My goal is to have a behavior tree that can run alone (autonomously) but also react to input from the player.

I'm making an AI for a hack and slash game where the AI will fight you, chase you, etc. and play defensive based on certain stats (health etc), but I want to add an additional layer; events from player. Say the player attacks, I want a random chance maybe 5-10% to trigger a dodge or step-back from the AI.

Currently I have its own selector running, but that is not sufficient, as it just has same priority or triggers on the same level as the other selectors / sequences in the tree. So how do I go about implementing a priority into the tree?

I want to have dodge, which should be labelled as maybe priority 1 if the event triggers, else it should just go about its usual business.


1 Answer 1


You did not specify engine, but any decent BT implementation should support some form of task abortion by a decorator, or at least re-running selection on demand.

Make dedicated event handling node right under the root selector, highest priority. Attach a decorator that checks if there is any pending event(s). Allow it to abort lower priority tasks. Put all relevant event handlers under that node.

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You probably want to add another decorator, that locks current event handler (ignores other events) for duration of the animation (e.g. dodge), but be aware of event priority (death > dodge) and relevance/expiration (e.g. chaining next dodge while current one runs).

How to implement required functionality from scratch.

Nodes should have an Abort method, and it must be deterministic: that is, once called, underlying processes (e.g. coroutines and threaded tasks) must be cancelled immediately. If that's not possible, then make Abort to be a task itself, although this is undesirable.

Decorators should recalculate their states once relevant underlying state changes. One way to do it is with Blackboard and Services. Blackboard is just a collection of variables relevant to the BT; blackboard must notify decorators when any variable changes. Service is blackboard update function that is ran periodically, it copies and/or recalculates relevant state from outside (the agent and the world) into blackboard.

(Note that I am using UE4 terminology here, not sure if standard; btw UE4 BTs are great, you may as well just copy UE's model).

Once relay/gate-type decorator recalculates it's state to new outcome that enables it's node, the decorator should ask BT to re-select current node, starting selection from decorator owner's parent, but only if currently running node is lower priority or child (otherwise BT would just select that higher-priority node again). Note that node need to be aborted only if BT selects another node, otherwise it should continue uninterrupted.

  • \$\begingroup\$ Thanks for the detailed post. I'm using Unity as the engine and coding it myself in c#. I like the task abortion way of doing it with a decorator checking for pending events. How would i go about implementing the "abort" portion of it? Use Threading or add an additional Nodestate? atm i have Failure, Success and Running. I find it hard to find a way to implement a non-blocking node-state atm :) ie running keeps going untill its either failure or success, i cant have it async atm. Thanks again for the reply! \$\endgroup\$
    – Angelo
    Commented Mar 21, 2021 at 22:30
  • \$\begingroup\$ @Angelo I did not realize that Unity has no built-in BT. I've edited my answer to include possible implementation. \$\endgroup\$ Commented Mar 22, 2021 at 20:02
  • \$\begingroup\$ @Angelo going async with C# should be easy, do not hesitate. Most tasks boil down to running on a timer and exiting if cancellation token is set. \$\endgroup\$ Commented Mar 22, 2021 at 20:12

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