update method
Functional state update — applies callback to the current
value and writes the result.
Equivalent to state = callback(state) but reads the underlying
field directly (bypassing state's reactive reportObserved):
an update is a write, not an observation, so the caller must
not be registered as a reactive dependent. Listener dispatch +
equality filtering run the same way as direct state = ...
writes.
Implementation
@protected
void update(StateUpdateCallback<TState> callback) {
_internalState.update(callback);
}