create a circular stream in Dart - stream

Question

I try to write a chat application to chat with a computer. The user can write a message and gets a response of the computer. A chat history might look like this:
user: Hi
computer: Hello
user: What's your name?
computer: Bot
...
My circular stream based design is inspired by the ideas of Cycle.js. I've got a stream of user messages, which get transformed to a stream of computer messages, which are in turn the input of the user message stream:
|----> user message stream ---->|
| |
transform transform
| |
|<-- computer message stream <--|
This code already works:
import 'dart:io';
import 'dart:async';
void main() {
cycle(computer, user);
}
typedef Stream<T> Transform<T>(Stream<T> input);
void cycle(Transform aToB, Transform bToA) {
var aProxy = new StreamController.broadcast();
var b = aToB(aProxy.stream);
var a = bToA(b);
aProxy.add('start'); // start with user
aProxy.addStream(a);
}
Stream<String> user(Stream<String> computerMessages) {
computerMessages = computerMessages.asBroadcastStream();
computerMessages.listen((message) => print('computer: $message'));
return computerMessages.map((message) {
stdout.write('user: ');
return stdin.readLineSync();
});
}
Stream<String> computer(Stream<String> userMessages) {
var messages = <String, String>{
"Hi": "Hello",
"What's your name?": "Bot"
};
return userMessages.map((m) => messages.containsKey(m) ? messages[m] : 'What?');
}
There is only one problem. You need a start value to get a circular stream running. Therefore, I put this line in my function cycle:
aProxy.add('start'); // start with user
Actually, this logic belongs into my function user, since cycle shouldn't know the initial value(s). Moreover, I don't like to print the initial value. It should only trigger the user input stream. Thus, I changed cycle and user:
void cycle(Transform aToB, Transform bToA) {
var aProxy = new StreamController.broadcast();
var b = aToB(aProxy.stream);
var a = bToA(b);
aProxy.addStream(a);
}
Stream<String> user(Stream<String> computerMessages) {
computerMessages = computerMessages.asBroadcastStream();
computerMessages.listen((message) => print('computer: $message'));
var requestInput = new StreamController<String>.broadcast();
requestInput.add('start'); // start with user
requestInput.addStream(computerMessages); // continue on computer response
return requestInput.stream.map((message) {
stdout.write('user: ');
return stdin.readLineSync();
});
}
But with this change my application terminates immediately with no message in stdout. What's wrong?

Answer 1

I found a solution. Create a normal instead of a broadcast StreamController in user:
Stream<String> user(Stream<String> computerMessages) {
computerMessages = computerMessages.asBroadcastStream();
computerMessages.listen((message) => print('computer: $message'));
var requestInput = new StreamController<String>();
requestInput.add('start'); // start with user
requestInput.addStream(computerMessages); // continue on computer response
return requestInput.stream.map((message) {
stdout.write('user: ');
return stdin.readLineSync();
});
}
Even though I found a solution, I sadly can not really explain what's the problem with a broadcast StreamController. null is successfully added by requestInput.add(null); but strangely neither requestInput.addStream(computerMessages); completes nor events of computerMessages are added to requestInput. Thus, requestInput is closed and the application terminates. I would appreciate if someone could provide further explanation.