Anything that manages state and must notify others of state changes has one of these, to allow others to register their desire to observe its state changes.
The current state
Receive a state-change request and if it is for a new state, pass it to observers
The actor whose state is changing
The state change event
Allow an observer to request notification
The state observer to register
Anything that manages state and must notify others of state changes has one of these, to allow others to register their desire to observe its state changes.