Android之Box2D学习（一）——HelloWorld

本文地址：http://blog.csdn.net/you_and_me12/article/details/7961573

2012-09-10

从今天开始学习Box2D,在java中是JBox2D(Download)，包的导入就不用说了吧！

文章后面给出了我写的helloworld，仅供参考

笔记：Ⅰ）Box2D由世界，物体等组成，Box2D能自动计算物体的位置，然后我们还需要在Android中绘制出来；

      Ⅱ）Box2D世界中的单位都是标准单位，秒、米、千克 etc. 需要按照比例缩放，不然速度很慢(代码中有转换)；

      Ⅲ）Box2D每帧的位移是2m/frame,不能超过这个速度，所以要进行缩放，即可放大速度；

      Ⅳ）注意坐标系的零点是在左上角，同时角度是以弧度制的；

                

坐标系                                  效果图

代码：

public class Hello_Box2d extends Activity {
	private World mWorld;
	private Handler mHandler;
	private View mView;
	private int screenH, screenW;
	private final float WORLD2PHONE_RATE = 5;
	private Runnable mRunnable = new Runnable() {
		@Override
		public void run() {
			mWorld.step(0.1f, 3, 3);//
			mView.invalidate();
			mHandler.postDelayed(this, 100);
		}
	};
	/** Called when the activity is first created. */
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		this.requestWindowFeature(Window.FEATURE_NO_TITLE); //remove title
		getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
		WindowManager.LayoutParams.FLAG_FULLSCREEN);// full screen
		//get the window width and height
		DisplayMetrics metric = new DisplayMetrics();
		getWindowManager().getDefaultDisplay().getMetrics(metric);
		screenH = metric.heightPixels;
		screenW = metric.widthPixels;
		//create view
		mView = new helloBox2dView(this);
		mView.setBackgroundColor(Color.WHITE);
		setContentView(mView);
	}

	@Override
	protected void onDestroy() {
		super.onDestroy();
		mHandler.removeCallbacks(mRunnable);
	}

	class helloBox2dView extends View {
		private Canvas mCanvas;
		private Body mBody;
		public helloBox2dView(Context context) {
			super(context);
			/** Creating a World */
			{
				Vec2 gravity = new Vec2(0f, 9.8f);// vector, the gravity direction, left and right, up and down
				mWorld = new World(gravity, false);
			}
			/** Creating a Ground Box */
			{
			// Bodies are built using the following steps:
				// Step 1. Define a body with position, damping, etc.
				BodyDef groundBodyDef = new BodyDef();
				groundBodyDef.position.set(0f, (screenH-screenW*3/4)/WORLD2PHONE_RATE);
				groundBodyDef.angle = (float) Math.asin(0.6);
				// Step 2. Use the world object to create the body.
				Body groundBody = mWorld.createBody(groundBodyDef);
				// Step 3. Define fixtures with a shape, friction, density, etc.
				PolygonShape groundBox = new PolygonShape();
				groundBox.setAsBox(screenW*5/4/WORLD2PHONE_RATE, 1f/WORLD2PHONE_RATE);
				// Step 4. Create fixtures on the body.
				groundBody.createFixture(groundBox, 0.0f);
				
				//border
				BodyDef groundBodyDef2 = new BodyDef();
				groundBodyDef2.position.set(screenW/WORLD2PHONE_RATE, 0f);
				groundBodyDef2.angle = (float) (0.5f*Math.PI);
				Body groundBody2 = mWorld.createBody(groundBodyDef2);
				PolygonShape groundBox2 = new PolygonShape();
				groundBox2.setAsBox(screenH/WORLD2PHONE_RATE, 1f/WORLD2PHONE_RATE);
				groundBody2.createFixture(groundBox2, 0.0f);
			}
			/** Creating a Dynamic Body */
			{
				BodyDef bodyDef = new BodyDef();
				bodyDef.type = BodyType.DYNAMIC;
				bodyDef.position.set(100f/WORLD2PHONE_RATE, 0f);
				mBody = mWorld.createBody(bodyDef);
				
				CircleShape dynamicBox = new CircleShape();
				dynamicBox.m_radius = 10/WORLD2PHONE_RATE;
				
				FixtureDef fixtureDef = new FixtureDef();
				fixtureDef.shape = dynamicBox;
				fixtureDef.density = 500f;
				fixtureDef.friction = 0.5f;
				fixtureDef.restitution = 0.8f;
				
				mBody.createFixture(fixtureDef);
			}
			mHandler = new Handler();
			mHandler.post(mRunnable);
		}
		@Override
		protected void onDraw(Canvas canvas) {
			super.onDraw(canvas);
			mCanvas = canvas;
			//draw ground
			drawGround();
			//draw box
			Vec2 position = mBody.getPosition();
			float angle = mBody.getAngle();
			drawBox(position.x*WORLD2PHONE_RATE, position.y*WORLD2PHONE_RATE, angle);
		}
		public void drawBox(float x, float y, float angle){
			Paint paint = new Paint();
			paint.setAntiAlias(true);
			paint.setColor(Color.RED);
			mCanvas.drawCircle(x, y, 10, paint);
			mCanvas.drawLine(x, y, (float)(x+10*Math.cos(angle)), (float)(y+10*Math.sin(angle)), new Paint());
		}
		public void drawGround(){
			Paint paint = new Paint();
			paint.setColor(Color.BLUE);
			paint.setStrokeWidth(10);
			mCanvas.drawLine(0, screenH-screenW*3/4, screenW, screenH, paint);
			mCanvas.drawLine(screenW, 0, screenW, screenH, paint);
		}
	}
}