flutter - 完成迭代循环后 map 为空

Question

我只需要在完成迭代循环后打印 map 的键数。

 import 'dart:collection';
import 'dart:convert';
import 'dart:ffi';
import 'package:flutter/services.dart';
import 'package:flutter_sample/model/GazStation.dart';
import 'package:flutter_sample/util/util.dart';
import 'package:geolocator/geolocator.dart';
import 'package:logger/logger.dart';

Future<GazStation> getNearestGazStation() async {
    List<GazStation> gazStationList = await getGazStationList();
    Position myPosition = await Geolocator()
        .getCurrentPosition(desiredAccuracy: LocationAccuracy.high);
    Map<double, GazStation> map = new HashMap();
    gazStationList.forEach((gazStation) {
      _getDistanceMetersToGazStation(myPosition, gazStation).then((distanceInMeters) {
        _logger.d("put_key = ${distanceInMeters}");
        map.putIfAbsent(distanceInMeters, () => gazStation);
      });
    });
    _logger.d("map_keys_count = ${map.keys.length}");
    return null;
  }

  Future<double> _getDistanceMetersToGazStation(Position myPosition, GazStation gazStation) async {
    var distance = Geolocator().distanceBetween(
        myPosition.latitude,
        double.parse(gazStation.Latitude),
        myPosition.longitude,
        double.parse(gazStation.Longitude));
    return distance;
  }

但是在日志中首先打印:

 map_keys_count = 0

并多次打印后

put_key = xxx

Answer 1

要正确填写 map ，您必须 await你的 future 。但是，填充 map 然后返回 null 可能不是该方法应该做的。这看起来更像是您正在寻找的东西:

Future<GazStation> getNearestGazStation() async {
    final gazStationList = await getGazStationList();
    final myPosition = await Geolocator().getCurrentPosition(desiredAccuracy: LocationAccuracy.high);

    GazStation closest = null;
    double closestDistance = double.maxFinite;

    for(var gazStation in gazStationList) {
      final distanceInMeters = await _getDistanceMetersToGazStation(myPosition, gazStation);

      if(distanceInMeters < closestDistance) {
         closestDistance = distanceInMeters;
         closest = gazStation;
      }
    }
    
    return closest;
  }