#Andengine##Box2D#-----前轮驱动赛车

一. 实现功能：

应用Andengine引擎+Box2D物理引擎，实现“前轮驱动”赛车.

二. 相关代码：

1. 在游戏场景中创建物理世界：

FixedStepPhysicsWorld mPhysicsWorld = new FixedStepPhysicsWorld(60, new Vector2(0f, 0f), false, 8, 3);  //创建物理世界，将重力设置为0
registerUpdateHandler(mPhysicsWorld); //在当前场景中注册该物理世界

2. 创建汽车主体部分：

public static final FixtureDef CAR_DEF = PhysicsFactory.createFixtureDef(1f, 1f, 1f, false);Sprite CarShape=new Sprite(); //创建汽车主体精灵
GameScene.attach(CarShape);	//将精灵附加到场景中Body carBody = PhysicsFactory.createBoxBody(Registry.sGameScene.mPhysicsWorld,CarShape,BodyDef.BodyType.DynamicBody,Registry.sGameScene.CAR_DEF);  //根据汽车主体精灵创建Body
carBody.setLinearDamping(1);   //设置线性方向上的阻尼
carBody.setAngularDamping(1);	//设置转动阻尼
mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(CarShape, carBody, true, true)); //将Sprite和Body进行连接

3. 创建四个轮胎：

public static final FixtureDef WHEEL_DEF = PhysicsFactory.createFixtureDef(1f, 1f, 1f, false);//右前轮（Sprite+Body）:
rightTopWheel=new Sprite(CarShape.getX()+CarShape.getWidth()*0.5f,CarShape.getY()+18,ResourceManager.getInstance().WheelRegion,ResourceManager.getInstance().vbom);
Registry.sGameScene.attachChild(rightTopWheel);  rightTopBody = PhysicsFactory.createBoxBody(Registry.sGameScene.mPhysicsWorld,rightTopWheel,BodyDef.BodyType.DynamicBody, Registry.sGameScene.WHEEL_DEF);
Registry.sGameScene.mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(rightTopWheel, rightTopBody, true, true));//左前轮（Sprite+Body）:
leftTopWheel=new Sprite(CarShape.getX()-CarShape.getWidth()*0.5f,CarShape.getY()+18,ResourceManager.getInstance().WheelRegion,ResourceManager.getInstance().vbom);
Registry.sGameScene.attachChild(leftTopWheel);leftTopBody = PhysicsFactory.createBoxBody(Registry.sGameScene.mPhysicsWorld,leftTopWheel,BodyDef.BodyType.DynamicBody, Registry.sGameScene.WHEEL_DEF);Registry.sGameScene.mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(leftTopWheel, leftTopBody, true, true));//右后轮（Sprite+Body）:
rightDownWheel=new Sprite(CarShape.getX()+CarShape.getWidth()*0.5f,CarShape.getY()-18,ResourceManager.getInstance().WheelRegion,ResourceManager.getInstance().vbom);
Registry.sGameScene.attachChild(rightDownWheel);rightDownBody = PhysicsFactory.createBoxBody(Registry.sGameScene.mPhysicsWorld,rightDownWheel,BodyDef.BodyType.DynamicBody, Registry.sGameScene.WHEEL_DEF);
Registry.sGameScene.mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(rightDownWheel, rightDownBody, true, true));//左后轮（Sprite+Body）
leftDownWheel=new Sprite(CarShape.getX()-CarShape.getWidth()*0.5f,CarShape.getY()-18,ResourceManager.getInstance().WheelRegion,ResourceManager.getInstance().vbom);
Registry.sGameScene.attachChild(leftDownWheel);leftDownBody = PhysicsFactory.createBoxBody(Registry.sGameScene.mPhysicsWorld,leftDownWheel,BodyDef.BodyType.DynamicBody, Registry.sGameScene.WHEEL_DEF);
Registry.sGameScene.mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(leftDownWheel, leftDownBody, true, true));

4.将轮胎与汽车主体进行关联：

(1). RevoluteJoint (旋转关节) ----前轮
因为前轮负责方向的引导（即转向，方向盘控制前轮），所以需要使用旋转关节

private RevoluteJoint mJointRightTop;  
private RevoluteJointDef rtjd; rtjd.initialize(carBody, rightTopBody, rightTopBody.getWorldCenter()); //将车体Body与右前轮Body进行连接，连接中心设为车胎中心
rtjd.enableMotor=true;   //旋转关节开启扭矩
rtjd.maxMotorTorque=100;  //最大扭矩设置为100
mJointRightTop= (RevoluteJoint)Registry.sGameScene.mPhysicsWorld.createJoint(rtjd);  //在物理世界中注册该关节

(2). PrismaticJoint(平行关节)----后轮
后轮只需要跟随车辆主体进行运动

PrismaticJointDef jd=new PrismaticJointDef();
Joint mJointRightDown=null;jd.initialize(carBody, rightDownBody, rightDownBody.getWorldCenter(),new Vector2(1,0)); //将右后轮Body与车主体Body进行平行关节连接
jd.enableLimit=true; //设置为跟随车动
jd.lowerTranslation=0;
jd.upperTranslation=0;
mJointRightDown=Registry.sGameScene.mPhysicsWorld.createJoint(jd);  //注册关节

5. 场景Update循环中控制车辆移动与转向：

*Box2D知识点： Body.getTransform() ---->获得Body的位置及速度，通过位置和速度能得到向量。也可以通过vals来简便获得
Body X轴的方向向量：
Vector2 d=new Vector2(Body.getTransform().vals[2],Body.getTransform().vals[3]);

Body Y轴的方向向量：
Vector2 d=new Vector2(Body.getTransform().vals[4],Body.getTransform().vals[5]);

public static double MAX_STEER_ANGLE = Math.PI/3;  //最大扭矩
public static float STEER_SPEED = 1.5f;    //转向速度
public static float HORSEPOWERS = 20f;     //马力public float engineSpeed;
public double steeringAngle;场景循环：（Update）
//前进
sideXR = (float)myCar.getRightTopBody().getTransform().vals[4];
sideYR = (float)myCar.getRightTopBody().getTransform().vals[5];
Vector2 rdirection=new Vector2(sideXR,sideYR);  //获得右前胎Y轴的方向向量
rdirection.mul(engineSpeed);         //方向向量*动力=作用力sideXL = (float)myCar.getLeftTopBody().getTransform().vals[4];
sideYL = (float)myCar.getLeftTopBody().getTransform().vals[5];
Vector2 ldirection=new Vector2(sideXL,sideYL);  //获得左前胎Y轴的方向向量
ldirection.mul(engineSpeed);		//方向向量*动力=作用力myCar.getRightTopBody().applyForce(ldirection, myCar.getRightTopBody().getPosition());  //施加右前胎作用力
myCar.getLeftTopBody().applyForce(rdirection, myCar.getLeftTopBody().getPosition());	//施加左前胎作用力//转向
float mspeed;
mspeed=(float)(steeringAngle-myCar.getmJointRightTop().getJointAngle());    //获得转向扭矩力
myCar.getmJointRightTop().setMotorSpeed(mspeed * STEER_SPEED);			//施加转向扭矩力mspeed=(float)(steeringAngle-myCar.getmJointLeftTop().getJointAngle());
myCar.getmJointLeftTop().setMotorSpeed(mspeed * STEER_SPEED);

6. 创建前进与转向按钮：

前进：
                engineSpeed=-CarManager.getInstance().HORSEPOWERS;

转向：（右转）
    steeringAngle=-CarManager.getInstance().MAX_STEER_ANGLE;

释放以后：
    engineSpeed=0;
    steeringAngle=0;

最后附上项目的demo供参考：

下载地址：http://pan.baidu.com/s/1jGkJbdc