AbstractCity.serializer()) {
        subclass(Capital::class, Capital.serializer())

val city = Capital("London", true)
db.collection("cities").document("UK").set(AbstractCity.serializer(), city) { 
    encodeDefaults = true
    serializersModule = module

val storedCity = db.collection("cities").document("UK").get().data(AbstractCity.serializer()) {
    serializersModule = module

Server Timestamp

FirestoreRealtime Databasekotlin @Serializable data class Post( // In case using Realtime Database. val timestamp = ServerValue.TIMESTAMP, // In case using Cloud Firestore. val timestamp: Timestamp = Timestamp.ServerTimestamp, // or val alternativeTimestamp = FieldValue.serverTimestamp, // or @Serializable(with = DoubleAsTimestampSerializer::class), val doubleTimestamp: Double = DoubleAsTimestampSerializer.serverTimestamp )firebase-firestorekotlin @Serializable data class PointOfInterest( val reference: DocumentReference, val location: GeoPoint ) val document = PointOfInterest( reference = Firebase.firestore.collection("foo").document("bar"), location = GeoPoint(51.939, 4.506) )

Polymorphic serialization (sealed classes)

This sdk will handle polymorphic serialization automatically if you have a sealed class and its children marked as Serializable. It will include a type property that will be used to discriminate which child class is the serialized.

You can change this type property by using the @FirebaseClassDiscrminator annotation in the parent sealed class:

sealed class Parent {
    data class Child(
        val property: Boolean
    ) : Parent

In combination with a SerialName specified for the child class, you have full control over the serialized data. In this case it will be:

  "class": "child",
  "property": true

Default arguments

To reduce boilerplate, default arguments are used in the places where the Firebase Android SDK employs the builder pattern:

UserProfileChangeRequest profileUpdates = new UserProfileChangeRequest.Builder()
        .setDisplayName("Jane Q. User")

        .addOnCompleteListener(new OnCompleteListener<Void>() {
            public void onComplete(@NonNull Task<Void> task) {
                if (task.isSuccessful()) {
                    Log.d(TAG, "User profile updated.")


user.updateProfile(displayName = "Jane Q. User", photoURL = "https://example.com/jane-q-user/profile.jpg")

Infix notation

To improve readability and reduce boilerplate for functions such as the Cloud Firestore query operators are built with infix notation:

citiesRef.whereEqualTo("state", "CA")
citiesRef.whereArrayContains("regions", "west_coast")
    Filter.equalTo("state", "CA"),
        Filter.equalTo("capital", true),
        Filter.greaterThanOrEqualTo("population", 1000000)


citiesRef.where { "state" equalTo "CA" }
citiesRef.where { "regions" contains "west_coast" }
citiesRef.where {
        "state" equalTo "CA",
            "capital" equalTo true,
            "population" greaterThanOrEqualTo 1000000

Operator overloading

In cases where it makes sense, such as Firebase Functions HTTPS Callable, operator overloading is used:

    val addMessage = functions.getHttpsCallable("addMessage")
    //In the official android Firebase SDK this would be addMessage.call(...)
    addMessage(mapOf("text" to text, "push" to true))


The Firebase Kotlin SDK provides a common API to access Firebase for projects targeting iOS, Android, JVM and JS meaning you can use Firebase directly in your common code. Under the hood, the SDK achieves this by binding to the respective official Firebase SDK for each supported platform.

It uses the Firebase Java SDK to support the JVM target. The library requires additional initialization compared to the official Firebase SDKs.

Accessing the underlying Firebase SDK

In some cases you might want to access the underlying official Firebase SDK in platform specific code, for example when the common API is missing the functionality you need. For this purpose each class in the SDK has android, ios and js properties which holds the equivalent object of the underlying official Firebase SDK. For JVM, as the firebase-java-sdk is a direct port of the Firebase Android SDK, is it also accessed via the android property.

These properties are only accessible from the equivalent target's source set. For example to disable persistence in Cloud Firestore on Android you can write the following in your Android specific code (e.g. androidMain or androidTest):

  Firebase.firestore.android.firestoreSettings = FirebaseFirestoreSettings.Builder(Firebase.firestore.android.firestoreSettings)

Running on iOS

On iOS the official Firebase iOS SDK in not linked as a transtive dependency. Therefore, any project using this SDK needs to link the actual Firestore SDK as well. This can be done through your preferred installation method (Cocoapods/SPM).

Similarly, tests require linking as well. Make sure to add the required frameworks to the search path of your test targets. This can be done by specifying a cocoapods block in your build.gradle:

cocoapods {
   pod("FirebaseCore") // Repeat for Firebase pods required by your project, e.g FirebaseFirestore for the `firebase-firestore` module.