Async - StreamController

 StreamController 

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english_words: ^4.0.0

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Source Code

import 'dart:async';
import 'package:flutter/material.dart';
import 'package:english_words/english_words.dart' as english_words;



void main() {
  runApp(  MaterialApp(
    title:" StreamController ",
    home:Scaffold(appBar: AppBar(title: Text(' StreamController ',),),
      body: Material(
        child: Center(
          child:
          StreamControllerExample()
          ,
        ),
      ) ,
    ),
  ));
}






class StreamControllerExample extends StatefulWidget {
  const StreamControllerExample({Key? key}) : super(key: key);

  @override
  _StreamControllerExampleState createState() =>
      _StreamControllerExampleState();
}

// The data class in our demo stream, contains just a message string and a
// timestamp.
class _Data {
  final String message;
  final DateTime timestamp;

  _Data({required this.message, required this.timestamp});
}

class _StreamControllerExampleState extends State<StreamControllerExample> {
  // The app puts value to _inputStreamController.sink, and renders widget from
  // _outputStreamController.stream.
  final _inputStreamController = StreamController<_Data>();
  final _outputStreamController = StreamController<Widget>();

  @override
  void initState() {
    super.initState();
    // This function connects the input and output stream controller, and does
    // the transformation (business logic) in between.
    void _onData(_Data data) {
      final widgetToRender = ListTile(
        title: Text(data.message),
        subtitle: Text(data.timestamp.toString()),
      );
      _outputStreamController.sink.add(widgetToRender);
    }

    _inputStreamController.stream.listen(_onData);
  }

  @override
  Widget build(BuildContext context) {
    return ListView(
      children: <Widget>[
        const Text(
            "StreamController is like a pipe with `sink` as input and `stream` "
                "as output. \n\n"
                "To put a new value to the stream, use `streamController.sink.add(newValue)`; "
                "to access the output stream, use `streamController.stream`.\n\n"
                "Note: we can't transform the stream's value inside the controller, "
                "the output is exactly the input. Instead, we can use two stream controllers, "
                "and put a function between the input controller and output controller.\n\n"
                "In this demo, the card renders the widget from _outputStreamController.stream "
                "and you can send a new random word to the "
                "_inputStreamController.sink by clicking the 'send' button.\n"),
        Card(
          elevation: 4.0,
          child: StreamBuilder<Widget>(
            stream: _outputStreamController.stream,
            builder: (BuildContext context, AsyncSnapshot<Widget> snapshot) {
              if (!snapshot.hasData) {
                return const ListTile(
                  leading: CircularProgressIndicator(),
                  title: Text('no data'),
                );
              }
              final Widget widgetToRender = snapshot.data!;
              return widgetToRender;
            },
          ),
        ),
        ElevatedButton.icon(
          icon: const Icon(Icons.send),
          label: const Text('Send random word to input stream'),
          onPressed: () => this._inputStreamController.sink.add(
            _Data(
              message: english_words.WordPair.random().asPascalCase,
              timestamp: DateTime.now(),
            ),
          ),
        )
      ],
    );
  }

  @override
  void dispose() {
    // Note: must explicitly close the stream controllers to release resource!
    _inputStreamController.close();
    _outputStreamController.close();
    super.dispose();
  }
}