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Why does a behavior tree decorator not run during execution?

I've got a behavior tree with a movement task in it. I have a decorator on the movement task node that aborts self if the movement gets too far from a given point. The problem is that the decorator conditional test isn't run until the movement task is completed. So basically the distance checking decorator isn't being run while the node its decorating IS being run.

The point is, you can't exit a node that is currently running, because the decorators that wrap it aren't being checked during the execute cycle. What should (I think) happen, is that during the execute update, decorators should be able to alter the returned execute value (i.e. instead of running, it should return failed/complete or whatever).

Reproduction: Make a BT with a moveto node in a sequence, have a decorator that should exit that movement (I use a distance from a starting point check as my test) and watch as the movement completes rather than stopping part-way through as expected.

Any advice welcome here.

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asked Sep 10 '14 at 04:13 PM in Bug Reports

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avatar image Ben Halliday STAFF Sep 15 '14 at 06:47 PM

Hi zoombapup,

I'm not seeing this behavior. Are you still experiencing this issue? Are you in 4.4.2? If so, could you post an image of your distance-from-starting-point decorator's event graph? Thanks!

avatar image zoombapup Sep 15 '14 at 07:18 PM

Hi Ben,

I've had a discussion with Mieszko about it and found an alternative. The problem seems to be that the decorator I'd written does not function the same as the Blackboard decorators. Essentially the blackboard decorators are woken by the observer on the blackboard values they are using, so as the values change the blackboard decorator is allowed to run and thus can exit the movement.

But my own decorators do not explicitly observe a blackboard value, but instead they read the value during execution of the condition. The problem is that I cannot find a node that allows me to add an observer callback for my own decorators that are then executed when the blackboard value I use changes.

So to reproduce, you would need to write your own decorator that checks a value in the blackboard and simply breakpoint when it gets called. It would get called before the node it decorates is run, and then when the node it decorates completes and not "during" that node's execution. Essentially I need some way of sayiing "when this node is executing, make sure to test my condition every time too", which CAN happen with the blackboard decorators because their observed values can run their conditions, but doesn't work for standard decorators.

I hope that is clear.

avatar image swiftwords Oct 20 '14 at 11:49 PM

Hi zoombapup and Ben,

I'm having the same issues with trying to write a Decorator that can exit a currently running node like MoveTo. Is there a fix for this, or do we need to just limit ourselves to the decorators that come by default?

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What zoombapup (hi Phil!) said is accurate. Currently there's no way to have a BP-implemented decorator that will be asked to evaluate it's condition every frame. I've taken a note and will add this option soon-ish (4.6?). How about if a decorator has Observer Aborts to non-`None` then its ReceiveConditionCheck gets called every frame? I guess that's what you expected up front, right? :D



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answered Oct 21 '14 at 08:32 PM

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MieszkoZ STAFF
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avatar image swiftwords Oct 21 '14 at 11:00 PM

That would be fantastic, Thanks for the reply!

avatar image zoombapup Oct 22 '14 at 08:25 PM

Yeah, basically it seems the right option is to be able to execute the condition as an inline check if you aren't observing something (i.e. its polled instead of observer-event based).

avatar image Deventico Mar 11 '15 at 10:07 PM

Does this work in 4.7? Can we have custom decorators that check every frame?

I seem to get that behavior, but wasn't sure if maybe I made a mistake.

avatar image MieszkoZ STAFF Mar 12 '15 at 10:53 AM

You need to overrider PerformConditionCheck or PerformConditionCheckAI and either have no BlackboardKeySelectors (which indicate you want to check condition on BB values' changes) or have bCheckConditionOnlyBlackBoardChanges set to false.

avatar image Deventico Mar 12 '15 at 06:07 PM

Thank you very much, I overlooked the checkbox and couldn't find anything in the documentation about it.

When I tried to use it, I ran into issues however...

I opened a new question for that.

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