Flutter StreamBuilder vs FutureBuilder

Both StreamBuilder and FutureBuilder have the same behavior: They listen to changes on their respective object. And trigger a new build when they are notified of a new value.

So in the end, their differences are how the object they listen to works.

Future is like Promise in JS or Task in c#. They are the representation of an asynchronous request. Futures have one and only one response. A common usage of Future is to handle HTTP calls. What you can listen to on a Future is its state. Whether it's done, finished with success, or had an error. But that's it.

Stream on the other hand is like async Iterator in JS. This can be assimilated to a value that can change over time. It usually is the representation of web-sockets or events (such as clicks). By listening to a Stream you'll get each new value and also if the Stream had an error or completed.

How each of them listens to changes in a dynamic list?

A Future can't listen to a variable change. It's a one-time response. Instead, you'll need to use a Stream.

FutureBuilder is used for one time response, like taking an image from Camera, getting data once from native platform (like fetching device battery), getting file reference, making an http request etc.

On the other hand, StreamBuilder is used for fetching some data more than once, like listening for location update, playing a music, stopwatch, etc.

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Here is full example mentioning both cases.

FutureBuilder solves a square value and returns the result after 5 seconds, till then we show progress indicator to the user.

StreamBuilder shows a stopwatch, incrementing _count value by 1 every second.

void main() => runApp(MaterialApp(home: HomePage()));

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  int _count = 0; // used by StreamBuilder

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          _buildFutureBuilder(),
          SizedBox(height: 24),
          _buildStreamBuilder(),
        ],
      ),
    );
  }

  // constructing FutureBuilder
  Widget _buildFutureBuilder() {
    return Center(
      child: FutureBuilder<int>(
        future: _calculateSquare(10),
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.done)
            return Text("Square = ${snapshot.data}");

          return CircularProgressIndicator();
        },
      ),
    );
  }

  // used by FutureBuilder
  Future<int> _calculateSquare(int num) async {
    await Future.delayed(Duration(seconds: 5));
    return num * num;
  }

  // constructing StreamBuilder
  Widget _buildStreamBuilder() {
    return Center(
      child: StreamBuilder<int>(
        stream: _stopwatch(),
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.active)
            return Text("Stopwatch = ${snapshot.data}");

          return CircularProgressIndicator();
        },
      ),
    );
  }

  // used by StreamBuilder
  Stream<int> _stopwatch() async* {
    while (true) {
      await Future.delayed(Duration(seconds: 1));
      yield _count++;
    }
  }
}