Question

I readed State Machine Ruby README.md at github.

I want to integrate SM with my Rails application.

However I am curious about what that line does:

before_transition :parked => any - :parked, :do => :put_on_seatbelt

Particulary this fragment looks like ‘magic’ to me:

any - :parked,

Soo, you subtract symbol from some kind of object(s) return by any helper.
How it is suppose to work and what exactly it does this entire line(before_transition ...)?

Answer 1

any is a singleton instance of the AllMathcher, which represents any state of your model. The minus (“-“) operator is actually an instance method of AllMatcher (reference here), which excludes the given state from the states of your model.

Therefore, if your model has states of :running, :stopped, and :parked, then any - :parked just returns states :running and :stopped (:parked is excluded).

And the meaning of the whole statement before_transition :parked => any - :parked, :do => :put_on_seatbelt is: before the model transits from :parked to any state but :parked, do the operation :put_on_seatbelt.