Firestore queries as LiveData

FirestoreLiveData.kt

inline fun <reified T> Query.asLiveData(noinline onError: ((Throwable) -> Unit) = {}): FirestoreQueryLiveData<T> {
    return FirestoreQueryLiveData(this, T::class.java, onError)
}

inline fun <reified T> DocumentReference.asLiveData(noinline onError: ((Throwable) -> Unit) = {}): FirestoreDocumentLiveData<T> {
    return FirestoreDocumentLiveData(this, T::class.java, onError)
}

class FirestoreQueryLiveData<T>(
    private val query: Query,
    private val clazz: Class<T>,
    private val onError: ((Throwable) -> Unit) = {}
) : LiveData<List<T>>() {

    private var listenerRegistration: ListenerRegistration? = null

    override fun onActive() {
        listenerRegistration?.remove()
        listenerRegistration = query.addSnapshotListener { snapshot, error ->
            if (error != null) {
                onError(error)
                return@addSnapshotListener
            }
            this.value = snapshot?.toObjects(clazz) // value caches the previous query -> list loads faster
        }
    }

    override fun onInactive() {
        listenerRegistration?.remove()
    }
}

class FirestoreDocumentLiveData<T>(
    private val docRef: DocumentReference,
    private val clazz: Class<T>,
    private val onError: ((Throwable) -> Unit) = {}
) : LiveData<T>() {

    private var listenerRegistration: ListenerRegistration? = null

    override fun onActive() {
        listenerRegistration?.remove()
        listenerRegistration = docRef.addSnapshotListener { snapshot, error ->
            if (error != null) {
                onError(error)
                return@addSnapshotListener
            }
            this.value = snapshot?.toObject(clazz)
        }
    }

    override fun onInactive() {
        listenerRegistration?.remove()
    }
}