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behv: A Behavior Tree implementation for the Godot Engine, in GDScript

`behv_godot` is a Godot Engine addon that adds a collection of nodes to the editor that facilitate the implementation of Behavior Trees. It is released under the terms of the MIT License.

This project exists because I (the author, Jeff Olson) had tried `quabug/godot_behavior_tree`. While it worked at first, I started seeing weird changes in behavior when I’d make minor tweaks in the BT that would disappear in subsequent runs. I traced this to some inconsistent behavior/race in the Sequence/Selector primitives. The C++ code was completely baffling to me, though. So, I just re-implemented BTs in pure-GDScript so that I could get unblocked in my own Godot project and move on, as opposed to learning all the idiomatic C++ needed for Godot plugin work. This repository is the result of that effort. I acknowledge and apologize for the fact that this is obvious NIH in action.

Installation

  • Clone this repository into the `res://addons` folder of your project (or use `git submodule`) and, I guess, restart Godot.

Design philosophy

  • Easy to use and reason about - just Behavior Trees!
  • Unobtrusive - to integrate an existing workflow
    • the BT primitives are implemented as Nodes that get added directly onto a scene; Actions are just normal nodes that you extend with a script
    • Build out a tree and attach it to your actor/entity sprite and just call `.tick()` during `_process()` or `_fixed_process()`
    • Uses a simple `Dictionary` value as the context object
  • Add add little as possible to Godot -
    • The system uses existing numeric constants (`OK`, `FAILED` and `ERR_BUSY`) as stand-ins for the existing “Success, Failure, Running” concepts in BT.
    • The entire thing is implemented as a pure-GDScript addon that can be easily added to a project without having to rebuild the Godot editor

Node types

  • `BehvRoot` Place this at the root of your tree at the AI/agent level. Accepts only a single child (probably some kind of composite)
  • Composite Types:
    • `BehvSequence` - Runs a collection of child nodes, stopping at the first failure or `ERR_BUSY`
      • Will return and complete if any child returns `FAILED`, returning `FAILED`
      • Will return `OK` if all children return `OK`
      • Will return if any child returns `ERR_BUSY`. Will resume at the `ERR_BUSY`-returning child
    • `BehvSelector` - Runs a collection of child nodes, stopping at the first success or `ERR_BUSY`
      • Will return and complete if any child returns `OK`, returning `OK`
      • Will return `FAILED` if all children return `FAILED`
      • Will return if any child returns `ERR_BUSY`. Will resume at the `ERR_BUSY`-returning child
  • Decorator Types:
    • Inverter
    • Succeeder
  • Leaf Types
    • Action
    • Condition