[Box2D]二.让乌龟做自由落体运动

点这看效果

创建复合刚体

使用b2PolygonShape类中和SetAsBox()方法相似的SetAsOrientedBox()增强方法来创建矩形，此方法有4个参数，

半宽长，半高长，中心，旋转角度，中心是一个b2Vec2对象，是相对坐标。

package {
	
	import Box2D.Collision.Shapes.b2PolygonShape;
	import Box2D.Common.Math.b2Vec2;
	import Box2D.Dynamics.b2Body;
	import Box2D.Dynamics.b2BodyDef;
	import Box2D.Dynamics.b2DebugDraw;
	import Box2D.Dynamics.b2FixtureDef;
	import Box2D.Dynamics.b2World;
	
	import flash.display.Sprite;
	import flash.events.Event;
	
	[SWF(width="640",height="480")]
	public class Tortoise extends Sprite {
		
		private var stageWidth:Number = 640;			//舞台宽度 
		private var stageHeight:Number = 480;			//舞台高度
		private var backgroundColor:uint = 0x333333;	//背景色
		
		private var world:b2World;				//Box2D世界
		private var worldScale:Number = 30;		//一米等于30像素
		private var timeStep:Number = 1 / 30;	//时间步,世界将在每一个时间步被更新
		private var velIterations:int = 10;		//速率约束解算器
		private var posIterations:int = 10;		//位置约束解算器
		
		public function Tortoise() {
			drawBackground();
			var gravity:b2Vec2 = new b2Vec2(0,9.81);	//重力
			//世界中的刚体静止时，可以允许他们进入睡眠状态,睡眠的刚体无需模拟
			var sleep:Boolean = true;
			world = new b2World(gravity,sleep);
			debugDraw();
			createFloor();
			createTortoise();
			addEventListener(Event.ENTER_FRAME,updateWorld);
		}
		
		//创建传说中的乌龟
		private function createTortoise():void {
			//乌龟壳由9个矩形组成，以下两个变量分别是矩形的半宽长，半高长
			var halfWidth:Number = 10 /worldScale;
			var halfHeight:Number = 10 /worldScale;
			
			//乌龟头的半宽长，半高长
			var headHalfWidth:Number = 7 /worldScale;
			var headHalfHeight:Number = 7 /worldScale;
			
			var x:Number = stageWidth/2/worldScale;
			var y:Number = 100/worldScale;
			var bodyDef:b2BodyDef = new b2BodyDef();//刚体定义
			bodyDef.type = b2Body.b2_dynamicBody;  //刚体类型为动态
			//设置刚体坐标,Box2D坐标用米表示，注册点在中心
			bodyDef.position.Set(x,y);
			
			var polygonShape:b2PolygonShape = new b2PolygonShape(); //创建多边形
			polygonShape.SetAsBox(halfWidth,halfHeight);//创建乌龟壳最中间的那个矩形
			
			var fixtureDef:b2FixtureDef = new b2FixtureDef(); //夹具(fixture)用于将形状绑定到刚体上
			fixtureDef.density = 1;		//密度
			fixtureDef.restitution = 0.4; //弹性系数
			fixtureDef.friction = 0.5;	//摩擦系数
			fixtureDef.shape = polygonShape;
		
			var theBody:b2Body = world.CreateBody(bodyDef);
			theBody.CreateFixture(fixtureDef);
			
			var pos:b2Vec2 = new b2Vec2(-20/worldScale,-20/worldScale); //相对坐标
			
			//创建乌龟壳剩余的8个矩形
			polygonShape.SetAsOrientedBox(halfWidth,halfHeight,new b2Vec2(-2 * halfWidth,-2 * halfHeight));
			theBody.CreateFixture(fixtureDef);
			polygonShape.SetAsOrientedBox(halfWidth,new b2Vec2(0,new b2Vec2(2 * halfWidth,-2 * halfHeight));
			theBody.CreateFixture(fixtureDef);
			
			polygonShape.SetAsOrientedBox(halfWidth,0));
			theBody.CreateFixture(fixtureDef);
			polygonShape.SetAsOrientedBox(halfWidth,0));
			theBody.CreateFixture(fixtureDef);
			
			polygonShape.SetAsOrientedBox(halfWidth,2 * halfHeight));
			theBody.CreateFixture(fixtureDef);
			polygonShape.SetAsOrientedBox(halfWidth,2 * halfHeight));
			theBody.CreateFixture(fixtureDef);
			
			//创建乌龟头
			polygonShape.SetAsOrientedBox(headHalfWidth,headHalfHeight,-(3 * halfHeight + headHalfHeight)));
			theBody.CreateFixture(fixtureDef);
			
			//创建乌龟的4只脚
			polygonShape.SetAsOrientedBox(8/worldScale,8/worldScale,new b2Vec2(-3 * halfWidth,-3 * halfHeight),Math.PI / 4);
			theBody.CreateFixture(fixtureDef);
			
			polygonShape.SetAsOrientedBox(8/worldScale,new b2Vec2(3 * halfWidth,3 * halfHeight),Math.PI / 4);
			theBody.CreateFixture(fixtureDef);
		}
		
		//创建地面
		private function createFloor():void {
			var bodyDef:b2BodyDef = new b2BodyDef();	//刚体定义
			bodyDef.type = b2Body.b2_staticBody;   //刚体类型为静态
			//设置刚体坐标,Box2D坐标用米来表示，注册点是中心
			bodyDef.position.Set(stageWidth/2/worldScale,(stageHeight-10)/worldScale);	
			
			var polygonShape:b2PolygonShape = new b2PolygonShape(); //创建多边形
			polygonShape.SetAsBox(stageWidth/2/worldScale,10/worldScale);//轴对称的矩形,半宽长和半高长
			
			var fixtureDef:b2FixtureDef = new b2FixtureDef(); //夹具(fixture)用于将形状绑定到刚体上
			fixtureDef.density = 1;			//密度
			fixtureDef.restitution = 0.3;	//弹性系数
			fixtureDef.friction = 0.1;		//摩擦系数
			fixtureDef.shape = polygonShape;
			
			var theFloorBody:b2Body = world.CreateBody(bodyDef); //创建刚体
			theFloorBody.CreateFixture(fixtureDef);
		}
		
		private function updateWorld(e:Event):void {
			world.Step(timeStep,velIterations,posIterations);
			world.ClearForces();
			world.DrawDebugData();
		}
		
		private function drawBackground():void {
			graphics.beginFill(backgroundColor);
			graphics.drawRect(0,stageWidth,stageHeight);
			graphics.endFill();
		}
		
		private function debugDraw():void {
			var debugDraw:b2DebugDraw = new b2DebugDraw(); //调试绘图类
			var debugSprite:Sprite = new Sprite(); //显示调试绘图数据的画布
			addChild(debugSprite);
			debugDraw.SetSprite(debugSprite); //告知debugSprite将要被用来显示调试绘图
			debugDraw.SetDrawScale(worldScale);
			//允许我们决定将在调试绘图（debug draw）中描绘的物
			//理实体的类型。此刻，我们只需要绘制形状
			debugDraw.SetFlags(b2DebugDraw.e_shapeBit);
			debugDraw.SetFillAlpha(0.5);
			world.SetDebugDraw(debugDraw);	
		}
		
	}
}