Flutter Firestore Run Transaction to make your UpVoting counter with FieldValue.arrayUnion

 Code:

import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

var myDocId, numberOfUpVoters;

void addUpVotingField() async {
  List<String> emptyList = new List();
  await FirebaseFirestore.instance.collection('YourCollection').add({
    "OtherField": "OtherValue",
    //... more fields
    "upVoters":
        FieldValue.arrayUnion(emptyList), //to store uid of users who up-voted
  }).then((DocumentReference documentReference) {
    myDocId = documentReference.id;
  });
}

void firestoreTransaction() {
  User user = FirebaseAuth.instance.currentUser;

  DocumentReference documentReference =
      FirebaseFirestore.instance.collection('YourCollection').doc(myDocId);

  FirebaseFirestore.instance.runTransaction((transaction) async {
    DocumentSnapshot documentSnapshot =
        await transaction.get(documentReference);

    if (documentSnapshot.data()['upVoters'].contains(user.uid)) {
      transaction.update(documentReference, <String, dynamic>{
        'upVoters': FieldValue.arrayRemove([user.uid])
      });
    } else {
      transaction.update(documentReference, <String, dynamic>{
        'upVoters': FieldValue.arrayUnion([user.uid])
      });
    }
  }).then((value) async {
    await FirebaseFirestore.instance
        .collection("YourCollection")
        .doc(myDocId)
        .get()
        .then((doc) {
      setState(() {// should run inside stateful widget
        numberOfUpVoters = doc['upVoters'].length;
      });
    });
  });
}