Flutter StreamBuilder vs FutureBuilder

Question

What is the main difference between StreamBuilder and FutureBuilder.

• What to use and when to use?

• What are the ta

Answer 1

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.

---

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++;
    }
  }
}

Answer 2

I find that sometimes real-world analogies work well for explaining / remembering concepts. Here's one - it's not perfect but it helps me.

Think that you are at one of those modern sushi restaurants where you have a belt going around the room with sushi boats on it. You just sit down and wait till one goes by, grab it and eat. But they also allow you to order carry out.

A Future is like the token with a number on it that they give you when you order takeout; you made the request, but the result is not yet ready but you have a placeholder. And when the result is ready, you get a callback (the digital board above the takeout counter shows your number or they shout it out) - you can now go in and grab your food (the result) to take out.

A Stream is like that belt carrying little sushi bowls. By sitting down at that table, you've "subscribed" to the stream. You don't know when the next sushi boat will arrive - but when the chef (message source) places it in the stream (belt), then the subscribers will receive it. The important thing to note is that they arrive asynchronously (you have no idea when the next boat/message will come) but they will arrive in sequence (i.e., if the chef puts three types of sushi on the belt, in some order -- you will see them come by you in that same order)

From a coding perspective -- both Futures and Streams help you deal with asynchrony (where things don't happen instantly, and you don't know when you will get a result after you make a request).

The difference is that Futures are about one-shot request/response (I ask, there is a delay, I get a notification that my Future is ready to collect, and I'm done!) whereas Streams are a continuous series of responses to a single request (I ask, there is a delay, then I keep getting responses until the stream dries up or I decide to close it and walk away).

Hope that helps.

Answer 3

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 is how the object they listen to works.

Future are 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 on a Future is it's state. Whether it's done, finished with a success, or had an error. But that's it.

Stream on the other hand are 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 click). 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.