setState() doesn't trigger build()

[feinstein]

I have a rather complicated widget, but it's general functionality is easy to explain. It starts to listen to a Stream in initState() and when the Stream emits a new value, it calls setState(). I know I could use a StreamBuilder, but this way I can wait for a Future to complete and then call setState(), whereas with StreamBuilder I need to return a new Widget synchronously.

AFAIK every call to setState() should trigger a build(), but when I go to InteliJ IDEA and click to Debug, sometimes the first time this Steam emits a value, setState() is called, but build() isn't. I checked mounted and it's true.

The docs don't say anything about the possibility of calling setState() and build() doesn't getting called right afterwards. Is this a bug on setState() or are the docs missing something?

My Flutter version:

Flutter 1.12.13+hotfix.5 • channel stable • https://github.com/flutter/flutter.git
Framework • revision 27321eb (6 weeks ago) • 2019-12-10 18:15:01 -0800
Engine • revision 2994f7e
Tools • Dart 2.7.0

[tekno0ryder]

Hi feinstein,
Can you post sample code of the problem?
You can use FutureBuilder inside your stream builder though

[feinstein]

Yeah, I know, it's just the code is so much simpler in the way it is right now.

I don't know if I will be able to shrink it down and reproduce it, it's kinda tedious to trigger it, as it needs a full build and an install to maybe trigger it, maybe not. I could send you the file by e-mail though, is this OK?

[tekno0ryder]

No problem, check the following sample, tried to listen to a stream and calling setState after doing a Future and it seems to work correctly

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int current = 0;

  @override
  void initState() {
    super.initState();
    streamListener();
  }

  Stream<int> _myStream() async* {
    for (var i = 0; i < 20; i++) {
      await Future.delayed(Duration(milliseconds: 1000));
      yield i;
    }
  }

  void streamListener() {
    _myStream().listen((onData) async {
      await Future.delayed(Duration(milliseconds: 500));
      current = onData;
      setState(() {});
    });
  }

  @override
  Widget build(BuildContext context) {
    print("Rebuild...");

    return Scaffold(
      appBar: AppBar(
        title: Text("Streams"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              "Value $current",
              style: TextStyle(fontSize: 24),
            )
          ],
        ),
      ),
    );
  }
}

[VladyslavBondarenko]

Hi @feinstein
is the issue actual?
could you edit the code above to provide a minimal complete reproducible code sample?

[feinstein]

Hi @VladyslavBondarenko, unfortunately this error is very hard to test as it's very intermittent and only happens after a new build and install, so testing it is tedious and slow, I will try to trigger it again this week and let you know.

[pedromassangocode]

Hi @feinstein
Once you have time please share the repro code.
Thank you

[TahaTesser]

Without additional information, we are unfortunately not sure how to resolve this issue.
We are therefore reluctantly going to close this bug for now.
Please don't hesitate to comment on the bug if you have any more information for us; we will reopen it right away!
Thanks for your contribution.