How to use dart.core.sink in flutter - flutter

Question

import 'package:flutter/material.dart';
import 'dart:async';
void main() => runApp(MyApp());
//Using Bloc
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return StreamBuilder(
stream: bloc.darkThemeEnabled,
initialData: false,
builder: (context, snapshot) => MaterialApp(
theme: snapshot.data ? ThemeData.dark() : ThemeData.light(),
home: HomePage(snapshot.data)),
);
}
}
class HomePage extends StatelessWidget {
final bool darkThemeEnabled;
HomePage(this.darkThemeEnabled);
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Dynamic Theme"),
),
body: Center(
child: Text("Hello World"),
),
drawer: Drawer(
child: ListView(
children: <Widget>[
ListTile(
title: Text("Dark Theme"),
trailing: Switch(
value: darkThemeEnabled,
onChanged: bloc.changeTheme,
),
)
],
),
),
);
}
}
class Bloc {
final _themeController = StreamController<bool>();
get changeTheme => _themeController.sink.add;
get darkThemeEnabled => _themeController.stream;
}
final bloc = Bloc();
1.A warning says to Close instances of dart.core.sink
2.Why dart.core.sink is used in flutter?
3.How can I solve this error
4.Its error documentation redirects me to this website link
5.I don't know how to use these methods in flutter please guide me

Answer 1

dart.core.sink is an interface that is implemented by Stream.
The warning is showing, because the dart compiler wants you to .close() your instance of a Stream. In this case that is your final _themeController = StreamController<bool>().
If you want to fix the warning, add
void dispose() {
_themeController.close();
}
to your Bloc class.
Just adding the method is not doing much, since it's not called. So you should change your main() method to call bloc.dispose() after runApp(MyApp()).

Answer 2

That error occur when missing close StreamController.
Simple way to fix:
Create abstract class:
abstract class Bloc {
void dispose();
}
Your bloc class implements Bloc, now you can close StreamController in dispose:
class ColorBloc implements Bloc {
// streams of Color
StreamController streamListController = StreamController<Color>.broadcast();
// sink
Sink get colorSink => streamListController.sink;
// stream
Stream<Color> get colorStream => streamListController.stream;
// function to change the color
changeColor() {
colorSink.add(getRandomColor());
}
// Random Colour generator
Color getRandomColor() {
Random _random = Random();
return Color.fromARGB(
_random.nextInt(256),
_random.nextInt(256),
_random.nextInt(256),
_random.nextInt(256),
);
}
// close Stream
#override
void dispose() {
streamListController.close();
}
}