It's very important to keep projects (such as libraries) as light as possible. This will improve the performance and increase the download time.

This is a great library, i suggest removing the unused resources such as the test examples to keep the library even lighter.

Currently, initializing an app with FirebaseOptions in Kotlin looks like this:

val options = FirebaseOptions.Builder()
            .setApplicationId("1:27992087142:android:ce3b6448250083d1") // Required for Analytics.
            .setApiKey("AIzaSyADUe90ULnQDuGShD9W23RDP0xmeDc6Mvw") // Required for Auth.
            .setDatabaseUrl("https://myproject.firebaseio.com") // Required for RTDB.
            .build()
FirebaseApp.initializeApp(this /* required */, options /* optional */, "secondary" /* optional */)

I propose initializing it like:

FirebaseApp.initializeApp {
    context = this // required
    name = "secondary" // optional
    options { // optional
        applicationdId = "1:27992087142:android:ce3b6448250083d1"
        apiKey = "AIzaSyADUe90ULnQDuGShD9W23RDP0xmeDc6Mvw"
        databaseUrl = "https://myproject.firebaseio.com"

        // Below are optional parameters not mentioned above
        gaTrackingId = "someId"
        gcmSenderId = "someId"
        storageBucket = "bucketUrl"
        projectId = "someId"
    }
}

As mentioned on our Contributing Guidelines, we try to follow the Android Kotlin Style Guide, but there are a few lines here not following that style guide and more "my-code-my-rules" might come up, since the project is open for new contributors.

I propose adding a linter to enforce these styling rules and have it as part of our CI automated build. My suggestion would be ktlint, but feel free to suggest a different one.

FirebaseUI is very helpful when developing Android apps. It already handles lots of boilerplate code, but with Kotlin it might be able to handle more. The FirebaseRecyclerViewAdapter for the realtime database is an example:

Database

See how it's currently written here. Here's my proposal:

val options = FirebaseRecyclerOptions.fromQuery<Chat>(query)

val adapter = FirebaseRecyclerAdapter.with<ChatHolder>(options)
  .viewHolder { _, parent ->
    val view = layoutInflater.inflate(R.layout.list_item, parent, false)
    ChatHolder(view)
  }
  .bind { position, model ->
    // Bind the Chat object to the views from the ChatHolder
  }
  .build()

The classes and methods in this library are not yet documented. We should create reference docs to help users of the library understand what each class and method is for.

Maybe use Dokka?

Testing is an integral part of the software development process. This library should be no exception.