// events could be used for this callback as well, but using private inner
// classes calling a private member is probably the simplest.
private void TransitionToState <T>() where T : DocumentState, new()
{
// save prior for a moment
DocumentState priorState = State;
// this can be lookup from map instead of new() if you need to keep them
// alive for some reason. I personally like flyweight states.
DocumentState nextState = new T();
// activate the new state. it will get notified so it can do any one-
// time setup
State = nextState;
State.EnterState(this);
// let the prior state know as well, so it can cleanup if needed
if (priorState != null)
{
priorState.ExitState();
}
}
