完成迭代循环后，映射为空 [英] Map is empty after finish iterate loop

问题描述

我只需要在完成迭代循环后再打印地图的键计数即可。

I need only after finish iterate loop then print map's key count.

 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;
  }

但在日志中先打印：

 map_keys_count = 0

并多次打印后

put_key = xxx

推荐答案

要正确填写地图，您必须正确地等待。但是，填充地图然后无论如何都返回null可能不是此方法应该做的。看起来更像是您要寻找的东西：

To fill you map correctly, you have to await your futures properly. However, filling a map and then returning null anyway is probably not what this method should do. This looks more like it could be what you are looking for:

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;
  }

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