Behavior Designer

Overview

Behavior Designer is a behavior tree implementation for Unity AI. This skill covers patterns for creating robust AI behaviors for enemies, NPCs, and game entities.

When to Use

• Enemy AI behavior
• NPC decision making
• Boss fight patterns
• Companion AI
• Any complex state-based AI

Core Concepts

Behavior Tree Structure

Selector (OR logic - first success wins)
├── Sequence (AND logic - all must succeed)
│   ├── Conditional (check condition)
│   └── Action (do something)
├── Sequence
│   ├── Conditional
│   └── Action
└── Action (fallback)

Custom Tasks

Action Task

using BehaviorDesigner.Runtime;
using BehaviorDesigner.Runtime.Tasks;

[TaskCategory("Combat")]
[TaskDescription("Attacks the current target")]
public class AttackTarget : Action
{
    [Tooltip("Reference to the ability system")]
    public SharedGameObject abilitySystemOwner;
    
    [Tooltip("The ability to activate")]
    public SharedString abilityTag;
    
    [Tooltip("Target to attack")]
    public SharedGameObject target;
    
    private IAbilitySystem _abilitySystem;
    private bool _abilityStarted;
    
    public override void OnStart()
    {
        _abilityStarted = false;
        
        if (abilitySystemOwner.Value == null)
        {
            Debug.LogError("AttackTarget: No ability system owner");
            return;
        }
        
        _abilitySystem = abilitySystemOwner.Value.GetComponent<IAbilitySystem>();
    }
    
    public override TaskStatus OnUpdate()
    {
        if (_abilitySystem == null || target.Value == null)
        {
            return TaskStatus.Failure;
        }
        
        if (!_abilityStarted)
        {
            if (!_abilitySystem.TryActivateAbilityByTag(abilityTag.Value))
            {
                return TaskStatus.Failure;
            }
            _abilityStarted = true;
        }
        
        // Check if ability is still running
        if (_abilitySystem.IsAbilityActive(abilityTag.Value))
        {
            return TaskStatus.Running;
        }
        
        return TaskStatus.Success;
    }
    
    public override void OnEnd()
    {
        // Cleanup if interrupted
        if (_abilityStarted && _abilitySystem != null)
        {
            _abilitySystem.CancelAbilityByTag(abilityTag.Value);
        }
    }
}

Conditional Task

[TaskCategory("Combat")]
[TaskDescription("Checks if target is within attack range")]
public class IsTargetInRange : Conditional
{
    public SharedGameObject target;
    public SharedFloat attackRange;
    public SharedTransform selfTransform;
    
    public override TaskStatus OnUpdate()
    {
        if (target.Value == null || selfTransform.Value == null)
        {
            return TaskStatus.Failure;
        }
        
        float distance = Vector3.Distance(
            selfTransform.Value.position,
            target.Value.transform.position
        );
        
        return distance <= attackRange.Value 
            ? TaskStatus.Success 
            : TaskStatus.Failure;
    }
}

Composite Task (Custom Selector)

[TaskCategory("Custom")]
[TaskDescription("Weighted random selection of children")]
public class WeightedSelector : Composite
{
    [Tooltip("Weights for each child (must match child count)")]
    public float[] weights;
    
    private int _currentChildIndex = -1;
    private int _executedChildIndex = -1;
    
    public override void OnStart()
    {
        _currentChildIndex = SelectWeightedChild();
        _executedChildIndex = -1;
    }
    
    public override int CurrentChildIndex()
    {
        return _currentChildIndex;
    }
    
    public override bool CanExecute()
    {
        return _currentChildIndex != _executedChildIndex 
            && _currentChildIndex < children.Count;
    }
    
    public override void OnChildExecuted(TaskStatus childStatus)
    {
        _executedChildIndex = _currentChildIndex;
    }
    
    public override void OnConditionalAbort(int childIndex)
    {
        _currentChildIndex = childIndex;
        _executedChildIndex = childIndex - 1;
    }
    
    private int SelectWeightedChild()
    {
        if (weights == null || weights.Length == 0)
        {
            return Random.Range(0, children.Count);
        }
        
        float totalWeight = 0f;
        for (int i = 0; i < Mathf.Min(weights.Length, children.Count); i++)
        {
            totalWeight += weights[i];
        }
        
        float randomValue = Random.Range(0f, totalWeight);
        float currentWeight = 0f;
        
        for (int i = 0; i < Mathf.Min(weights.Length, children.Count); i++)
        {
            currentWeight += weights[i];
            if (randomValue <= currentWeight)
            {
                return i;
            }
        }
        
        return 0;
    }
}

Decorator Task

[TaskCategory("Decorators")]
[TaskDescription("Repeats child until condition is false")]
public class RepeatWhile : Decorator
{
    public SharedBool condition;
    
    private TaskStatus _childStatus = TaskStatus.Inactive;
    
    public override bool CanExecute()
    {
        return condition.Value && _childStatus != TaskStatus.Running;
    }
    
    public override void OnChildExecuted(TaskStatus childStatus)
    {
        _childStatus = childStatus;
    }
    
    public override TaskStatus Decorate(TaskStatus status)
    {
        if (!condition.Value)
        {
            return TaskStatus.Success;
        }
        
        return TaskStatus.Running;
    }
    
    public override void OnEnd()
    {
        _childStatus = TaskStatus.Inactive;
    }
}

Shared Variables

Defining Shared Variables

// In your behavior tree component or global variables
public class EnemyBehaviorVariables : MonoBehaviour
{
    [Header("References")]
    public SharedGameObject self;
    public SharedTransform selfTransform;
    public SharedGameObject currentTarget;
    
    [Header("Combat")]
    public SharedFloat attackRange = 2f;
    public SharedFloat detectionRange = 10f;
    public SharedFloat health;
    public SharedBool isInCombat;
    
    [Header("Movement")]
    public SharedVector3 homePosition;
    public SharedVector3 targetPosition;
    public SharedFloat moveSpeed = 5f;
}

Custom Shared Variable Type

[Serializable]
public class SharedAbilityData : SharedVariable<AbilityData>
{
    public static implicit operator SharedAbilityData(AbilityData value)
    {
        return new SharedAbilityData { Value = value };
    }
}

// Usage in task
public class UseAbility : Action
{
    public SharedAbilityData ability;
    
    public override TaskStatus OnUpdate()
    {
        if (ability.Value == null)
            return TaskStatus.Failure;
            
        // Use ability data
        return TaskStatus.Success;
    }
}

Global Variables

public class AIGlobalVariables : MonoBehaviour
{
    public static AIGlobalVariables Instance { get; private set; }
    
    private GlobalVariables _globalVariables;
    
    private void Awake()
    {
        Instance = this;
        _globalVariables = GlobalVariables.Instance;
    }
    
    public void SetPlayerReference(GameObject player)
    {
        _globalVariables.SetVariable("Player", (SharedGameObject)player);
    }
    
    public void SetCombatState(bool inCombat)
    {
        _globalVariables.SetVariable("GlobalCombatActive", (SharedBool)inCombat);
    }
    
    public GameObject GetPlayer()
    {
        var playerVar = _globalVariables.GetVariable("Player") as SharedGameObject;
        return playerVar?.Value;
    }
}

Common Patterns

Target Selection

[TaskCategory("Targeting")]
public class FindClosestEnemy : Action
{
    public SharedGameObject result;
    public SharedFloat searchRadius;
    public SharedLayerMask targetLayers;
    public SharedTransform selfTransform;
    
    private Collider[] _hitColliders = new Collider[20];
    
    public override TaskStatus OnUpdate()
    {
        if (selfTransform.Value == null)
            return TaskStatus.Failure;
        
        int hitCount = Physics.OverlapSphereNonAlloc(
            selfTransform.Value.position,
            searchRadius.Value,
            _hitColliders,
            targetLayers.Value
        );
        
        if (hitCount == 0)
        {
            result.Value = null;
            return TaskStatus.Failure;
        }
        
        float closestDistance = float.MaxValue;
        GameObject closest = null;
        
        for (int i = 0; i < hitCount; i++)
        {
            var col = _hitColliders[i];
            if (col.gameObject == selfTransform.Value.gameObject)
                continue;
            
            float dist = Vector3.Distance(
                selfTransform.Value.position,
                col.transform.position
            );
            
            if (dist < closestDistance)
            {
                closestDistance = dist;
                closest = col.gameObject;
            }
        }
        
        result.Value = closest;
        return closest != null ? TaskStatus.Success : TaskStatus.Failure;
    }
}

Movement Tasks

[TaskCategory("Movement")]
public class MoveToTarget : Action
{
    public SharedGameObject target;
    public SharedFloat stoppingDistance = 1f;
    public SharedFloat moveSpeed = 5f;
    
    private IAstarAI _ai;
    private Transform _transform;
    
    public override void OnStart()
    {
        _ai = GetComponent<IAstarAI>();
        _transform = transform;
    }
    
    public override TaskStatus OnUpdate()
    {
        if (target.Value == null)
            return TaskStatus.Failure;
        
        if (_ai != null)
        {
            _ai.destination = target.Value.transform.position;
            
            if (_ai.reachedEndOfPath)
                return TaskStatus.Success;
                
            return TaskStatus.Running;
        }
        
        // Fallback simple movement
        Vector3 direction = (target.Value.transform.position - _transform.position).normalized;
        _transform.position += direction * moveSpeed.Value * Time.deltaTime;
        
        float distance = Vector3.Distance(
            _transform.position,
            target.Value.transform.position
        );
        
        return distance <= stoppingDistance.Value 
            ? TaskStatus.Success 
            : TaskStatus.Running;
    }
    
    public override void OnEnd()
    {
        if (_ai != null)
        {
            _ai.isStopped = true;
        }
    }
}

Patrol Pattern

[TaskCategory("Movement")]
public class Patrol : Action
{
    public SharedTransformList waypoints;
    public SharedInt currentWaypointIndex;
    public SharedFloat waypointReachedDistance = 1f;
    
    private IAstarAI _ai;
    
    public override void OnStart()
    {
        _ai = GetComponent<IAstarAI>();
        
        if (waypoints.Value == null || waypoints.Value.Count == 0)
        {
            return;
        }
        
        SetDestinationToCurrentWaypoint();
    }
    
    public override TaskStatus OnUpdate()
    {
        if (waypoints.Value == null || waypoints.Value.Count == 0)
            return TaskStatus.Failure;
        
        if (_ai == null)
            return TaskStatus.Failure;
        
        // Check if reached waypoint
        float distance = Vector3.Distance(
            transform.position,
            waypoints.Value[currentWaypointIndex.Value].position
        );
        
        if (distance <= waypointReachedDistance.Value)
        {
            // Move to next waypoint
            currentWaypointIndex.Value = 
                (currentWaypointIndex.Value + 1) % waypoints.Value.Count;
            SetDestinationToCurrentWaypoint();
        }
        
        return TaskStatus.Running;
    }
    
    private void SetDestinationToCurrentWaypoint()
    {
        if (_ai != null && waypoints.Value.Count > currentWaypointIndex.Value)
        {
            _ai.destination = waypoints.Value[currentWaypointIndex.Value].position;
            _ai.isStopped = false;
        }
    }
}

Combat State Machine

// Tree structure for combat AI
/*
Selector (Root)
├── Sequence [Flee when low health]
│   ├── IsHealthLow
│   └── FleeFromTarget
├── Sequence [Attack when in range]
│   ├── HasTarget
│   ├── IsTargetInRange
│   └── Selector [Choose attack]
│       ├── Sequence [Special attack if ready]
│       │   ├── IsSpecialReady
│       │   └── UseSpecialAttack
│       └── UseBasicAttack
├── Sequence [Chase target]
│   ├── HasTarget
│   └── MoveToTarget
└── Patrol [Default behavior]
*/

[TaskCategory("Combat")]
public class IsHealthLow : Conditional
{
    public SharedFloat currentHealth;
    public SharedFloat maxHealth;
    public SharedFloat lowHealthThreshold = 0.2f;
    
    public override TaskStatus OnUpdate()
    {
        float healthPercent = currentHealth.Value / maxHealth.Value;
        return healthPercent <= lowHealthThreshold.Value 
            ? TaskStatus.Success 
            : TaskStatus.Failure;
    }
}

Conditional Aborts

Self Abort Pattern

// Use AbortType.Self to re-evaluate when conditions change
[TaskCategory("Combat")]
public class HasTarget : Conditional
{
    public SharedGameObject target;
    
    // Set abort type in inspector to Self
    // This will re-evaluate the branch when target changes
    
    public override TaskStatus OnUpdate()
    {
        return target.Value != null 
            ? TaskStatus.Success 
            : TaskStatus.Failure;
    }
}

Lower Priority Abort

// Use AbortType.LowerPriority to interrupt lower priority branches
[TaskCategory("Combat")]
public class IsUnderAttack : Conditional
{
    public SharedBool underAttack;
    
    // Set abort type to LowerPriority
    // Will interrupt patrol/idle when attacked
    
    public override TaskStatus OnUpdate()
    {
        return underAttack.Value 
            ? TaskStatus.Success 
            : TaskStatus.Failure;
    }
}

Integration with GAS

Ability Task Integration

[TaskCategory("Abilities")]
public class ActivateAbilitiesByTag : Action
{
    public SharedGameObject owner;
    public SharedString abilityTag;
    public SharedBool waitForCompletion = true;
    
    private AbilitySystemComponent _asc;
    private bool _activated;
    
    public override void OnStart()
    {
        _activated = false;
        
        if (owner.Value != null)
        {
            _asc = owner.Value.GetComponent<AbilitySystemComponent>();
        }
    }
    
    public override TaskStatus OnUpdate()
    {
        if (_asc == null)
            return TaskStatus.Failure;
        
        if (!_activated)
        {
            var spec = _asc.TryActivateAbilitiesByTag(
                GameplayTag.FromString(abilityTag.Value)
            );
            
            if (spec == null)
                return TaskStatus.Failure;
            
            _activated = true;
            
            if (!waitForCompletion.Value)
                return TaskStatus.Success;
        }
        
        // Wait for ability to complete
        if (_asc.HasActiveAbilityWithTag(GameplayTag.FromString(abilityTag.Value)))
        {
            return TaskStatus.Running;
        }
        
        return TaskStatus.Success;
    }
}

Debugging

Behavior Tree Debugging

public class BehaviorTreeDebugger : MonoBehaviour
{
    [SerializeField] private BehaviorTree _behaviorTree;
    [SerializeField] private bool _logTaskChanges = true;
    
    private void OnEnable()
    {
        if (_behaviorTree != null)
        {
            _behaviorTree.OnBehaviorStart += OnBehaviorStart;
            _behaviorTree.OnBehaviorRestart += OnBehaviorRestart;
            _behaviorTree.OnBehaviorEnd += OnBehaviorEnd;
        }
    }
    
    private void OnDisable()
    {
        if (_behaviorTree != null)
        {
            _behaviorTree.OnBehaviorStart -= OnBehaviorStart;
            _behaviorTree.OnBehaviorRestart -= OnBehaviorRestart;
            _behaviorTree.OnBehaviorEnd -= OnBehaviorEnd;
        }
    }
    
    private void OnBehaviorStart(Behavior behavior)
    {
        if (_logTaskChanges)
        {
            Debug.Log($"[BT] {name} Started");
        }
    }
    
    private void OnBehaviorRestart(Behavior behavior)
    {
        if (_logTaskChanges)
        {
            Debug.Log($"[BT] {name} Restarted");
        }
    }
    
    private void OnBehaviorEnd(Behavior behavior)
    {
        if (_logTaskChanges)
        {
            Debug.Log($"[BT] {name} Ended");
        }
    }
}

Best Practices

1. Use shared variables - For data passing between tasks
2. Keep tasks simple - Single responsibility
3. Use conditional aborts - For responsive AI
4. Cache component references - In OnStart
5. Handle null gracefully - Return Failure on null
6. Use task categories - Organize custom tasks
7. Profile behavior trees - Can be expensive
8. Use external trees - For reusable behaviors
9. Document task descriptions - Use TaskDescription attribute
10. Test edge cases - Target dies, interrupted, etc.

Troubleshooting

Issue	Solution
Task never completes	Check for infinite Running state
Variables not synced	Verify shared variable binding
Abort not working	Check abort type setting
Performance issues	Reduce tree complexity, cache refs
Null reference	Add null checks in OnUpdate
Task not found	Check TaskCategory attribute