In behavior trees, how do you retain your place in a sequence?

Question

For context, I am coding a naive implementation of behavior trees myself, in C#. Let's say I have two scenarios that I want to create behaviors for. Both AI's will traverse/patrol four specific locations (markers), in order, and then repeat the path.

AI 1) Upon seeing the player, freeze. Once the player is out of sight, begin patrolling again, but from the very first marker (i.e. restart).

AI 2) Upon seeing the player, freeze. Once the player is out of sight, begin patrolling again, but from whatever marker it was previously on before being interrupted by the player (i.e. carry on).

I would expect both of these behaviors to be modeled by first checking for the player, and if that fails proceeding further down to a Sequence node that contains four children, each being a "traverse to maker" behavior.

In other words -> after a behavior is interrupted by a node higher up the tree, how do I retain the current place in a sequence and carry on when we finally make it back down the tree again?

Answer 1

Put all the information needed to restore the state you're interrupting into some structure representing the context of the current action. This is likely at least:

• a reference to the tree about to be interrupted (if you have more than one tree capable of processing at a time)
• a reference to the node about to be interrupted
• state specific to that node (e.g., destination if a "move to target" node, et cetera)

You may want to stuff in some extra information like the time of interruption, so that you can do some logic to enable AI to disregard old actions if their interruption ended up lasting more than X seconds, or whatever.

Put all that kind of stuff in something called, say, InterruptedActionContext. When a node is about to be interrupted, build one of those contexts from the node that is currently executing and stuff it onto a stack of interrupted actions, and then proceed with the interrupting node.

Whenever a node naturally completes (or a branch of the tree naturally completes; whatever criteria is most important to you), check the interruption stack to see if you should pop something from it and return to a previously-interrupted behavior. Otherwise continue as you would normally.