The State Machine

We add a new state DoorLocked and the new messages Lock and Unlock:

Notice the diamond node in the state diagram — this represents a conditional transition where the outcome depends on runtime data: The unlock operation can succeed (transitioning to DoorClosed) if the condition PinCorrect is met - or fail (staying in DoorLocked) when the condition PinIncorrect is met.

In the transition table the conditional transitions are expressed by the new “Guard” column. For most transitions however, this column is empty — these are unconditional transitions that always succeed.

Guard labels are not strictly required. Transitions can have multiple target states without explicit labels - like in the Countdown example. But the labels can be very useful to make the code and the state diagram more readable.