checkTimeout method
Check if the current state has timed out.
Implementation
ZModemEvent? checkTimeout() {
final timeout = _stateTimeouts[_state];
if (timeout == null) return null;
if (_stateEnteredAt == null) return null;
if (clock.now().difference(_stateEnteredAt!) > timeout) {
_enqueue(ZModemHeader.fin());
_state = ZModemState.closed;
_stateEnteredAt = clock.now();
return ZTimeoutEvent(_state.name);
}
return null;
}