Behaviour of Sequence Node

[thassa2s]

Hello Daniel,
I observed that for a sequence node, py_trees library does not recheck the status of completed child nodes on successive ticks. Wouldn't this remove the reactivity of behaviour trees? Is this desired or planned to be this way?

Behaviour trees -- at least those used in robot behaviour and task planning -- would require this reactivity. For example, actions and their preconditions are attached to a sequence node. An action is started when the preconditions are fulfilled and should be terminated when these no longer hold. For this, it is necessary to recheck the status of the preconditions (which are the left nodes) at every tick. I have written a small patch to make py_trees sequence nodes (behaviour) function this way.

I just wanted to check with you if the behaviour of the sequence node was desired to be this way or whether the library would in the future include also a more reactive or responsive version of sequential behaviours.

Best regards,
Teena

[stonier]

Aye, a sequence without memory ... coming very soon. See the recently started thread in #313. You can also achieve a similar effect with a parallel (so long as your preconditions are binary, i.e. return S or F only), e.g.

+ Parallel (SuccessOnAll)
  + Sequence
    - PreCondition 1
    - PreCondition 2
  - Action

You can relax the binary condition on the preconditions by using SuccessOnSelected(action, synchronise=False).