Sequential read issue

[vd3d]

Hi,

I try to have a sequential read of every message, because I have a protocol to follow, it is in 3 steps:

• connect
• auth
• subscribe

And so I wish to:

• connect + read the connection response
• send an "auth" message + read the auth response
• subscribe + listen to all the other messages

Here is what I tried:

WebSocketChannel channel = WebSocketChannel.connect(Uri.parse(...));
dynamic connectionResponse = await channel.stream.last;
_handleStreamError(jsonDecode(connectionResponse));

String authPayload = '{"action": "auth", "key": "' + Secrets.alpaca.OAuthClientId + '", "secret": "' + Secrets.alpaca.OAuthClientSecret + '"}';
channel.sink.add(authPayload);
dynamic authResponse = await channel.stream.last;
_handleStreamError(jsonDecode(authResponse));

But I got an exception at the last 'channel.stream.last' : Unhandled Exception: Bad state: Stream has already been listened to

Any idea why, and how to manage this ?

Thanks

[KammererTob]

The underyling Stream of the WebSocketChannel is a single-subscription stream. Meaning it can only be listened to once. By using stream.last twice, you try to listen to it twice, which isn't allowed. Here is an excerpt from the documentation:

A single-subscription stream allows only a single listener during the whole lifetime of the stream. It doesn't start generating events until it has a listener, and it stops sending events when the listener is unsubscribed, even if the source of events could still provide more. The stream created by an async* function is a single-subscription stream, but each call to the function creates a new such stream.

Listening twice on a single-subscription stream is not allowed, even after the first subscription has been canceled.

https://api.flutter.dev/flutter/dart-async/Stream-class.html