on
external fun onSnapshot(reference: DocumentReference, next: (snapshot: DocumentSnapshot) -> Unit, error: (error: Throwable) -> Unit): Unsubscribe
external fun onSnapshot(reference: DocumentReference, options: Json, next: (snapshot: DocumentSnapshot) -> Unit, error: (error: Throwable) -> Unit): Unsubscribe
external fun onSnapshot(reference: Query, next: (snapshot: QuerySnapshot) -> Unit, error: (error: Throwable) -> Unit): Unsubscribe
external fun onSnapshot(reference: Query, options: Json, next: (snapshot: QuerySnapshot) -> Unit, error: (error: Throwable) -> Unit): Unsubscribe