Question

我使用数组存储“市场位置数据”，并将数据保存在Data.dart文件中

Data.dart

class Market{
  static List dataMarket = [
    Data(
        nameDest = 'Alfamart',
        latitudeDest: -6.163891,
        longitudeDest: 106.628915),
    Data(
        nameDest = 'Minis'
        latitudeDest: -6.211269,
        longitudeDest: 106.615574),
    Data(
        nameDest = 'Star Market'
        latitudeDest: -6.223254,
        longitudeDest:  106.703905),
    Data(
        nameDest = 'Allmark'
        latitudeDest: -6.163891,
        longitudeDest: 106.628915),
    Data(
        nameDest = 'Eig Store'
        latitudeDest: -6.178293, 
        longitudeDest: 106.631862),
  ];
}

class Data{
  String nameDest;
  var latitudeDest;
  var longitudeDest;


  Data({
     this.nameDest
     this.latitudeDest,
     this.longitudeDest});
}

我使用“市场位置数据”来计算用户和市场之间的距离。而且我还想显示最近的市场订单

MyListMarket.dart

import 'package:example/Data.dart';
import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:math' show cos, sqrt, asin;

import 'package:geolocator/geolocator.dart';


class TabMarket extends StatefulWidget{
  @override
  _TabMarketState createState() => _TabMarketState();
}

class _TabMarketState extends State<TabMarket> {
  var lastLatitude;
  var lastLongitude;


  @override
  void initState() {
    super.initState();
    _initUserLocation();
  }


  Future<void> _initUserLocation() async {
    try {
      Position position = await Geolocator()
          .getLastKnownPosition(desiredAccuracy: LocationAccuracy.best);
      lastLatitude = position.latitude;
      lastLongitude = position.longitude;
    } catch (e) {
      print("Permission Denied");
    }
  }



  @override
  Widget build(BuildContext context) {
    final dataInstance = Market.dataMarket;
    var newSortRange = [];

    if (lastLatitude == null || lastLongitude == null){
      for(int i = 0; i >= dataInstance.length; i++) {
        newSortRange[i] = 0;
      }
    }else{for (int i = 0; i >= dataInstance.length; i++) {
      //calculating distance with lat1, lon1, lat2, lon2

      var p = 0.017453292519943295;
      var c = cos;
      var a = 0.5 - c((dataInstance[i].latitudeDest - lastLatitude) * p) / 2 +
              c(lastLatitude * p) * c(dataInstance[i].latitudeDest * p) *
              (1 - c((dataInstance[i].longitudeDest - lastLongitude) * p)) / 2;
      newSortRange[i] = 12742 * asin(sqrt(a));
    }}
    newSortRange.sort();
    //i wanna sort this data from the smallest number
    //already tried with newSortRange.sort((a,b) => a.compareTo(b));

    return Scaffold(
      backgroundColor: Color.fromRGBO(246, 246, 246, 1),
      body: FutureBuilder(
        future: _initUserLocation(),
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          return ListView.separated(
              key: PageStorageKey<String>('aKey'),
              physics: BouncingScrollPhysics(),
              itemCount: newSortRange.length,
              itemBuilder: (context, index) {

                return Material(
                  color: Color.fromRGBO(251, 251, 251, 1),
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      ListTile(
                        leading: Text(dataInstance[index].nameDest)
                        trailing: Text(newSortRange[index].toStringAsFixed(2) +
                            "\nKM",
                          style: new TextStyle(fontSize: 13,),
                          textAlign: TextAlign.right,),
                      ),
                    ],),);
              },

              separatorBuilder: (context, index) {
                return Divider(
                  height: 0.0,
                  thickness: 0.7,
                );
              }
          );
        },
      ),
    );
  }
}

所以我尝试用for循环计算距离并将其保存到新数组中

我的代码没有错误，但没有列表视图就什么也没有出现。也许将数据存储到数组中的方法是错误的？我该如何解决？

Answer 1

由于_initUserLocation方法是异步的，因此即使您以initstate进行调用，也无法基于build方法中的值来计算其他值，因为_initUserLocation方法无法完成其任务。

在将来的构建器中，您必须计算可以看到实际数据的所有内容，因为将来的构建器会在过程完成时提供数据。

您必须从future方法返回任何数据，以便futurebuilder现在可以了解方法的完整计算。

我对您的代码进行了许多更改，效果很好。

让我知道它是否对您有用，如果您不了解任何代码部分，也可以随时提出任何问题。

完整演示：

 class _DeleteWidgetState extends State<DeleteWidget> {
  var lastLatitude;
  var lastLongitude;
  final dataInstance = Market.dataMarket;
  var newSortRange = [];

  @override
  void initState() {
    super.initState();
    //_initUserLocation();
  }

  Future<int> _initUserLocation() async {
    try {
      await Future.delayed(Duration(seconds: 1));
      Position position = await Geolocator()
             .getLastKnownPosition(desiredAccuracy: LocationAccuracy.best);
      lastLatitude = position.latitude;
      lastLongitude = position.longitude;
    } catch (e) {
      print("Permission Denied");
    }
    return 1;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Color.fromRGBO(246, 246, 246, 1),
      body: FutureBuilder(
        future: _initUserLocation(),
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          if (!snapshot.hasData) {
            return CircularProgressIndicator();
          }
          if (lastLatitude == null || lastLongitude == null) {
            for (int i = 0; i < dataInstance.length; i++) {
              newSortRange[i] = 0;
            }
          } else {
            for (int i = 0; i < dataInstance.length; i++) {
              //calculating distance with lat1, lon1, lat2, lon2

              var p = 0.017453292519943295;
              var c = cos;
              var a = 0.5 -
                  c((dataInstance[i].latitudeDest - lastLatitude) * p) / 2 +
                  c(lastLatitude * p) *
                      c(dataInstance[i].latitudeDest * p) *
                      (1 -
                          c((dataInstance[i].longitudeDest - lastLongitude) *
                              p)) /
                      2;
              newSortRange.add(12742 * asin(sqrt(a)));
            }
          }
          newSortRange.sort();
          print(newSortRange);
          return ListView.separated(
              key: PageStorageKey<String>('aKey'),
              physics: BouncingScrollPhysics(),
              itemCount: newSortRange.length,
              itemBuilder: (context, index) {
                return Material(
                  color: Color.fromRGBO(251, 251, 251, 1),
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      ListTile(
                        trailing: Text(
                          newSortRange[index].toStringAsFixed(2) + "\nKM",
                          style: new TextStyle(
                            fontSize: 13,
                          ),
                          textAlign: TextAlign.right,
                        ),
                      ),
                    ],
                  ),
                );
              },
              separatorBuilder: (context, index) {
                return Divider(
                  height: 0.0,
                  thickness: 0.7,
                );
              });
        },
      ),
    );
  }
}

class Market {
  static List dataMarket = [
    Data(latitudeDest: -6.163891, longitudeDest: 106.628915),
    Data(latitudeDest: -6.211269, longitudeDest: 106.615574),
    Data(latitudeDest: -6.223254, longitudeDest: 106.703905),
    Data(latitudeDest: -6.163891, longitudeDest: 106.628915),
    Data(latitudeDest: -6.178293, longitudeDest: 106.631862),
  ];
}

class Data {
  var latitudeDest;
  var longitudeDest;

  Data({this.latitudeDest, this.longitudeDest});
}

Flutter：数据排序（来自数组数据）的结果未显示在ListView中

1 个答案: