如何在 Dart 中获得车辆的速度 [重复]

Posted 2023-02-23

【中文标题】如何在 Dart 中获得车辆的速度 [重复]

【英文标题】：How to get speed of a vehicle in Dart [duplicate]

【发布时间】：2021-01-22 12:06:53

【问题描述】：

我在 Dart 有一个小项目，我正在使用库 location。 使用该库，我尝试使用 GPS 位置计算车辆的速度。 问题是每次运行项目时控制台都会出现错误：

The following NoSuchMethodError was thrown building ListenLocationWidget(dirty, state: _ListenLocationState#55ce8):
The getter 'speed' was called on null.
Receiver: null
Tried calling: speed

这是我的小项目的链接： https://github.com/flo/VehicleSpeed

这是我计算位置之间速度的代码：

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:location/location.dart';

class ListenLocationWidget extends StatefulWidget 
  const ListenLocationWidget(Key key) : super(key: key);

  @override
  _ListenLocationState createState() => _ListenLocationState();


class _ListenLocationState extends State<ListenLocationWidget> 
  final Location location = Location();

  LocationData _location;
  StreamSubscription<LocationData> _locationSubscription;
  String _error;

  @override
  void initState() 
    super.initState();
    _listenLocation();
  

  Future<void> _listenLocation() async 
    _locationSubscription =
        location.onLocationChanged.handleError((dynamic err) 
      setState(() 
        _error = err.code;
      );
      _locationSubscription.cancel();
    ).listen((LocationData currentLocation) 
      setState(() 
        _error = null;
        _location = currentLocation;
      );
    );
  

  Future<void> _stopListen() async 
    _locationSubscription.cancel();
  

  Widget _calculateSpeedBetweenLocations() 
    return Text(
      '$_location.speed != null && _location.speed * 3600 / 1000 > 0 ? (_location.speed * 3600 / 1000).toStringAsFixed(2) : 0 KM/h',
      style: TextStyle(
        color: Colors.lightGreen[500],
        fontSize: 20,
        letterSpacing: 4,
      ),
    );
  

  @override
  Widget build(BuildContext context) 
    return Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      children: <Widget>[
        _calculateSpeedBetweenLocations(),
      ],
    );
  

这是我的主屏幕：

import 'package:vehicle_speed_demo/location/permissionStatus.dart';
import 'package:vehicle_speed_demo/location/serviceEnabled.dart';
import 'package:vehicle_speed_demo/location/getLocation.dart';
import 'package:vehicle_speed_demo/location/listenLocation.dart';
import 'package:flutter/material.dart';

class HomeScreen extends StatefulWidget 
  @override
  _HomeScreenState createState() => _HomeScreenState();


class _HomeScreenState extends State<HomeScreen> 
  @override
  void initState() 
    super.initState();
  

  @override
  void dispose() 
    super.dispose();
  

  Widget build(BuildContext context) 
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        resizeToAvoidBottomInset: false,
        backgroundColor: Colors.grey[900],
        appBar: _appBar(),
        body: _buildUI(),
      ),
    );
  

  Widget _appBar() 
    return AppBar(
      backgroundColor: Colors.purple[800],
      elevation: 0.0,
      title: Text(
        'My App',
        style: TextStyle(
          color: Colors.white,
          fontSize: 30.0,
        ),
      ),
      automaticallyImplyLeading: false,
    );
  

  Widget _buildUI() 
    return Container(
        child: Padding(
      padding: const EdgeInsets.all(10),
      child: Column(
        children: <Widget>[
          Wrap(
            spacing: 30.0, // gap between sections
            runSpacing: 30.0, // gap between lines
            alignment: WrapAlignment.spaceAround,
            children: <Widget>[
              Divider(color: Colors.white, height: 20),
              _buildSpeedSection(),
            ],
          ),
        ],
      ),
    ));
  

  Widget _buildSpeedSection() 
    return Container(
      height: 150,
      width: 170,
      child: Column(
        children: <Widget>[
          Text(
            'SPEED',
            style: TextStyle(
              color: Colors.white,
              fontSize: 30,
            ),
          ),
          PermissionStatusWidget(),
          ServiceEnabledWidget(),
          GetLocationWidget(),
          ListenLocationWidget(),
        ],
      ),
    );
  

任何帮助将不胜感激！

【问题讨论】：

您可能还想查看***.com/questions/63017280/…，因为您的问题是您没有等待异步调用完成，因此您的变量在使用时仍然为空。

【参考方案1】：

也许，_location.speed 导致了问题，因为最初，_location 为空。未来需要一些时间来执行，所以总结一下，您正在尝试访问 _location 的 speed 属性，该属性一开始为空。

试试这个：

Widget _calculateSpeedBetweenLocations() 

// Check if location is null
if(_location == null) return Text("Hold on!");

return Text(
  '$_location.speed != null && _location.speed * 3600 / 1000 > 0 ? (_location.speed * 3600 / 1000).toStringAsFixed(2) : 0 KM/h',
  style: TextStyle(
    color: Colors.lightGreen[500],
    fontSize: 20,
    letterSpacing: 4,
  ),
);

【讨论】：

谢谢先生！ if 解决了我的问题。