Custom LiveData to works with FirebaseFirestore snapshots

FirestoreLiveData.kt

import androidx.lifecycle.LiveData
import com.google.firebase.firestore.DocumentSnapshot
import com.google.firebase.firestore.FirebaseFirestore
import com.google.firebase.firestore.ListenerRegistration
import com.google.firebase.firestore.QuerySnapshot

/**
 * Custom [LiveData] to works with [FirebaseFirestore] snapshots
 *
 * @param collection The current collection's name
 * @param transformer Transform [DocumentSnapshot] to [T]
 * @param query Query conditions
 */
class FirestoreLiveData<T : Any>(
    private val collection: String,
    private val transformer: Transformer<T>,
    private val query: (CollectionReference.() -> Query)? = null
) : LiveData<List<T>>() {

    interface Transformer<T> {
        fun transform(document: DocumentSnapshot): T
    }

    /**
     * Firebase Firestore
     */
    private val db by lazy { FirebaseFirestore.getInstance() }

    /**
     * Snapshot listener
     */
    private var listener: ListenerRegistration? = null

    override fun onActive() {
        subscribe()
    }

    override fun onInactive() {
        unSubscribe()
    }

    /**
     * Subscribe to snapshot updates
     */
    private fun subscribe() {
        unSubscribe()

        listener = db.collection(collection)
            .run {
                query?.invoke(this) ?: this
            }
            .addSnapshotListener { querySnapshot, firebaseFirestoreException ->
                querySnapshot?.let(this::onSnapshotGot)
            }
    }

    private fun onSnapshotGot(snapshot: QuerySnapshot) {
        value = snapshot.documents.mapNotNull {
            try {
                transformer.transform(it)
            } catch (error: Throwable) {
                error.printStackTrace()
                null
            }
        }
    }

    /**
     * Unsubscribe from updates
     */
    private fun unSubscribe() {
        listener?.remove()
    }
}