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State Machine from Scratch

In most languages and frameworks, good State Machine libraries exist, like Stateless. Use them when possible. When it’s not possible, there are principles you need to have in your head before you start.

tl;dr: A FromState can go to a ToState when some Trigger causes a Transition only if some GuardCondition is met.

Decide whether you’re building a generic state machine or a specialised one. Are you going to need other, differently-configured StateMachines in this solution? Or is this the only one? Are you building a library or a user story? Maybe you want to write the simplest thing that works, and refactor later.

This is not a definitive guide; it’s more a like a public notebook page :)

A StateMachine Class

Can have:

Naive Implementation

If you know up front that all states have the same triggers then these could be methods of the SM:

class StateMachine
	- State
	- Model
	- ForwardTransitions<State,State>
	- BackTransitions<State,State>
	+ Forward()
	+ Back()
	+ Reset()

And the Forward subroutine checks that the transition exists and the condition is met:

void Forward()
	Transition = ForwardTransitions.For(State)
	If Transition?.ConditionMet(Model)
		State = Transition.NextState

Even lazier, if you don’t want to shape out a Transition class but instead are maintaining a list of <FromState, ToState> and want to hard-code the guard conditions to get an 80% solution:

void Forward()
	Allow = False
	Switch (State)
		Case WelcomeState:
			Allow = Model.Name.IsSomething
		Case AgeState:
			Allow = Model.Age >= 18
	If Allow
		Transition = ForwardTransitions.For(State)
		State = Transition.NextState

More Sophisticated

In any more complex scenario, you won’t be able to hard-code either the triggers or the guard clauses. They should be given to the StateMachine at config time; triggers could be Enums and Guard clauses could be lambdas (or Func or similar in C#).

Doing a Transition when a Trigger happens

Your code which calls the StateMachine trigger needs to know the result. Two ways to acheive this are:


Like I said, not a definitive guide, but hopefully future me/someone else finds this helpful to get a quick overview of state machine code concepts.