Flutter websocket onOpen? - flutter

Question

I'm trying to create a websocket connection in Flutter.
I followed this tutorial and this
My query is that how can we know once the websocket connection is established? Like there is onOpen event in Javascript.
My code in flutter is:
void main() async {
final channel = IOWebSocketChannel.connect('wss://my.server.org');
channel.stream.listen(
(dynamic message) {
debugPrint('message $message');
},
onDone: () {
debugPrint('ws channel closed');
},
onError: (error) {
debugPrint('ws error $error');
},
);
}

Answer 1

This is what I used in one project to connect to AWS AppSync service. The communication is implemented over websockets.
What you do is parse the messages you receive through web socket. Messages will carry the data, but also there are messages to tell you that the connection was established, keep alive message etc.
I don't remember if the messages below are AppSync specific, or are applicable to any websocket server - but it should at least give you an idea how this could work.
var channel=IOWebSocketChannel.connect(url, protocols: ['graphql-ws']);
channel.sink.add(json.encode({"type": "connection_init"}));
channel.stream.listen((event) {
var e = json.decode(event);
switch (e['type']) {
case 'connection_ack':
wsTimeoutInterval = e['payload']['connectionTimeoutMs'];
var register = {
'id': uniqueKey,
'payload': {
'data': json.encode(query),
'extensions': {'authorization': header}
},
'type': 'start'
};
var payload = json.encode(register);
channel.sink.add(payload);
break;
case 'data':
// calling a callback function Function(Map<String, dynamic>) listener
listener(e);
break;
case 'ka':
print('Keep alive message received!!!');
break;
case 'start_ack':
print('Ws Channel: Subscription started');
break;
default:
print('Unimplemented event received $event');
}
}, onError: (error, StackTrace stackTrace) {
// error handling
print('Ws Channel: $error');
}, onDone: () {
// communication has been closed
channel.sink.close();
print('Ws Channel: Done!');
});