<转载>android box2d物理引擎示例

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Box2d是一个很出名的2d物理引擎，大家可以google之。Box2d有C++，flash和Java等版本。android可以直接使用java版本的Jbox2d，但因为Jbox2d的图形渲染是使用processing库来实现的，所以，在android中使用Jbox2d的时候，图形渲染的工作就只能自己来写了。因为网上关于box2d的资料真的非常的少，特别是关于图形绘制方面，所以，虽然程序写得不是很好，还是贴上来了，先看截图：

程序很简单：蓝色的是地面，从上面掉下两个绿球和一个红色的方块，它们之间的碰撞都由box2d引擎自己来完成。

（这程序运行起来有点卡，我也正在解决这个问题，有朋友知道答案的也请分享一下啦！）

代码如下：

package com.ray.test;

import org.jbox2d.collision.AABB;
import org.jbox2d.collision.CircleDef;
import org.jbox2d.collision.PolygonDef;
import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.Body;
import org.jbox2d.dynamics.BodyDef;
import org.jbox2d.dynamics.World;

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.view.Window;
import android.view.WindowManager;

public class MyBox2d extends Activity {

   private final static int RATE = 10;//屏幕到现实世界的比例 10px：1m;
   private AABB worldAABB;//创建一个管理碰撞的世界
   private World world;
   private float timeStep = 1/60;//模拟的的频率
   private int iterations = 10;//迭代越大，模拟约精确，但性能越低
   private Body body,body2,body3;
   private MyView myView;
   private Handler mHandler;
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams. FLAG_FULLSCREEN ,
        WindowManager.LayoutParams. FLAG_FULLSCREEN);

        worldAABB = new AABB();

       //上下界，以屏幕的左上方为原点，如果创建的刚体到达屏幕的边缘的话，会停止模拟
        worldAABB.lowerBound.set(-100.0f,-100.0f)；
        worldAABB.upperBound.set(100.0f, 100.0f);//注意这里使用的是现实世界的单位

        Vec2 gravity = new Vec2(0.0f,10.0f);
        boolean doSleep = true;


        world = new World(worldAABB, gravity, doSleep);//创建世界

        createBox(160, 470, 160, 10, true);
        createBox1(160, 150, 160, 10, false);

        createCircle(160, 100, 10);
        createCircle1(150, 60, 10);
        timeStep = 1.0f/60.0f;
        iterations = 10;

        myView = new MyView(this);
        setContentView(myView);
        mHandler = new Handler();
        mHandler.post(update);
    }

    private Runnable update = new Runnable() {
        public void run() {
               world.step(timeStep, iterations);//开始模拟
               Vec2 position = body.getPosition();
               Vec2 position1 = body2.getPosition();
               Vec2 position2 = body3.getPosition();
               myView.x=position.x*RATE;
               myView.y=position.y*RATE;

               myView.x1=position1.x*RATE;
               myView.y1=position1.y*RATE;

               myView.x2=position2.x*RATE;
               myView.y2=position2.y*RATE;
                myView.update();
                mHandler.postDelayed(update, (long)timeStep*1000);
        }
    };

    public void createBox(float x,float y,float half_width,float half_height,
                     boolean isStatic){
       PolygonDef shape = new PolygonDef();
       if(isStatic){shape.density = 0;}
       else{shape.density = 2.0f;}
       shape.friction = 0.8f;
       shape.restitution = 0.3f;
       shape.setAsBox(half_width/RATE, half_height/RATE);

       BodyDef bodyDef = new BodyDef();
       bodyDef.position.set(x/RATE, y/RATE);
       Body body1= world.createBody(bodyDef);
       body1.createShape(shape);
       body1.setMassFromShapes();
    }

    public void createCircle(float x,float y,float radius){
       CircleDef shape = new CircleDef();
       shape.density = 7;
       shape.friction = 0.2f;
       shape.radius = radius/RATE;

       BodyDef bodyDef = new BodyDef();
       bodyDef.position.set(x/RATE, y/RATE);
       body2 = world.createBody(bodyDef);
       body2.createShape(shape);
       body2.setMassFromShapes();
    }

    public void createCircle1(float x,float y,float radius){
       CircleDef shape = new CircleDef();
       shape.density = 7;
       shape.friction = 0.2f;
       shape.radius = radius/RATE;

       BodyDef bodyDef = new BodyDef();
       bodyDef.position.set(x/RATE, y/RATE);
       body3 = world.createBody(bodyDef);
       body3.createShape(shape);
       body3.setMassFromShapes();
    }

    public void createBox1(float x,float y,float half_width,float half_height,
                   boolean isStatic){
       PolygonDef shape = new PolygonDef();
       if(isStatic){shape.density = 0;}
       else{shape.density = 2.0f;}
       shape.friction = 0.3f;
       shape.setAsBox(half_width/RATE, half_height/RATE);

       BodyDef bodyDef = new BodyDef();
       bodyDef.position.set(x/RATE, y/RATE);
       body= world.createBody(bodyDef);
       body.createShape(shape);
       body.setMassFromShapes();
    }

    class MyView extends View{
       Canvas canvas;
       public float x=160,y=150;
       public float x1=160,y1=100;
       public float x2=150,y2=60;
       public MyView(Context context) {
           super(context);
       }

       public void drawBox(float x,float y){
           Paint mPaint = new Paint();
           mPaint.setAntiAlias(true);
           mPaint.setColor(Color.RED);
           canvas.drawRect(x-160, y-10, x+160, y+10, mPaint);
       }

       public void drawGround(){
           Paint mPaint = new Paint();
           mPaint.setAntiAlias(true);
           mPaint.setColor(Color.BLUE);
           canvas.drawRect(0, 460, 320, 480, mPaint);
       }

       public void drawCircle(float x1,float y1){
           Paint mPaint = new Paint();
           mPaint.setAntiAlias(true);
           mPaint.setColor(Color.GREEN);
           canvas.drawCircle(x1, y1, 10, mPaint);
       }

       public void update(){
           postInvalidate();
       }

       protected void onDraw(Canvas canvas) {
           super.onDraw(canvas);
           this.canvas = canvas;
           drawGround();
           drawBox(x, y);
           drawCircle(x1, y1);
           drawCircle(x2, y2);
       }


    }

}

程序运行的时候需要加载Jbox2d的库，可到以下地址下载（使用的是不带渲染部分的库jbox2d-2.0.1-library-only.jar）：

http://sourceforge.net/projects/jbox2d/

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