Question

我的目标是对与用户地理位置相比的机架列表进行排序。这是我的代码：

constructor(props)

问题出在for循环中，变量import 'package:geolocator/geolocator.dart'; import 'rack.dart'; import 'dart:async'; class RackList { List<Rack> _racks; RackList(this._racks); get racks => _racks; factory RackList.fromJson(Map<String,dynamic> json){ var list = json['racks'] as List; Future<int> calculateDistance(var element) async{ var startLatitude=45.532; var startLongitude=9.12246; var endLatitude=element.latitude; var endLongitude=element.longitude; var dist = await Geolocator().distanceBetween( startLatitude, startLongitude, endLatitude, endLongitude); var distance=(dist/1000).round(); return distance; } list = list.map((i) => Rack.fromJson(i)).toList(); for (int i = 0; i < list.length; i++) { var dist = calculateDistance(list[i]); print(dist); //prints Instance of 'Future<int>' list[i].distance=dist; //crash } list.sort((a, b) => a.distance.compareTo(b.distance)); return RackList(list); } }是dist类型，不能分配给Future<int>。如何将该值转换为普通的int？

我已经尝试过@Nuts的解决方案，但是：

list[i].distance

就像在周期外，我丢失了var distances = new List(); for (int i = 0; i < list.length; i++) { calculateDistance(list[i]).then((dist) { distances.add(dist); print(distances[i]); //print the correct distance }); } print("index 0 "+distances[0].toString()); //prints nothing列表中的所有值

Answer 1

import 'package:geolocator/geolocator.dart';
import 'rack.dart';
import 'dart:async';
class RackList {

  List<Rack> _racks;
  RackList(this._racks);
  get racks => _racks;

  factory RackList.fromJson(Map<String,dynamic> json){
    var list = json['racks'] as List;

     calculateDistance(var element) async{
      var startLatitude=45.532;
      var startLongitude=9.12246;
      var endLatitude=element.latitude;
      var endLongitude=element.longitude;
      var dist = await Geolocator().distanceBetween(
          startLatitude,
          startLongitude,
          endLatitude,
          endLongitude);
      int distance=(dist/1000).round();
      return distance;
    }

    list = list.map((i) => Rack.fromJson(i)).toList();

      for (int i = 0; i < list.length; i++) {
        calculateDistance(list[i]).then((dist){

        print("${dist}"); //prints Instance of 'Future<int>'
        list[i].distance=dist; //crash
      });
    }

    list.sort((a, b) => a.distance.compareTo(b.distance));
    return RackList(list);
  }
}

Answer 2

还可以：

var dist =  await calculateDistance(list[i]);

它将等待Future返回int值。

另一个解决方案是：

calculateDistance(list[i]).then((dist) {list[i].distance=dist;})

Future完成后，运行功能。

在Flutter中将Future <int>转换为int

2 个答案: