行为树（Behavior Trees）是一种在游戏开发中广泛使用的AI设计模式，主要用于描述AI的行为和决策过程，实现更加智能和自然的游戏AI。它由多个节点组成，每个节点代表一个行为或决策，按照特定的方式连接在一起，形成一个树状结构。
在行为树中，根节点是AI的起点，通过遍历子节点来决策AI的行为。节点有以下三种状态：成功（Success）、失败（Failure）和运行（Running）。前两个通知其父节点其操作是成功还是失败。第三种意味着尚未确定成功或失败，并且该节点仍在运行。下次树被选择时，该节点将再次被选择，此时它将再次有机会成功，失败或继续运行。
行为树的节点有以下几种主要原型：

  1. 组合控制节点（Composite）：一种将多个子节点组合在一起的节点，用于实现复杂的行为和决策逻辑。主要包括次序节点（Sequencer）和选择节点（Selector）。次序节点并行执行多个子节点，直到所有子节点都返回成功或者任意一个子节点返回失败为止。选择节点按照顺序执行子节点，当某个子节点返回成功时，停止执行并返回成功。

  2. 修饰节点（Decorator）：一种特殊的节点，它不执行具体的行为或决策，而是修饰其它节点的行为或决策。主要包括逆变节点（Inverter）和重复节点（Repeater）。逆变节点可以将子节点的结果倒转，比如子节点返回了失败，则这个修饰节点会向上返回成功，以此类推。重复节点重复执行其子节点指定的次数或者一直重复执行，直到其子节点返回成功或者失败。

  3. 叶节点（Leaf）：树的最末端，就是这些AI实际上去做事情的命令。

行为树通过模拟树状结构来描述AI的行为和决策过程，使得AI的行为更加灵活、自然和智能。
代码实现

using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{public class BehaviourTree : MonoBehaviour{private Node root = null;private Blackboard blackboard;public Node Root{get{return root;}set{root= value;}}void Start(){Init();}void Update(){if(root!=null&& gameObject!=null){root.Evaluate(gameObject.transform,blackboard);}}protected virtual void Init(){blackboard = new Blackboard();}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{//行为树共享数据public class Blackboard{private Dictionary<string,object> Data;public Blackboard(){Data = new Dictionary<string, object>();}//获取数据public T Get<T>(string key){if(Data.ContainsKey(key)){return (T)Data[key];}return default;}//添加数据public void Add<T>(string key,T value){if(Data.ContainsKey(key)){Data[key] = value;}else{Data.Add(key,value);}}//删除数据public void Remove(string key){if(Data.ContainsKey(key)){Data.Remove(key);}else{Debug.Log("数据不存在 "+key);}}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{//状态类型public enum Status{Running,//运行中Failure,// 失败Success,//成功}//节点基类public abstract class Node{protected Node parent;protected Status status;Status Status{get{return status;}set{status = value;}}public Node(){}//声明节点状态返回方法public abstract Status Evaluate(Transform transform,Blackboard blackboard);}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{//复合节点类public abstract class Composite : Node{//子节点列表protected List<Node> children = new List<Node>();protected int index;//子节点下标计数protected Composite(List<Node> children){index = 0;foreach(var child in children) {this.children.Add(child);}}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{//修饰器节点public abstract class Decorator : Node{//子节点列表protected List<Node> children;protected Decorator(List<Node> children){children = new List<Node>(){};foreach(var child in children) {this.children.Add(child);}}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{//选择节点，所有子节点都为失败则失败public class Selector : Composite{public Selector(List<Node> children) : base(children){}//所有子节点都为失败则失败public override Status Evaluate(Transform transform, Blackboard blackboard){if(index>=children.Count){index = 0;return Status.Success;}var status = children[index].Evaluate(transform,blackboard);switch(status){case Status.Success:index = 0;return Status.Success;case Status.Failure:index+=1;if(index>=children.Count){index = 0;return Status.Failure;}return Status.Running;case Status.Running:return Status.Running;default:return Status.Failure;}}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{//顺序节点，所有子节点成功才成功public class Sequencer : Composite{public Sequencer(List<Node> children) : base(children){}//所有子节点成功才成功public override Status Evaluate(Transform transform, Blackboard blackboard){if(index>=children.Count){index = 0;return Status.Success;}var status = children[index].Evaluate(transform,blackboard);switch(status){case Status.Success:index+=1;if(index>=children.Count){index = 0;return Status.Success;}return Status.Running;case Status.Failure:return Status.Failure;case Status.Running:return Status.Running;default:return Status.Failure;}}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{//任务节点,这里会处理对象具体行为逻辑（叶节点）public abstract class Task : Node{}}

简单使用
实现敌人在两点之间巡逻，人物靠近会变红温并停止移动，人物远离时继续巡逻

using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;using System.ComponentModel.Design.Serialization;using System.Linq;namespace BehaviourTree{public class EnemyBT : BehaviourTree{Vector3 aPos = new Vector3(4.07999992f,-2.21000004f,-2);Vector3 bPos = new Vector3(4.07999992f,1.65999997f,-2)protected override void Init(){//调用基类初始化base.Init();//创建变红节点TurnRed turnRed = new TurnRed();//创建巡逻节点Patrol patrol = new Patrol(aPos,bPos);//创建查找节点FindObject findObject = new FindObject();//创建侦查节点子节点序列Node[] spyChildren = {findObject,turnRed};//创建顺序节点用于执行侦查行为EnemySequencer enemySequencer = new EnemySequencer(spyChildren.ToList());//创建根节点子节点序列Node[] selectorChildren = {enemySequencer,patrol};//创建选择节点用于处理侦查和巡逻行为EnemySelector enemySelector = new EnemySelector(selectorChildren.ToList());//将选择节点设置为根节点Root = enemySelector;}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{public class EnemySelector : Selector{public EnemySelector(List<Node> children) : base(children){}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;namespace BehaviourTree{public class EnemySequencer : Sequencer{public EnemySequencer(List<Node> children) : base(children){}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;using DG.Tweening;namespace BehaviourTree{public class FindObject : Task{float radius = 5f;int layer = 512;public override Status Evaluate(Transform transform, Blackboard blackboard){Collider2D obj = Physics2D.OverlapCircle(transform.position,radius,layer);if(obj!=null){return Status.Success; }transform.gameObject.GetComponent<SpriteRenderer>().DOColor(Color.white,1);return Status.Failure;}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;using DG.Tweening;namespace BehaviourTree{//在aPoint和bPoint之间来回移动public class Patrol : Task{Vector3 aPoint;Vector3 bPoint;float speed = 4f;Vector3 target;public Patrol(Vector3 aPos,Vector3 bPos){aPoint = aPos;bPoint = bPos;target = aPoint;}public override Status Evaluate(Transform transform, Blackboard blackboard){if(Vector2.Distance(transform.position,target)<0.1f){if(target == aPoint){target = bPoint;}else{target = aPoint;}}else{transform.position = Vector2.MoveTowards(transform.position,target,Time.deltaTime*speed);}return Status.Success;}}}
using System.Collections;using System.Collections.Generic;using UnityEngine;using BehaviourTree;using DG.Tweening;namespace BehaviourTree{//变红public class TurnRed : Task{Vector3 bPoint;public override Status Evaluate(Transform transform, Blackboard blackboard){transform.gameObject.GetComponent<SpriteRenderer>().DOColor(Color.red,1);return Status.Success;}}}