Firebase Query as LiveData

FirestoreLiveData.kt

/**
 * Copyright 2019 Google LLC.
 * SPDX-License-Identifier: Apache-2.0
 */

package com.example.firestore

import android.util.Log
import androidx.lifecycle.LiveData
import com.google.firebase.firestore.*

private const val TAG = "FirestoreLiveData"

fun <T> Query.toLiveData(convert: QuerySnapshot.() -> T): LiveData<T> {
    return object : LiveData<T>(), EventListener<QuerySnapshot> {

        private var listenerRegistration: ListenerRegistration? = null

        override fun onEvent(snapshot: QuerySnapshot?, e: FirebaseFirestoreException?) {
            if (e != null) {
                Log.w(TAG, "Listen failed", e)
                return
            }
            if (snapshot != null && !snapshot.isEmpty) {
                value = convert(snapshot)
            }
        }

        override fun onActive() {
            listenerRegistration = addSnapshotListener(this)
        }

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

SampleRepository.kt

/**
 * Copyright 2019 Google LLC.
 * SPDX-License-Identifier: Apache-2.0
 */

package com.example.firestore

import androidx.lifecycle.LiveData
import com.google.firebase.firestore.FirebaseFirestore
import com.google.firebase.firestore.Query

class SampleRepository(
    private val firestore: FirebaseFirestore
) {
    fun liveMessages(): LiveData<List<String>> {
        return firestore.collection("messages")
            .orderBy("timestamp", Query.Direction.DESCENDING)
            .limit(25)
            .toLiveData {
                documents.map { d ->
                    d["text"] as String
                }
            }
    }
}