在上一篇里，我們學習了“自主角色”的一些基本行為：尋找(seek)、避開(flee)、到達(arrive)、追捕(pursue)、躲避(evade)、漫游(wander)。這一篇將繼續學習其它更復雜，更高級的行為。

一、對象回避(object avoidance)

對象回避的正式解釋為：角色預測出對象的行動路徑，然后避開他們。

也可以通俗的描述為：假如有一個"灰太狼抓喜羊羊"的游戲場景，“喜羊羊"在草地上四處游蕩的時候要盡量避免被隨處找羊的"灰太狼"抓住。好象聽起來并不復雜，跟前面提到的"避開(flee)"或"躲避(evade)"甚至第一章提到的碰撞檢測差不多，只要調頭走開、改變路線甚至檢測這二者是否發生碰撞即可。

但如果仔細考慮的話，這個并不象上面想的這么簡單，如果“羊”跟“狼”的距離足夠遠(或者“狼”運動得太慢)，以至于“狼”在預測的時間范圍內根本不可能抓住“羊”，那么“羊”可能根本無需調整方向（或僅需做小的調整）；如果“狼”突然出現在離“羊”很近的地方，“羊”做出的反應是急轉90度，換個方向跑開。（問：為什么不是轉180度反方向跑呢？答：如果大家經常看動物世界里非洲草原上“獵豹追羚羊"的片段，應該就能理解了，大多數情況下，急轉彎比反向跑，更能有效避開覓食者）另外，該行為的另一個特征是預測可能要發生的碰撞，而非實際發生的碰撞，所以碰撞檢測也不太適合。

ok，直接看算法示意圖吧：

首先把目標（障礙）物體認為是一個有半徑范圍的圓形對象(當然這是一種簡化問題的理想模型)；

然后得出自己與目標的距離向量difference；

接下來將自身的速度向量單位化，得到單位向量header（即長度為1，且與速度同方向的向量）；

計算header與difference的點積(也叫點乘、內積、標量積)，得到一個值dotProd = |header| * |difference| * cos(θ) = |difference| * cos(θ) (注：header為單位向量，大小為1，所以可省去)，很明顯如果該值小于0，則表示障礙物就在前方，準備做調整；

將header放大一個系數，模擬出自身的觸角feeler(相當于物體自身向前方伸出去一個觸須試探試探)；

將difference投影在feeler上，得到向量projection，并進一步得到距離dist(即projection末端與difference末端的距離)；

如果dist小于圓半徑，且projection長度小于feeler的長度（即觸角碰到了目標了），則轉90度逃開；

private var _avoidDistance:Number=300;//發現障礙物的有效視野 private var _avoidBuffer:Number=20;//機車在準備避開時，自身和障礙物間的預留距離。public function set avoidDistance(value:Number):void {_avoidDistance=value; } public function get avoidDistance():Number {return _avoidDistance; }public function set avoidBuffer(value:Number):void {_avoidBuffer=value; } public function get avoidBuffer():Number {return _avoidBuffer; }//對象回避 public function avoid(circles: Array):void {for (var i: int=0; i < circles.length; i++) {var circle:Circle=circles[i] as Circle;var heading:Vector2D=_velocity.clone().normalize();// 障礙物和機車間的位移向量var difference:Vector2D=circle.position.subtract(_position);var dotProd:Number=difference.dotProd(heading);// 如果障礙物在機車前方if (dotProd>0) {// 機車的“觸角”var feeler:Vector2D=heading.multiply(_avoidDistance);// 位移在觸角上的映射var projection:Vector2D=heading.multiply(dotProd);// 障礙物離觸角的距離var dist:Number=projection.subtract(difference).length;// 如果觸角（在算上緩沖后）和障礙物相交// 并且位移的映射的長度小于觸角的長度// 我們就說碰撞將要發生，需改變轉向if (dist < circle.radius + _avoidBuffer && projection.length < feeler.length) {// 計算出一個轉90度的力var force:Vector2D=heading.multiply(_maxSpeed);force.angle+=difference.sign(_velocity)*Math.PI/2;// 通過離障礙物的距離，調整力度大小，使之足夠小但又能避開force=force.multiply(1.0-projection.length/feeler.length);// 疊加于轉向力上_steeringForce=_steeringForce.add(force);// 剎車——轉彎的時候要放慢機車速度，離障礙物越接近，剎車越狠。_velocity=_velocity.multiply(projection.length/feeler.length);}}} }

將以上代碼加入SteeredVehicle.as，當然，在測試前還要有一個Circle類來模擬障礙物

package {import flash.display.Sprite;public class Circle extends Sprite {private var _radius:Number;private var _color:uint;private var _vx:Number;private var _vy:Number;public function Circle(radius:Number, color:uint = 0x000000) {_radius=radius;_color=color;graphics.lineStyle(0, _color);graphics.beginFill(_color);graphics.drawCircle(0, 0, _radius);graphics.endFill();}public function get radius():Number {return _radius;}public function get position():Vector2D {return new Vector2D(x, y);}public function get vx():Number {return _vx;}public function set vx(value:Number):void {_vx = value;}public function get vy():Number {return _vy;}public function set vy(value:Number):void {_vy = value;}} }

測試：

package {import flash.display.Sprite;import flash.display.StageAlign;import flash.display.StageScaleMode;import flash.events.Event;public class AvoidTest extends Sprite {private var _vehicle:SteeredVehicle;private var _circles:Array;private var _numCircles:int=5;public function AvoidTest():void {stage.align=StageAlign.TOP_LEFT;stage.scaleMode=StageScaleMode.NO_SCALE;_vehicle = new SteeredVehicle(0xff0000);_vehicle.edgeBehavior=Vehicle.BOUNCE;addChild(_vehicle);//初始化障礙物_circles = new Array();for (var i:int = 0; i < _numCircles; i++) {var c:Circle=new Circle(Math.random()*30+15,0x0000ff);c.x=Math.random()*stage.stageWidth;c.y=Math.random()*stage.stageHeight;c.vx=Math.random()-0.5;c.vy=Math.random()-0.5;if (c.x<c.radius) {c.x=c.radius;} else if (c.x>stage.stageWidth-c.radius) {c.x=stage.stageWidth-c.radius;}if (c.y<c.radius) {c.y=c.radius;} else if (c.y>stage.stageHeight-c.radius) {c.y=stage.stageHeight-c.radius;}addChild(c);_circles.push(c);}addEventListener(Event.ENTER_FRAME, onEnterFrame);}private function onEnterFrame(e:Event):void { _vehicle.wander();//處理障礙物的運動以及邊界反彈for (var i:int = 0; i < _numCircles; i++) {var c:Circle=_circles[i] as Circle;c.x+=c.vx;c.y+=c.vy;if (c.x<c.radius) {c.x=c.radius;c.vx*=-1;} else if (c.x>stage.stageWidth-c.radius) {c.x=stage.stageWidth-c.radius;c.vx*=-1;}if (c.y<c.radius) {c.y=c.radius;c.vy*=-1;} else if (c.y>stage.stageHeight-c.radius) {c.y=stage.stageHeight-c.radius;c.vy*=-1;}}_vehicle.avoid(_circles);_vehicle.update();}} }

二、路徑跟隨(path following)

對于玩過星際之類游戲的朋友們，這種行為應該最熟悉了。隨便選一個神族的狂熱者(俗稱叉叉兵)，然后在幾個指定的位置點擊一下，它們就會沿著指定的位置來回巡邏。這就是路徑跟隨：角色盡可能的沿著指定的路徑移動。

在算法上的處理很簡單：用數組保存一組位置(每個位置其實就是一個Vector2D對象)，然后加一個索引變量(指針)，用于指示當前移動到了哪一個位置，最終將機車以seek行為移動以下一個位置。

但有一個要注意的細節：seek行為會讓機車最終在目標位置來回反復運動停不下來，為了讓代碼能知道機車已經經過了當前位置，接下來應該去下一個位置，需要引入一個距離閾值，用于判斷機車是否已經接近目標點。

private var _pathIndex:int=0;//路徑索引private var _pathThreshold:Number=20;//路徑跟隨中的距離閾值public function set pathIndex(value:int):void {_pathIndex=value;}public function get pathIndex():int {return _pathIndex;}public function set pathThreshold(value:Number):void {_pathThreshold=value;}public function get pathThreshold():Number {return _pathThreshold;}public function set avoidDistance(value:Number):void {_avoidDistance=value;}public function get avoidDistance():Number {return _avoidDistance;}public function set avoidBuffer(value:Number):void {_avoidBuffer=value;}public function get avoidBuffer():Number {return _avoidBuffer;}//路徑跟隨public function followPath(path:Array,loop:Boolean=false):void {var wayPoint:Vector2D=path[_pathIndex];if (wayPoint==null) {return;}if (_position.dist(wayPoint)<_pathThreshold) {if (_pathIndex>=path.length-1) {if (loop) {_pathIndex=0;}} else {_pathIndex++;}}if (_pathIndex>=path.length-1&&! loop) {arrive(wayPoint);} else {seek(wayPoint);}}

測試：

package {import flash.display.Sprite;import flash.display.StageAlign;import flash.display.StageScaleMode;import flash.events.Event;import flash.events.MouseEvent;public class PathTest extends Sprite {private var _vehicle:SteeredVehicle;private var _path:Array;public function PathTest() {stage.align=StageAlign.TOP_LEFT;stage.scaleMode=StageScaleMode.NO_SCALE;_vehicle=new SteeredVehicle ; addChild(_vehicle);_path=new Array ;stage.addEventListener(MouseEvent.CLICK,onClick);addEventListener(Event.ENTER_FRAME,onEnterFrame);}private function onEnterFrame(event:Event):void {_vehicle.followPath(_path,true);_vehicle.update();}private function onClick(event:MouseEvent):void {graphics.lineStyle(0,0,.25);if (_path.length==0) {graphics.moveTo(mouseX,mouseY);}graphics.lineTo(mouseX,mouseY);graphics.drawCircle(mouseX,mouseY,10);graphics.moveTo(mouseX,mouseY);_path.push(new Vector2D(mouseX,mouseY));}} }

拿鼠標在上面隨便點一下，就能看到效果了

三、群落(flock)行為

群落行為是指類似鳥群這樣的復合行為。它由三個子行為組成：

分離(separation)：鳥群中每個角色都試著和相鄰角色保持一定的距離(即：一只鳥與其它鳥太靠近時，主動退讓一定的距離，以避免碰到)
凝聚(cohesion)：每個角色盡量不掉隊，不落下太遠(即：盡量向鳥群靠攏)
隊列(alignment)：每個角色盡可能與相鄰角色行動于同一方向（即：每只鳥的速度方向可能不完全相同，但大致跟隊伍的總體方向一致）

借用一句李建忠老師名言：原代碼就是最好的設計！ 直接上代碼吧：

private var _inSightDist:Number=200;//視野距離private var _tooCloseDist:Number=60;//防止群落靠得太近的安全距離//群落行為public function flock(vehicles:Array):void {var averageVelocity:Vector2D=_velocity.clone();//平均速度變量var averagePosition:Vector2D=new Vector2D ;//平均位置變量var inSightCount:int=0;//在視野中的機車數量for (var i:int=0; i<vehicles.length; i++) {var vehicle:Vehicle=vehicles[i] as Vehicle;if (vehicle!=this&&inSight(vehicle)) { //如果其它機車在視野中//累加速度與位置averageVelocity=averageVelocity.add(vehicle.velocity);averagePosition=averagePosition.add(vehicle.position);//如果其它機車太靠近，則避開(即分離行為[separation]的體現)if (tooClose(vehicle)) {flee(vehicle.position);}inSightCount++; //累加在視野中的機車數}}if (inSightCount>0) {//計算平均位置averageVelocity=averageVelocity.divide(inSightCount);averagePosition=averagePosition.divide(inSightCount);seek(averagePosition);//向中心位置靠攏(即凝聚行為[cohesion]的體現)_steeringForce.add(averageVelocity.subtract(_velocity));//根據平均速度校準自身速度(即隊列[alignment]行為的體現)}}public function set inSightDist(vaule:Number):void {_inSightDist=vaule;}public function get inSightDist():Number {return _inSightDist;}public function set tooCloseDist(value:Number):void {_tooCloseDist=value;}public function get tooCloseDist():Number {return _tooCloseDist;}//判斷(身后的其它)機車是否在視野范圍內public function inSight(vehicle:Vehicle):Boolean {if (_position.dist(vehicle.position)>_inSightDist) {return false;}//---->start 下面這一段代碼甚至去掉也行，不過去掉后，群落的行為將有所不同var heading:Vector2D=_velocity.clone().normalize();var difference:Vector2D=vehicle.position.subtract(_position); var dotProd:Number=difference.dotProd(heading);if (dotProd<0) {return false;}//<-----endreturn true;}public function tooClose(vehicle:Vehicle):Boolean {return _position.dist(vehicle.position)<_tooCloseDist;}

重點關注下inSight方法，它直接影響到群落的行為，示意圖如下：

先檢測二只鳥的距離是否足夠近，然后僅關注身后的其它鳥。

測試代碼：

package {import flash.display.Sprite;import flash.display.StageAlign;import flash.display.StageScaleMode;import flash.events.Event;public class FlockTest extends Sprite {private var _vehicles:Array;private var _numVehicles:int=20;public function FlockTest() {stage.align=StageAlign.TOP_LEFT;stage.scaleMode=StageScaleMode.NO_SCALE;_vehicles=new Array ;for (var i:int=0; i<_numVehicles; i++) {var vehicle:SteeredVehicle=new SteeredVehicle(Math.random()*0xffffff);vehicle.position=new Vector2D(Math.random()*stage.stageWidth,Math.random()*stage.stageHeight);vehicle.velocity=new Vector2D(Math.random()*20-10,Math.random()*20-10);vehicle.edgeBehavior=Vehicle.BOUNCE;_vehicles.push(vehicle);addChild(vehicle);}addEventListener(Event.ENTER_FRAME,onEnterFrame);}private function onEnterFrame(event:Event):void {for (var i:int=0; i<_numVehicles; i++) {_vehicles[i].flock(_vehicles);_vehicles[i].update();}}} }

如果把inSight中檢測其它鳥是否在身后的代碼去掉，即簡化成：

public function inSight(vehicle:Vehicle):Boolean {if (_position.dist(vehicle.position)>_inSightDist) {return false;} return true;}

預測一下最終的效果：這樣相當于只要距離小于閾值的其它鳥，其速度和位置都會被計算在內，最終整個群落將始終聚集在一定的范圍內，不會發生分離，從而體現出了另外一種群落效果。

?最后，給出Vehicle.as及SteeredVehicle.as的完整代碼

package {import flash.display.Sprite;public class Vehicle extends Sprite {//邊界行為:是屏幕環繞(wrap)，還是反彈{bounce}protected var _edgeBehavior:String=WRAP;//質量protected var _mass:Number=1.0;//最大速度protected var _maxSpeed:Number=10;//坐標protected var _position:Vector2D;//速度protected var _velocity:Vector2D;//邊界行為常量public static const WRAP:String="wrap";public static const BOUNCE:String="bounce";public function Vehicle(color:uint=0xffffff) {_position=new Vector2D ;_velocity=new Vector2D ;draw(color);}protected function draw(color:uint=0xffffff):void {graphics.clear();graphics.lineStyle(0);graphics.beginFill(color);graphics.moveTo(10,0);graphics.lineTo(-10,5);graphics.lineTo(-10,-5);graphics.lineTo(10,0);graphics.endFill();}public function update():void {//設置最大速度_velocity.truncate(_maxSpeed);//根據速度更新坐標向量_position=_position.add(_velocity); //處理邊界行為if (_edgeBehavior==WRAP) {wrap();} else if (_edgeBehavior==BOUNCE) {bounce();}//更新x,y坐標值x=position.x;y=position.y;//處理旋轉角度rotation=_velocity.angle*180/Math.PI;}//反彈private function bounce():void {if (stage!=null) {if (position.x>stage.stageWidth) {position.x=stage.stageWidth;velocity.x*=-1;} else if (position.x<0) {position.x=0;velocity.x*=-1;}if (position.y>stage.stageHeight) {position.y=stage.stageHeight;velocity.y*=-1;} else if (position.y<0) {position.y=0;velocity.y*=-1;}}}//屏幕環繞private function wrap():void {if (stage!=null) {if (position.x>stage.stageWidth) {position.x=0;}if (position.x<0) {position.x=stage.stageWidth;}if (position.y>stage.stageHeight) {position.y=0;}if (position.y<0) {position.y=stage.stageHeight;}} }//下面的都是屬性定義public function set edgeBehavior(value:String):void {_edgeBehavior=value;}public function get edgeBehavior():String {return _edgeBehavior;}public function set mass(value:Number):void {_mass=value;}public function get mass():Number {return _mass;}public function set maxSpeed(value:Number):void {_maxSpeed=value;}public function get maxSpeed():Number {return _maxSpeed;}public function set position(value:Vector2D):void {_position=value;x=_position.x;y=_position.y;}public function get position():Vector2D {return _position;}public function set velocity(value:Vector2D):void {_velocity=value;}public function get velocity():Vector2D {return _velocity;}override public function set x(value:Number):void {super.x=value;_position.x=x;}override public function set y(value:Number):void {super.y=value;_position.y=y;}} } package {import flash.display.Sprite;//(具有)轉向(行為的)機車public class SteeredVehicle extends Vehicle {private var _maxForce:Number=1;//最大轉向力private var _steeringForce:Vector2D;//轉向速度private var _arrivalThreshold:Number=100;//到達行為的距離閾值(小于這個距離將減速)private var _wanderAngle:Number=0;private var _wanderDistance:Number=10;private var _wanderRadius:Number=5;private var _wanderRange:Number=1;private var _avoidDistance:Number=300;//發現障礙物的有效視野private var _avoidBuffer:Number=20;//機車在準備避開時，自身和障礙物間的預留距離。private var _pathIndex:int=0;//路徑索引private var _pathThreshold:Number=20;//路徑跟隨中的距離閾值private var _inSightDist:Number=200;//視野距離private var _tooCloseDist:Number=60;//防止群落靠得太近的安全距離//群落行為public function flock(vehicles:Array):void {var averageVelocity:Vector2D=_velocity.clone();//平均速度變量var averagePosition:Vector2D=new Vector2D ;//平均位置變量var inSightCount:int=0;//在視野中的機車數量for (var i:int=0; i<vehicles.length; i++) {var vehicle:Vehicle=vehicles[i] as Vehicle;if (vehicle!=this&&inSight(vehicle)) { //如果其它機車在視野中//累加速度與位置averageVelocity=averageVelocity.add(vehicle.velocity);averagePosition=averagePosition.add(vehicle.position);//如果其它機車太靠近，則避開(即分離行為[separation]的體現)if (tooClose(vehicle)) {flee(vehicle.position);}inSightCount++; //累加在視野中的機車數}}if (inSightCount>0) {//計算平均位置averageVelocity=averageVelocity.divide(inSightCount);averagePosition=averagePosition.divide(inSightCount);seek(averagePosition);//向中心位置靠攏(即凝聚行為[cohesion]的體現)_steeringForce.add(averageVelocity.subtract(_velocity));//根據平均速度校準自身速度(即隊列[alignment]行為的體現)}}public function set inSightDist(vaule:Number):void {_inSightDist=vaule;}public function get inSightDist():Number {return _inSightDist;}public function set tooCloseDist(value:Number):void {_tooCloseDist=value;}public function get tooCloseDist():Number {return _tooCloseDist;}//判斷(身后的其它)機車是否在視野范圍內public function inSight(vehicle:Vehicle):Boolean {if (_position.dist(vehicle.position)>_inSightDist) {return false;}//---->start 下面這一段代碼甚至去掉也行，不過去掉后，群落的行為將有所不同var heading:Vector2D=_velocity.clone().normalize();var difference:Vector2D=vehicle.position.subtract(_position); var dotProd:Number=difference.dotProd(heading);if (dotProd<0) {return false;}//<-----endreturn true;}public function tooClose(vehicle:Vehicle):Boolean {return _position.dist(vehicle.position)<_tooCloseDist;}public function set pathIndex(value:int):void {_pathIndex=value;}public function get pathIndex():int {return _pathIndex;}public function set pathThreshold(value:Number):void {_pathThreshold=value;}public function get pathThreshold():Number {return _pathThreshold;}public function set avoidDistance(value:Number):void {_avoidDistance=value;}public function get avoidDistance():Number {return _avoidDistance;}public function set avoidBuffer(value:Number):void {_avoidBuffer=value;}public function get avoidBuffer():Number {return _avoidBuffer;}//路徑跟隨public function followPath(path:Array,loop:Boolean=false):void {var wayPoint:Vector2D=path[_pathIndex];if (wayPoint==null) {return;}if (_position.dist(wayPoint)<_pathThreshold) {if (_pathIndex>=path.length-1) {if (loop) {_pathIndex=0;}} else {_pathIndex++;}}if (_pathIndex>=path.length-1&&! loop) {arrive(wayPoint);} else {seek(wayPoint);}}//對象回避public function avoid(circles:Array):void {for (var i:int=0; i<circles.length; i++) {var circle:Circle=circles[i] as Circle;var heading:Vector2D=_velocity.clone().normalize();// 障礙物和機車間的位移向量var difference:Vector2D=circle.position.subtract(_position);var dotProd:Number=difference.dotProd(heading);// 如果障礙物在機車前方if (dotProd>0) {// 機車的“觸角”var feeler:Vector2D=heading.multiply(_avoidDistance);// 位移在觸角上的映射var projection:Vector2D=heading.multiply(dotProd);// 障礙物離觸角的距離var dist:Number=projection.subtract(difference).length;// 如果觸角（在算上緩沖后）和障礙物相交// 并且位移的映射的長度小于觸角的長度// 我們就說碰撞將要發生，需改變轉向if (dist<circle.radius+_avoidBuffer&&projection.length<feeler.length) {// 計算出一個轉90度的力var force:Vector2D=heading.multiply(_maxSpeed);force.angle+=difference.sign(_velocity)*Math.PI/2;// 通過離障礙物的距離，調整力度大小，使之足夠小但又能避開force=force.multiply(1.0-projection.length/feeler.length);// 疊加于轉向力上_steeringForce=_steeringForce.add(force);// 剎車——轉彎的時候要放慢機車速度，離障礙物越接近，剎車越狠。_velocity=_velocity.multiply(projection.length/feeler.length);}}}}//漫游public function wander():void {var center:Vector2D=velocity.clone().normalize().multiply(_wanderDistance);var offset:Vector2D=new Vector2D(0);offset.length=_wanderRadius;offset.angle=_wanderAngle;_wanderAngle+=Math.random()-0.5*_wanderRange;var force:Vector2D=center.add(offset);_steeringForce=_steeringForce.add(force);}public function set wanderDistance(value:Number):void {_wanderDistance=value;}public function get wanderDistance():Number {return _wanderDistance;}public function set wanderRadius(value:Number):void {_wanderRadius=value;}public function get wanderRadius():Number {return _wanderRadius;}public function set wanderRange(value:Number):void {_wanderRange=value;}public function get wanderRange():Number {return _wanderRange;}public function set arriveThreshold(value:Number):void {_arrivalThreshold=value;}public function get arriveThreshold():Number {return _arrivalThreshold;}//構造函數public function SteeredVehicle(color:uint=0xffffff) {_steeringForce=new Vector2D ;super(color);}public function set maxForce(value:Number):void {_maxForce=value;}public function get maxForce():Number {return _maxForce;}//尋找(Seek)行為public function seek(target:Vector2D):void {var desiredVelocity:Vector2D=target.subtract(_position);desiredVelocity.normalize();desiredVelocity=desiredVelocity.multiply(_maxSpeed);//注：這里的_maxSpeed是從父類繼承得來的var force:Vector2D=desiredVelocity.subtract(_velocity);_steeringForce=_steeringForce.add(force);}//避開(flee)行為public function flee(target:Vector2D):void {var desiredVelocity:Vector2D=target.subtract(_position);desiredVelocity.normalize();desiredVelocity=desiredVelocity.multiply(_maxSpeed);var force:Vector2D=desiredVelocity.subtract(_velocity);_steeringForce=_steeringForce.subtract(force);//這是唯一也seek行為不同的地方，一句話解釋：既然發現了目標，那就調頭就跑吧！}//到達(arrive)行為public function arrive(target:Vector2D):void {var desiredVelocity:Vector2D=target.subtract(_position);desiredVelocity.normalize();var dist:Number=_position.dist(target);if (dist>_arrivalThreshold) {desiredVelocity=desiredVelocity.multiply(_maxSpeed);} else {desiredVelocity=desiredVelocity.multiply(_maxSpeed*dist/_arrivalThreshold);}var force:Vector2D=desiredVelocity.subtract(_velocity);_steeringForce=_steeringForce.add(force);}//追捕(pursue)行為public function pursue(target:Vehicle):void {var lookAheadTime:Number=position.dist(target.position)/_maxSpeed;//假如目標不動，追捕者開足馬力趕過去的話，計算需要多少時間var predictedTarget:Vector2D=target.position.add(target.velocity.multiply(lookAheadTime));seek(predictedTarget);}//躲避(evade)行為public function evade(target:Vehicle):void {var lookAheadTime:Number=position.dist(target.position)/_maxSpeed;var predictedTarget:Vector2D=target.position.add(target.velocity.multiply(lookAheadTime));flee(predictedTarget);}override public function update():void {_steeringForce.truncate(_maxForce);//限制為最大轉向速度，以避免出現突然的大轉身_steeringForce=_steeringForce.divide(_mass);//慣性的體現_velocity=_velocity.add(_steeringForce);_steeringForce=new Vector2D ;super.update();}} }

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