flutter firestore:如何计算map中所有数组中的所有元素

dependencies:
  cloud_firestore: ^2.5.3

import 'package:cloud_firestore/cloud_firestore.dart';

final FirebaseFirestore firestore = FirebaseFirestore.instance;

int calculateSumOfArrays() {
  int sum = 0;
  
  firestore.collection('data').doc('documentId').get().then((snapshot) {
    Map<String, dynamic> data = snapshot.data() as Map<String, dynamic>;
    
    if (data.containsKey('arrayMap')) {
      Map<String, dynamic> arrayMap = data['arrayMap'] as Map<String, dynamic>;
      
      arrayMap.values.forEach((array) {
        if (array is List) {
          array.forEach((element) {
            if (element is int) {
              sum += element;
            }
          });
        }
      });
    }
  });
  
  return sum;
}