Async - StreamBuilder

 StreamBuilder 

Source Code

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



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






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

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

class StreamBuilderExampleState extends State<StreamBuilderExample> {
  // The timer's value shown on app's UI.
  int _timerValue = 0;
  bool _paused = true;
  // A stream that updates every second. The stream's value is the number of
  // seconds elapsed since the app is started. We simply print this stream value
  // out in command-line. The value shown on UI is this._timerValue.
  final Stream<int> _periodicStream =
  Stream.periodic(const Duration(milliseconds: 1000), (i) => i);
  // Record of the latest stream value that we saw. Because the StreamBuilder is
  // rebuilt when we call setState(), and in the re-build we shouldn't increment
  // this._timerValue if the latest snapshot's value hasn't changed.
  int _previousStreamValue = 0;

  @override
  Widget build(BuildContext context) {
    return Container(
      alignment: Alignment.center,
      margin: const EdgeInsets.all(8),
      child: StreamBuilder(
        stream: this._periodicStream,
        builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
          if (snapshot.hasData) {
            if (snapshot.data != this._previousStreamValue) {
              print('Latest snapshot from stream: ${snapshot.data}');
              this._previousStreamValue = snapshot.data!;
              if (!_paused) {
                this._timerValue++;
              }
            }
          }
          return Column(
            mainAxisSize: MainAxisSize.min,
            children: <Widget>[
              const Text("StreamBuilders can listen to a stream, and update UI "
                  "according to the stream's latest snapshot value.\n\n"
                  "In this demo we build a simple timer app by listening to a "
                  "stream that updates every second.\n"),
              Card(child: _buildTimerUI()),
            ],
          );
        },
      ),
    );
  }

  Widget _buildTimerUI() {
    return Column(
      children: <Widget>[
        Text(
          '$_timerValue',
          style: Theme.of(context).textTheme.headline4,
        ),
        ButtonBar(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            IconButton(
              icon: Icon(this._paused ? Icons.play_arrow : Icons.pause),
              onPressed: () => setState(() => this._paused = !this._paused),
            ),
            IconButton(
              icon: const Icon(Icons.stop),
              onPressed: () => setState(
                    () {
                  this._timerValue = 0;
                  this._paused = true;
                },
              ),
            ),
          ],
        ),
      ],
    );
  }
}