Flutter infinite loop when StreamBuilder inside LayoutBuilder

Issue

So i am making a page with a LayoutBuilder as described here

Inside the LayoutBuilder i put a StreamBuilder with a TextField powered by the bloc class SignupFormBloc. The stream is a BehaviorSubject

When someone put something in the input it trigger the onChanged function which is the sink for my stream. So i add the value in the stream then i pass the value in a StreamTransformer to validate the value and then i let the StreamBuilder to build the TextField again with an error message(if value not valid).

This is were the problem starts.

When i click on the TextField and enter something it starts an infinite loop like this:

  • The StreamBuilder sees the new value in the stream
  • The StreamBuilder try to rebuild TextField
  • Some how this triggers the LayoutBuilder builder function
  • The LayoutBuilder builder function builds again the StreamBuilder
  • StreamBuilder find a value in stream(because of the BehaviorSubject)
  • and all start again from the first bulled in an endless loop

Hint: If i change the BehaviorSubject to a PublishSubject everything is ok

Hint 2: If i remove the StreamBuilder completely and just let a blank TextField, you can see that in every entry the LayoutBuilder builder function run. Is that a normal behavior?

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:rxdart/rxdart.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(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {


  SignupFormBloc _signupFormBloc;

  @override
  void initState() {
    super.initState();
    _signupFormBloc = SignupFormBloc();
  }

  @override
  Widget build(BuildContext context) {
    print('Build Run!!!!!');
    return Scaffold(
      appBar: AppBar(

        title: Text(widget.title),
      ),
      body: LayoutBuilder(
        builder: (BuildContext context, BoxConstraints viewportConstraints) {
          print('Layout Builder!!!');
          return SingleChildScrollView(
            child: ConstrainedBox(
              constraints: BoxConstraints(
                minHeight: viewportConstraints.maxHeight,
              ),
              child: IntrinsicHeight(
                child:         StreamBuilder<String>(
                  stream: _signupFormBloc.emailStream,
                  builder: (context, AsyncSnapshot<String> snapshot) {

                    return TextField(
                      onChanged: _signupFormBloc.onEmailChange,
                      keyboardType: TextInputType.emailAddress,
                      decoration: InputDecoration(
                        hintText: 'Email',
                        contentPadding: const EdgeInsets.symmetric(horizontal: 15, vertical: 18),
                        filled: true,
                        fillColor: Colors.white,
                        errorText: snapshot.error,
                        border: new OutlineInputBorder(
                          borderSide: BorderSide.none
                        ),
                      ),

                    );
                  }
                ),
              ),
            ),
          );
        },
      )
    );
  }

  @override
  void dispose() {
    _signupFormBloc?.dispose();
    super.dispose();
  }

}


class SignupFormBloc  {

  ///
  /// StreamControllers
  ///
  BehaviorSubject<String> _emailController = BehaviorSubject<String>();


  ///
  /// Stream with Validators
  ///
  Observable<String> get emailStream => _emailController.stream.transform(StreamTransformer<String,String>.fromHandlers(handleData: (email, sink){

    final RegExp emailExp = new RegExp(r"^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$");

    if (!emailExp.hasMatch(email) || email.isEmpty){
      print('has error');
      sink.addError('Email format is invalid');
    } else {
      sink.add(email);
    }
  }));


  ///
  /// Sinks
  ///
  Function(String) get onEmailChange => _emailController.sink.add;


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



}

Solution

This happens because of a misuse of streams.

The culprit is this line:

Observable<String> get emailStream => _emailController.stream.transform(...);

The issue with this line is that it creates a new stream every time.

This means that bloc.emailStream == bloc.emailStream is actually false.

When combined with StreamBuilder, it means that every time something asks StreamBuilder to rebuild, the latter will restart the listening process from scratch.


Instead of a getter, you should create the stream once inside the constructor body of your BLoC:

class MyBloc {
  StreamController _someController;
  Stream foo;

  MyBloc() {
    foo = _someController.stream.transform(...);
  }
}

Answered By – Rémi Rousselet

Answer Checked By – David Goodson (FlutterFixes Volunteer)

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