How to transition between child and parent states in HSM?

[drvelociraptor]

Basically I have a movement state and an attack state as a child. I want to change from movement to attack and viceversa. Can this be done? It seems add_transition just works with direct child states, and change_active_state gives me this error: "Unable to perform transition to a state that is not a child of this HSM."... Don't know what to do because I can't find anything on the docs, all examples seem to be for basic FSM.

[limbonaut]

Can you provide an example of your state hierarchy?
And correct - transitions can be declared for the direct children only. Nested HSM can have their own transitions. (Nested) HSM has initial_state which dictates which substate becomes active when it is entered. If not set, it defaults to the first child.

[drvelociraptor]

Just tryied to add_transition to terra and this happens: "Invalid call. Nonexistent function 'add_transition' in base 'LimboState'." In your example "terra" was a LimboState?

[limbonaut]

Just tryied to add_transition to terra and this happens: "Invalid call. Nonexistent function 'add_transition' in base 'LimboState'." In your example "terra" was a LimboState?

I think, "terra" should be LimboHSM, and in your case it is LimboState

[limbonaut]

State machine is not just in "leaf" state. It is in each state that is currently active, so in case of active atacTF it would be in the following states: LimboHSM, terra, atacTF. However, terra should be LimboHSM type, not LimboState. LimboState can't have child states.

[limbonaut]

I assume there’s a reason for this, but can I ask why it works this way?

LimboState is not a composite state, so it cannot contain other states as children (you could use class inheritance instead if you want to share some functionality). Adding more states under vanilla LimboState not going to work. And LimboHSM is a composite that can have exactly one child substate active. We call it active substate. Think of it like a switch - only one of the children states is allowed to be active. And this is what transitions do - switch the active substate on a level. LimboHSM is a state itself (inherits from LimboState), so you can nest them like so:

- LimboHSM1
--- LimboHSM2
----- LimboState1
----- LimboState2
--- LimboState

So in other words,

• LimboState is a leaf
• LimboHSM is a container (aka composite), can be nested

LimboHSM can define its own logic as well, so if LimboState1 is active, LimboHSM2 is also active, so is LimboHSM1. But by definition, HSM must be in one of its substates.

You can read more here: https://statecharts.dev/glossary/compound-state.html
We are not exactly following state charts, but in this case, most details are pretty much how we do it.

[drvelociraptor]

Thanks for the explanation! It all makes sense now. States cannot have childs and HSM act like state folders. Here the correct structure that works for me now: