on method
Implementation
SocketServer on(String event, Function listener) {
if (event == 'open' && listener is SocketOpenCallback) {
_onOpen = listener;
} else if (event == 'close' && listener is SocketCloseCallback) {
_onClose = listener;
} else if (event == 'error' && listener is SocketErrorCallback) {
_onError = listener;
} else if (event == 'message' && listener is SocketMessageCallback) {
_onMessage = listener;
}
return this;
}