Behavior tree basics

Learn the behavior tree structure and node types.

Behavior Tree Architecture

A Behavior Tree is a decision-making model that organizes complex AI behaviors into a hierarchical tree. Each evaluated node reports one of three states: Success, Failure, or Running.

public enum NodeState {
    Success,
    Failure,
    Running
}

The Running status conveys that a time-consuming action, such as walking toward an objective, is actively in progress.

Sequence and Selector Nodes

Two composite control nodes govern tree branching: Sequence (AND logic) and Selector (OR logic).

// Sequence: All child steps must return Success
// [Spot Enemy] -> [Close Distance] -> [Shoot]

// Selector: Halts on the first successful child
// [Flee if Low Health] OR [Patrol Waypoints]

Combining these two building blocks yields adaptive and nuanced artificial intelligence behaviors.

Exercise Behavior tree node state

Define a Start method. Inside, declare string dugumDurumu = "Success"; and print "Dugum sonucu: " + dugumDurumu to the console.

Show example solution

using UnityEngine;

public class BehaviorTreeTester : MonoBehaviour
{
    void Start()
    {
        string dugumDurumu = "Success";
        Debug.Log("Dugum sonucu: " + dugumDurumu);
    }
}

Test yourself

Which three states can a Behavior Tree node return upon evaluation?
- Red, Green, Blue
- Play, Pause, Stop
- Success, Failure, Running
- Low, Medium, High
Show answer

Success, Failure, Running — Behavior tree nodes return Success on completion, Failure on rejection, and Running when an action is ongoing.

Node states enum declaration

public enum NodeState { Success, Failure, ____ }

Show answer

public enum NodeState { Success, Failure, Running }

Behavior tree components

Sequence

All child steps must succeed (AND)
Selector

Completes on first successful child (OR)
Success

Node successfully completing its task
Running

Node still executing the current task

Next lesson: State machine

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