Impara Streams for Real-Time Flutter UIs | Asynchronous Dart in Flutter

Dart Streams are a powerful way to handle asynchronous sequences of data. In Flutter, you use Streams to listen for real-time updates—such as user actions, network responses, or sensor data—and update your UI instantly as new events arrive. Unlike Future, which returns a single value, a Stream delivers many values over time, making it ideal for chat messages, search suggestions, or live feeds.

main.dart


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

void main() => runApp(StreamDemoApp());

class StreamDemoApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: StreamCounterWidget(),
    );
  }
}

class StreamCounterWidget extends StatefulWidget {
  @override
  _StreamCounterWidgetState createState() => _StreamCounterWidgetState();
}

class _StreamCounterWidgetState extends State<StreamCounterWidget> {
  final StreamController<int> _controller = StreamController<int>();
  int _counter = 0;

  @override
  void initState() {
    super.initState();
    // Emit a new value every second
    Timer.periodic(Duration(seconds: 1), (timer) {
      _controller.sink.add(++_counter);
      if (_counter >= 5) timer.cancel();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Stream Counter')),
      body: Center(
        child: StreamBuilder<int>(
          stream: _controller.stream,
          builder: (context, snapshot) {
            if (snapshot.hasData) {
              return Text('Counter: [${snapshot.data}',
                  style: TextStyle(fontSize: 24));
            } else {
              return Text('Waiting for data...',
                  style: TextStyle(fontSize: 24));
            }
          },
        ),
      ),
    );
  }

  @override
  void dispose() {
    _controller.close();
    super.dispose();
  }
}

When you work with Streams in Flutter, you will often use a StreamController to add data to a stream and manage its lifecycle. Subscribing to a stream gives you a StreamSubscription, which you must manage carefully—especially in stateful widgets—to avoid resource leaks or unexpected behavior.

main.dart


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

void main() => runApp(SubscriptionDemoApp());

class SubscriptionDemoApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: SubscriptionWidget(),
    );
  }
}

class SubscriptionWidget extends StatefulWidget {
  @override
  _SubscriptionWidgetState createState() => _SubscriptionWidgetState();
}

class _SubscriptionWidgetState extends State<SubscriptionWidget> {
  final StreamController<String> _controller = StreamController<String>();
  StreamSubscription<String>? _subscription;
  String _latest = "No data";

  @override
  void initState() {
    super.initState();
    _subscription = _controller.stream.listen((data) {
      setState(() {
        _latest = data;
      });
    });
    // Simulate incoming data
    Future.delayed(Duration(seconds: 1), () {
      _controller.add("Hello from Stream!");
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('StreamSubscription Example')),
      body: Center(child: Text(_latest, style: TextStyle(fontSize: 24))),
    );
  }

  @override
  void dispose() {
    _subscription?.cancel();
    _controller.close();
    super.dispose();
  }
}

Note

Always cancel your StreamSubscriptions in the dispose method of a stateful widget. Failing to do so can cause memory leaks and unexpected behavior, as the widget may continue to listen to data even after it is removed from the widget tree.

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Sezione 2. Capitolo 2

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main.dart


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

void main() => runApp(StreamDemoApp());

class StreamDemoApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: StreamCounterWidget(),
    );
  }
}

class StreamCounterWidget extends StatefulWidget {
  @override
  _StreamCounterWidgetState createState() => _StreamCounterWidgetState();
}

class _StreamCounterWidgetState extends State<StreamCounterWidget> {
  final StreamController<int> _controller = StreamController<int>();
  int _counter = 0;

  @override
  void initState() {
    super.initState();
    // Emit a new value every second
    Timer.periodic(Duration(seconds: 1), (timer) {
      _controller.sink.add(++_counter);
      if (_counter >= 5) timer.cancel();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Stream Counter')),
      body: Center(
        child: StreamBuilder<int>(
          stream: _controller.stream,
          builder: (context, snapshot) {
            if (snapshot.hasData) {
              return Text('Counter: [${snapshot.data}',
                  style: TextStyle(fontSize: 24));
            } else {
              return Text('Waiting for data...',
                  style: TextStyle(fontSize: 24));
            }
          },
        ),
      ),
    );
  }

  @override
  void dispose() {
    _controller.close();
    super.dispose();
  }
}

main.dart


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

void main() => runApp(SubscriptionDemoApp());

class SubscriptionDemoApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: SubscriptionWidget(),
    );
  }
}

class SubscriptionWidget extends StatefulWidget {
  @override
  _SubscriptionWidgetState createState() => _SubscriptionWidgetState();
}

class _SubscriptionWidgetState extends State<SubscriptionWidget> {
  final StreamController<String> _controller = StreamController<String>();
  StreamSubscription<String>? _subscription;
  String _latest = "No data";

  @override
  void initState() {
    super.initState();
    _subscription = _controller.stream.listen((data) {
      setState(() {
        _latest = data;
      });
    });
    // Simulate incoming data
    Future.delayed(Duration(seconds: 1), () {
      _controller.add("Hello from Stream!");
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('StreamSubscription Example')),
      body: Center(child: Text(_latest, style: TextStyle(fontSize: 24))),
    );
  }

  @override
  void dispose() {
    _subscription?.cancel();
    _controller.close();
    super.dispose();
  }
}

Note

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Grazie per i tuoi commenti!

Sezione 2. Capitolo 2