state-machinespring-statemachine

State machine: In which state is an action on a transition performed?

I'm just reading into the theory of state machines. Please consider this:

             event[guard]/action
State A -----------------------------> State B

Here's my question: If I define a transition between states A and B, with event, guard, and action, like in the above "picture"; and furthermore the event is received and the guard expression evaluates to true, then: will the action be performed while my object is in state A, or B?

In other words, do I need the action to be configured to be performable in state A, or B (let's assume I want to choose only one state in which the action can be performed)?

Google finds tell me that the action will be performed at the exact time of transitioning; but my brain has problems to accept it: imo my object needs to be in a certain state while the action is being performed (just because my object needs to always be in a certain state). And the performing of the action may take a while.

Related: What happens if an error occurs during the performing of the action. Will my object stay in state A, or will it transition to state B anyways (remember that the event was received and the guard expression evaluated to true)?

Solution

  • This is fairly easy to check with a custom state machine listener, in which you override the corresponding methods for entering/exiting state and transitions.

    will the action be performed while my object is in state A, or B?

    Your action (which is on transition) will be performed while you're in state A. The order of what happens is the following:

    Started transition
State Entered: A
SM changed states from:null to: A
Ended transition
---
Executing guard logic
Started transition
Executing normal action //action is executed before exiting State A
State exited: A
State Entered: B
SM changed states from:A to: B
Ended transition

    What happens if an error occurs during the performing of the action. Will my object stay in state A, or will it transition to state B anyways

    You will stay in State A.

    As you can see in the above output, the exit of the state happens after the action is executed (successfully). If an exception occurs before that you will still be in state A.