Flutter 获取用户位置实时错误：setter 'latitude' was called on null

Question

我是新手。我正在尝试获取用户位置的实时信息。为此，我使用了位置插件 (https://pub.dev/packages/location)。

问题是地图在分配位置之前被填充，并返回错误：

在 null 上调用了 setter 'latitude'。

这是主要的飞镖：

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'dart:async';
import 'package:flutter/services.dart';
import 'package:location/location.dart';
import './models/userLocation.dart';

void main() => runApp(Maps());

class Maps extends StatefulWidget {
  @override
  _MapsState createState() => _MapsState();
}

class _MapsState extends State<Maps> {

  LocationData currentLocation;

  UserLocation _currentLocation;



  var location = new Location();


  String error;


  void _onMapCreated(GoogleMapController controller) {
    mapController = controller;
  }
  GoogleMapController mapController;

  void initState() {
    super.initState();

    initMapState();


  }

  initMapState() async {
    try {
      var userLocation = await location.getLocation();
      _currentLocation = UserLocation(
        latitude: userLocation.latitude,
        longitude: userLocation.longitude,
      );

    } on PlatformException catch (e) {
      if (e.code == 'PERMISSION_DENIED') {
        error = 'Permission denied';
      }
      return _currentLocation;

    }

    location.onLocationChanged().listen((LocationData currentLocation) {
      print(currentLocation.latitude);
      print(currentLocation.longitude);

    });
  }

  @override
  Widget build(BuildContext context) {

    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Create new trip'),
          backgroundColor: Colors.green[700],
        ),
        body: GoogleMap(
          onMapCreated: _onMapCreated,
          initialCameraPosition: CameraPosition(
            target:
                LatLng(_currentLocation?.latitude,_currentLocation?.longitude),
            zoom: 11.0,
          ),
        ),
      ),
    );
  }
}

以下是定位模型：

class UserLocation {
   double latitude;
   double longitude;
  UserLocation({this.latitude, this.longitude});
}

感谢您的支持。

谢谢，

编辑：

更新的工作文件：

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'dart:async';
import 'package:flutter/services.dart';
import 'package:location/location.dart';
import './models/userLocation.dart';

void main() => runApp(Maps());

class Maps extends StatefulWidget {
  @override
  _MapsState createState() => _MapsState();
}

class _MapsState extends State<Maps> {


  Location location = new Location();
  UserLocation userLocation;
  StreamSubscription<LocationData> positionSubscription;

  UserLocation _currentLocation;


  String error;


  void _onMapCreated(GoogleMapController controller) {
    mapController = controller;
  }
  GoogleMapController mapController;

  @override
  void initState() {
  super.initState();
  positionSubscription = location
      .onLocationChanged()
      .handleError((error) => print(error))
      .listen(
        (newPosition) => setState(() {
          _currentLocation = UserLocation(
            latitude: newPosition.latitude,
            longitude: newPosition.longitude,
          );
        }),
      );
}

@override
void dispose() {
  positionSubscription?.cancel();
  super.dispose();
}


  @override
  Widget build(BuildContext context) {

    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Create new trip'),
          backgroundColor: Colors.green[700],
        ),
        body: GoogleMap(
          onMapCreated: _onMapCreated,
          initialCameraPosition: CameraPosition(
            target:
                LatLng(_currentLocation.latitude,_currentLocation.longitude),
            zoom: 11.0,
          ),
        ),
      ),
    );
  }
}

Answer 1

• 因此，由于您的 initState 未使用 @override，因此此方法是 不叫

• 您不能在 onInit 回调中使用异步调用。这是错误的。再一次，你 错过了@override。

• 根据文档，您无需致电 getLocation，因为您 已经订阅了位置信息。

理想情况下，你需要这样的东西：

UserLocation userPosition;
StreamSubscription<LocationData> positionSubscription;

@override
void initState() {
  super.initState();
  positionSubscription = location
      .onLocationChanged()
      .handleError((error) => print(error))
      .listen(
        (newPosition) => setState(() {
          _currentLocation = UserLocation(
            latitude: newPosition.latitude,
            longitude: newPosition.longitude,
          );
        }),
      );
}

@override
void dispose() {
  positionSubscription?.cancel();
  super.dispose();
}

Answer 2

在initMapState();之后写super.initState()，因为在调用super.initState()之后直接调用build()，不会等待initMapState()完成执行

void initState() {
    initMapState();
    super.initState();
  }