Introduction

Reactive programming in Flutter leverages asynchronous data flows—known as streams—to update UI in response to state changes. Instead of manually calling setState, you subscribe widgets to streams that emit events. This approach decouples business logic from presentation, making apps more maintainable. In this tutorial, you’ll learn how to create, consume, and dispose streams for straightforward state management, all with concise Dart code.

Understanding Streams in Dart

A Dart Stream is a sequence of asynchronous data events. You can think of it like a conveyor belt that delivers values over time. There are two main types:

• Single-subscription streams—ideal for one listener.

• Broadcast streams—allow multiple listeners simultaneously.

To produce events, you typically use a StreamController. The controller provides a sink for input and exposes a stream for output.

Key methods:

• controller.add(value) pushes new data.

• controller.close() signals completion.

• Always close your controller to prevent memory leaks.

Building a Reactive Counter with StreamBuilder

Let’s build a simple counter that emits an integer each time a button is pressed.

import 'dart:async';

class CounterBloc {
  final _controller = StreamController<int>();
  int _count = 0;

  Stream<int> get countStream => _controller.stream;
  void increment() {
    _controller.add(++_count);
  }

  void dispose() {
    _controller.close();
  }
}

Here, CounterBloc holds a private counter and exposes a Stream. Calling increment() emits the next value. Always call dispose() when the widget is removed to close the stream.

Displaying Stream Data with StreamBuilder

Flutter’s StreamBuilder widget listens to a stream and rebuilds when new data arrives. Embed it in your UI as follows:

import 'package:flutter/material.dart';

class CounterPage extends StatefulWidget {
  @override
  _CounterPageState createState() => _CounterPageState();
}

class _CounterPageState extends State<CounterPage> {
  final bloc = CounterBloc();

  @override
  void dispose() {
    bloc.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: StreamBuilder<int>(
          stream: bloc.countStream,
          initialData: 0,
          builder: (_, snapshot) => Text(
            'Count: ${snapshot.data}',
            style: TextStyle(fontSize: 32),
          ),
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: bloc.increment,
        child: Icon(Icons.add),
      ),
    );
  }
}

In this snippet:

• stream binds your stream of events.

• initialData sets the starting UI state.

• The builder callback rebuilds the Text widget whenever the stream emits a new value.

Best Practices for Stream Management

• Always dispose controllers in dispose().

• Choose single-subscription streams for unique flows; use broadcast when multiple widgets need the same data.

• Consider packages like rxdart for advanced operators (map, debounce, merge).

• Structure your code with the BLoC (Business Logic Component) pattern: separate UI from logic, group sinks and streams in one class.

This results in testable, modular components. If you’re streaming data from Firebase, use FirebaseFirestore.instance.collection('items').snapshots() directly in a StreamBuilder for real-time syncing.

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Conclusion

Reactive programming with streams in Flutter allows you to build responsive, maintainable applications by treating state changes as events. By encapsulating logic in classes like BLoCs, exposing streams, and consuming them via StreamBuilder, you achieve a clean separation of concerns and seamless UI updates. As you grow more comfortable, explore RxDart operators and higher-level state management libraries that build on streams.