The protocol which was selected during ALPN protocol negotiation.
Returns null if one of the peers does not have support for ALPN, did not specify a list of supported ALPN protocols or there was no common protocol between client and server.
String get selectedProtocol;
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Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dart.dev/stable/2.5.0/dart-io/SecureSocket/selectedProtocol.html