firebase - 如何从列表元素中获取单个值

Question

我正在尝试将名为“data”的列表中的一个值写入 firestore，该列表有两个元素和两个值，mileage 和 geodistance。目前 firestore 正在将里程和地理距离写入 firestore，而实际上我只需要将里程写入 firestore。目前我正在使用 final mileager = data.elementAt(0);将元素放入 firestore，但是当我只需要元素中的单个值时，它会上传元素中的所有内容。如何从此列表中获取单个值并将其写入 firestore？基本上我使用地理定位器来查看哪个点最近，然后我想将该点的“里程”写入 firestore。下图显示了当前的里程数。这很接近，但我希望它只是一个字段和值，而不是 map 。

import 'dart:async';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:geolocator/geolocator.dart';
import 'package:intl/intl.dart';

final _firestore = FirebaseFirestore.instance;

class Geodistancer {
  final int? mileage;
  final double geodistance;

  const Geodistancer({
    required this.mileage,
    required this.geodistance,
  });
}

Future<Position> getLocation() async {
  var currentLocation;
  try {
    currentLocation = await Geolocator.getCurrentPosition(
        desiredAccuracy: LocationAccuracy.best);
  } catch (e) {
    currentLocation = null;
  }
  return currentLocation;
}

Future<void> backgroundUpdate() async {
  Firebase.initializeApp();
  FirebaseAuth auth = FirebaseAuth.instance;
  Position _fetchedUserLocation = await Geolocator.getCurrentPosition(
      desiredAccuracy: LocationAccuracy.high);

  getLocation().then((position) async {
    _fetchedUserLocation = position;
    final distanceInMiles1 = Geolocator.distanceBetween(
        _fetchedUserLocation.latitude, _fetchedUserLocation.longitude,
        45.4906999, -122.49727);
    final distanceInMiles2 = Geolocator.distanceBetween(
        _fetchedUserLocation.latitude, _fetchedUserLocation.longitude, 45.48144,
        -122.50433);

    List<dynamic> data = [
      {"mileage": 1, "geodistance": distanceInMiles1},
      {"mileage": 2, "geodistance": distanceInMiles2},
    ];
    data.sort((a, b) => a["geodistance"].compareTo(b["geodistance"]));

    User firebaseUser = (auth.currentUser)!;
    String uid = firebaseUser.uid;
    DateTime dateTime = DateTime.now();
    String dateString = DateFormat().format(dateTime);
    double latitude = _fetchedUserLocation.latitude.toDouble();
    double longitude = _fetchedUserLocation.longitude.toDouble();
    final speedInMilesPerHour = _fetchedUserLocation.speed * 2.23694;
    final mileager = data.elementAt(0);

    _firestore
        .collection('locations')
        .add({
      'Uid': uid,
      'DateTime': dateString,
      'Mileage': mileager,
      'Speed': speedInMilesPerHour.round().toString() + ' MPH',
      'position': GeoPoint(latitude, longitude)
    });
  });
}

Answer 1

改成这样:

final mileager = data.elementAt(0)["mileage"];

因为之前，您正确获取了索引为 0 的元素，但该元素是一个 Map。您想要的是该元素中索引 0 处的键“里程”处的值。该元素应该作为单个字段而不是 map 发布到 firebase，您应该很好。

保持其余代码不变。