registerState method
Sets the underlying state object and registers this object's state listeners to be notified of updates to the state's stream.
Implementation
void registerState(S state) {
if (identical(_state, state)) return;
if (_state != null) _state.dispose();
_state = state;
_state.controller = this;
_state.subscribeToMutations(onStateMutate,
onError: onStateError, onDone: onStateDone);
this.publishUpdate();
}