How to use dart.core.sink in 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();
    }
  

  }