A Data Source using LiveData.

livedata-endtoend-datasource.kt

@Singleton
class NearbyUsersDataSource @Inject constructor() {
    // Ideally, those should be constructor-injected.
    val geoFire = GeoFire(FirebaseDatabase.getInstance().getReference("geofire"))
    val geoLocation = GeoLocation(0.0, 0.0)
    val radius = 100.0
    
    val geoQuery = geoFire.queryAtLocation(geoLocation, radius)
    
    // Listener for receiving GeoLocations
    val listener: GeoQueryEventListener = object : GeoQueryEventListener {
        val map = mutableMapOf<Key, GeoLocation>()
        override fun onKeyEntered(key: String, location: GeoLocation) {
            map[key] = location
        }
        override fun onKeyExited(key: String) {
            map.remove(key)
        }
        override fun onKeyMoved(key: String, location: GeoLocation) {
            map[key] = location
        }
        override fun onGeoQueryReady() {
            _locations.value = State.Ready(map.toMap())
        }
        override fun onGeoQueryError(e: DatabaseError) {
            _locations.value = State.Error(map.toMap(), e.toException())
        }
    }

    // Listen for changes only while observed
    private val _locations = object : MutableLiveData<State>() {
        override fun onActive() {
            geoQuery.addGeoQueryEventListener(listener)
        }

        override fun onInactive() {
            geoQuery.removeGeoQueryEventListener(listener)
        }
    }

    // Expose read-only LiveData
    val locations: LiveData<State> by this::_locations
    
    sealed class State(open val value: Map<Key, GeoLocation>) {
        data class Ready(
            override val value: Map<Key, GeoLocation>
        ) : State(value)
        
        data class Error(
            override val value: Map<Key, GeoLocation>,
            val exception: Exception
        ) : State(value)
    }
}