Behaviour Tree

Behaviour Tree runs ScriptableObject tasks and services from a Behaviour component and provides a node editor for authoring trees. It uses the sibling Behaviour Vars package for shared values.

Package id: com.deathbygravitystudio.behaviour.tree (0.1.0). Display name: Behaviour Tree. Runtime assembly: Behaviour.Tree. Editor assembly: Behaviour.Tree.Editor.

• Tree assets (Behaviours/Tree create menu, or Behaviour > Tree > Create Tree Asset...). A new asset starts with a root Composite node. Tree extends ILayer from Behaviour Vars, so each asset holds the shared variable list.
• Behaviours.BT.Behaviour ticks one tree. With playOnAwake, Awake builds a RuntimeTreeData from the assigned asset. The first Update starts the tree; later frames call runtimeTree.update with Time.deltaTime while the component is not paused. pause, resume, and restart are public. pause ends the running node; resume and restart start it again.
• Tasks return State.Running, State.Success, or State.Failed. Built-in composites: Sequence, Selector, Repeat, Invert, Convert, and Random. Built-in leaves: Wait and Fail. Built-in conditions: CheckBool and CheckProbability.
• Per-node services (IService) tick while the node is running, at ticksPerSecond (default 1). A service tick that returns a state other than Running ends that node update with that state. The built-in service is Timeout.
• Custom tasks are Task subclasses. [RuntimeData(typeof(...))] (from Behaviour Vars) supplies a RuntimeTaskData subclass for per-run state. [TaskColor("hex")] draws a colour bar on the node. [BTVar(ValueType)] on a string or int field draws a popup of matching variable names from the tree open in the Behaviour Tree window.
• Window > Behaviour Tree Window lays out nodes, draws links, and shows a properties pane with the tree's variables and the selected task's inspector. Right-click a node to delete it, copy or paste its task, encapsulate it, move it back among siblings, set its task, add a service, add a child (on a composite), or remove the task. Set task lists Task subclasses from every loaded assembly. Add service lists IService subclasses from the Behaviour Tree assembly.
• In play mode, with a selected GameObject that has an IBehaviourTreeProvider, node frames tint from the last returned state. Task.debugBreak calls Debug.Break when the returned state matches debugBreakCondition.
• IBehaviourTreeProvider exposes one or more trees to that window. Behaviour implements it for its single tree. setFloat, setInt, setBool, setString, and setTrigger (and the matching getters, except trigger) forward to the runtime variable collection by name or by string.GetHashCode().

package.json does not set unity or unityRelease. The editor sources use C# 9 (is not patterns, or patterns, and switch expressions). Unity compiles C# 9 from 2021.2 onward, so that is the oldest editor this package can be imported into as written.

This package depends on com.deathbygravitystudio.behaviour.vars at version 0.1.0 (Behaviour.Tree references Behaviour.Vars; the editor assembly also references Behaviour.Vars.Editor). Unity does not clone that sibling repository from the version dependency alone. Add both git URLs.

In Package Manager, Add package from git URL for each line, or merge them into Packages/manifest.json:

{
  "dependencies": {
    "com.deathbygravitystudio.behaviour.tree": "https://github.com/denchi/UnityBehaviorTree.git",
    "com.deathbygravitystudio.behaviour.vars": "https://github.com/denchi/UnityBehaviorVars.git"
  }
}

Vars package: https://github.com/denchi/UnityBehaviorVars.git

1. Create a tree with Behaviour > Tree > Create Tree Asset....
2. Open Window > Behaviour Tree Window and select the asset. On the Tree tab, add a bool named alert (default false). Right-click the root, Set task... to Behaviours/BT/Sequence, then Add node twice and set those children to Behaviours/BT/Conditions/CheckBool (varName = alert, varValue = true) and to the CountTo task below.
3. Add Behaviours.BT.Behaviour to a GameObject, assign the tree asset, and leave playOnAwake enabled. SetTree only builds runtime data when the component's tree field is already set, which is how Awake uses the inspector reference.
4. Enter play mode. The sequence fails CheckBool until something sets alert. The component below does that on Space, and restarts the tree on R.

CountTo is a custom task. Each run keeps a counter on its runtime data and succeeds once the counter reaches target.

using Behaviours;
using Behaviours.BT;
using Behaviours.BT.Runtime;

[RuntimeData(typeof(CountTo.CountData))]
public class CountTo : Task
{
    public int target = 3;

    public class CountData : RuntimeTaskData
    {
        public int count;
    }

    public override void start(RuntimeTaskData taskData)
    {
        ((CountData)taskData).count = 0;
    }

    public override State update(RuntimeTaskData taskData, float dt)
    {
        var data = (CountData)taskData;
        data.count++;
        return data.count >= target ? State.Success : State.Running;
    }
}

using UnityEngine;

public class TreeRunner : MonoBehaviour
{
    Behaviours.BT.Behaviour treeBehaviour;

    void Awake()
    {
        treeBehaviour = GetComponent<Behaviours.BT.Behaviour>();
    }

    void Update()
    {
        if (Input.GetKeyDown(KeyCode.Space))
            treeBehaviour.setBool("alert", true);

        if (Input.GetKeyDown(KeyCode.R))
            treeBehaviour.restart();
    }
}

setBool looks up a variable copied from the tree asset at init. A missing name logs a warning from Behaviour Vars and leaves the tree unchanged. The int overloads use the same hash as RuntimeValueData: name.GetHashCode(). setTrigger sets that bool for three frames, and only if the Behaviour GameObject is active.

ICondition.update calls check once per tick. Subclass ICondition and override check for a condition that does not span frames. Subclass Task and override start, update, and end for anything else. Subclass IService and override onServiceStarted, tick, and onServiceEnded for work that runs beside a node.

These repositories are separate packages from Death By Gravity Studio. They share Behaviour Vars (ILayer, RuntimeValueCollection, RuntimeDataAttribute, and the editor types GenericWindow and ValueGUI). This tree package's assembly references only Behaviour.Vars / Behaviour.Vars.Editor.

HFSM, Graph, and Dialogs do not declare a dependency on this tree package. Install them the same way, by git URL, only when you use them.

Released under the MIT License. Copyright (c) 2026 Denis Chiriac / Death By Gravity Studio.