将存储在Firebase中的值相加，并在FutureBuilder中显示总数

import 'package:flutter/material.dart';
import 'package:firebase_database/firebase_database.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('Firebase Sum Example')),
        body: SumDisplay(),
      ),
    );
  }
}

class SumDisplay extends StatefulWidget {
  @override
  _SumDisplayState createState() => _SumDisplayState();
}

class _SumDisplayState extends State<SumDisplay> {
  Future<int> _getSum() async {
    final databaseReference = FirebaseDatabase.instance.reference();
    DataSnapshot dataSnapshot = await databaseReference.child('values').once();
    int sum = 0;
    dataSnapshot.value.forEach((key, value) {
      sum += value as int;
    });
    return sum;
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<int>(
      future: _getSum(),
      builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
        if (snapshot.connectionState == ConnectionState.waiting) {
          return Center(child: CircularProgressIndicator());
        } else if (snapshot.hasError) {
          return Center(child: Text('Error: ${snapshot.error}'));
        } else {
          return Center(child: Text('Total Sum: ${snapshot.data}'));
        }
      },
    );
  }
}