How to get the page is not disposed - flutter

Question

I have application which has mappage using location
class _MapPageState extends State<MapPage> {
LocationData currentLocation;
Location _locationService = new Location();
#override
void initState(){
super.initState();
_locationService.onLocationChanged().listen((LocationData result) async {
setState(() {
print(result.latitude);
print(result.longitude);
currentLocation = result;
});
});
}
In this case, setState() works well when mappage is shown.
However after mappage is disposed, there comes error like this.
E/flutter ( 6596): This error happens if you call setState() on a State object for a widget that no longer appears in the widget tree (e.g., whose parent widget no longer includes the widget in its build). This error can occur when code calls setState() from a timer or an animation callback.
E/flutter ( 6596): The preferred solution is to cancel the timer or stop listening to the animation in the dispose() callback. Another solution is to check the "mounted" property of this object before calling setState() to ensure the object is still in the tree.
So, I have two ideas.
Remove onLocationChanged() listener when page is disposed.
Check if State is disposed or not before setState()
How can I solve this??

Answer 1

You can copy paste two files below and directly replace official example's code
https://github.com/Lyokone/flutterlocation/tree/master/location/example/lib
After Navigate to ListenLocationWidget page,
you can call _stopListen() in dispose()
code snippet
class _MyHomePageState
...
RaisedButton(
child: Text('Open route'),
onPressed: () {
Navigator.push(
context,
MaterialPageRoute<void>(
builder: (context) => ListenLocationWidget()),
);
},
),
PermissionStatusWidget(),
Divider(height: 32),
ServiceEnabledWidget(),
Divider(height: 32),
GetLocationWidget(),
Divider(height: 32),
//ListenLocationWidget()
class _ListenLocationState extends State<ListenLocationWidget> {
...
StreamSubscription<LocationData> _locationSubscription;
String _error;
#override
void initState() {
print("initState");
super.initState();
_listenLocation();
}
#override
void dispose() {
print("stopListen");
_stopListen();
super.dispose();
}
Future<void> _listenLocation() async {
_locationSubscription =
location.onLocationChanged.handleError((dynamic err) {
setState(() {
_error = err.code;
});
_locationSubscription.cancel();
}).listen((LocationData currentLocation) {
setState(() {
print("setState");
_error = null;
_location = currentLocation;
});
});
}
Future<void> _stopListen() async {
_locationSubscription.cancel();
}
working demo
full code ListenLocationWidget
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() {
print("initState");
super.initState();
_listenLocation();
}
#override
void dispose() {
print("stopListen");
_stopListen();
super.dispose();
}
Future<void> _listenLocation() async {
_locationSubscription =
location.onLocationChanged.handleError((dynamic err) {
setState(() {
_error = err.code;
});
_locationSubscription.cancel();
}).listen((LocationData currentLocation) {
setState(() {
print("setState");
_error = null;
_location = currentLocation;
});
});
}
Future<void> _stopListen() async {
_locationSubscription.cancel();
}
#override
Widget build(BuildContext context) {
return SafeArea(
child: Container(
color: Colors.white,
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
Text(
'Listen location: ' + (_error ?? '${_location ?? "unknown"}'),
style: Theme.of(context).textTheme.body2,
),
Row(
children: <Widget>[
Container(
margin: const EdgeInsets.only(right: 42),
child: RaisedButton(
child: const Text('Listen'),
onPressed: _listenLocation,
),
),
RaisedButton(
child: const Text('Stop'),
onPressed: _stopListen,
)
],
),
],
),
),
);
}
}
full code main.dart
import 'package:flutter/material.dart';
import 'package:location/location.dart';
import 'package:url_launcher/url_launcher.dart';
import 'get_location.dart';
import 'listen_location.dart';
import 'permission_status.dart';
import 'service_enabled.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Location',
theme: ThemeData(
primarySwatch: Colors.amber,
),
home: const MyHomePage(title: 'Flutter Location Demo'),
);
}
}
class MyHomePage extends StatefulWidget {
const MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
final Location location = Location();
Future<void> _showInfoDialog() {
return showDialog<void>(
context: context,
builder: (BuildContext context) {
return AlertDialog(
title: const Text('Demo Application'),
content: SingleChildScrollView(
child: ListBody(
children: <Widget>[
const Text('Created by Guillaume Bernos'),
InkWell(
child: Text(
'https://github.com/Lyokone/flutterlocation',
style: TextStyle(
decoration: TextDecoration.underline,
),
),
onTap: () =>
launch('https://github.com/Lyokone/flutterlocation'),
),
],
),
),
actions: <Widget>[
FlatButton(
child: const Text('Ok'),
onPressed: () {
Navigator.of(context).pop();
},
),
],
);
},
);
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
actions: <Widget>[
IconButton(
icon: Icon(Icons.info_outline),
onPressed: _showInfoDialog,
)
],
),
body: Container(
padding: const EdgeInsets.all(32),
child: Column(
children: <Widget>[
RaisedButton(
child: Text('Open route'),
onPressed: () {
Navigator.push(
context,
MaterialPageRoute<void>(
builder: (context) => ListenLocationWidget()),
);
},
),
PermissionStatusWidget(),
Divider(height: 32),
ServiceEnabledWidget(),
Divider(height: 32),
GetLocationWidget(),
Divider(height: 32),
//ListenLocationWidget()
],
),
),
);
}
}