Android上使用Box2d

Box2d是一個非常優秀的開源2D物理引擎，可以模擬出非常真實的物理效果，今天我們介紹如何在Andoird下使用Box2d：

1.

Box2d原來使用c++寫的，不過已經有了Java版本的：JBox2D，可以在http://sourceforge.net/projects/jbox2d/下載，一共有兩個：

JBox2d-2.0.1.zip和JBox2D-2.0.1-b250-Full.jar。我的版本是2.0.1

2.

新建一個Android Project，如下所示：

2.

修改Jbox2dtestActivity.java文件如下：

1. package com.test.jbox2d;
3. import org.jbox2d.collision.AABB;
4. import org.jbox2d.collision.CircleDef;
5. import org.jbox2d.collision.PolygonDef;
6. import org.jbox2d.common.Vec2;
7. import org.jbox2d.dynamics.Body;
8. import org.jbox2d.dynamics.BodyDef;
9. import org.jbox2d.dynamics.World;
11. import android.app.Activity;
12. import android.content.Context;
13. import android.graphics.Canvas;
14. import android.graphics.Color;
15. import android.graphics.Paint;
16. import android.os.Bundle;
17. import android.os.Handler;
18. import android.view.View;
19. import android.view.Window;
20. import android.view.WindowManager;
22. public class Jbox2dtestActivity extends Activity
23. {
24. private final static int RATE = 10; //屏幕到現實世界的比例 10px：1m;
25. private AABB worldAABB; //創建 一個管理碰撞的世界
26. private World world;
27. private float timeStep = 1/60; //模擬的的頻率
28. private int iterations = 10; //迭代越大，模擬約精確，但性能越低
29. private Body body,body2,body3;
30. private MyView myView;
31. private Handler mHandler;
33. public void onCreate(Bundle savedInstanceState)
34. {
35. super.onCreate(savedInstanceState);
36. requestWindowFeature(Window.FEATURE_NO_TITLE);
37. getWindow().setFlags(WindowManager.LayoutParams. FLAG_FULLSCREEN ,
38. WindowManager.LayoutParams. FLAG_FULLSCREEN);
40. worldAABB = new AABB();
42. //上下界，以屏幕的左上方為 原點，如果創建的剛體到達屏幕的邊緣的話，會停止模擬
43. worldAABB.lowerBound.set(-100.0f,-100.0f);
44. worldAABB.upperBound.set(100.0f, 100.0f);//注意這裡使用的是現實世界的單位
46. Vec2 gravity = new Vec2(0.0f,10.0f);
47. boolean doSleep = true;
49. world = new World(worldAABB, gravity, doSleep);//創建世界
51. createBox(160, 470, 160, 10, true);
52. createBox1(160, 150, 160, 10, false);
54. createCircle(160, 100, 10);
55. createCircle1(150, 60, 10);
56. timeStep = 1.0f/60.0f;
57. iterations = 10;
59. myView = new MyView(this);
60. setContentView(myView);
61. mHandler = new Handler();
62. mHandler.post(update);
63. }
65. private Runnable update = new Runnable()
66. {
67. public void run()
68. {
69. world.step(timeStep, iterations);//開始模擬
70. Vec2 position = body.getPosition();
71. Vec2 position1 = body2.getPosition();
72. Vec2 position2 = body3.getPosition();
73. myView.x=position.x*RATE;
74. myView.y=position.y*RATE;
76. myView.x1=position1.x*RATE;
77. myView.y1=position1.y*RATE;
79. myView.x2=position2.x*RATE;
80. myView.y2=position2.y*RATE;
81. myView.update();
82. mHandler.postDelayed(update, (long)timeStep*1000);
83. }
84. };
86. public void createBox(float x, float y, float half_width, float half_height, boolean isStatic)
87. {
88. PolygonDef shape = new PolygonDef();
89. if(isStatic)
90. {
91. shape.density = 0;
92. }
93. else
94. {
95. shape.density = 2.0f;
96. }
97. shape.friction = 0.8f;
98. shape.restitution = 0.3f;
99. shape.setAsBox(half_width/RATE, half_height/RATE);
101. BodyDef bodyDef = new BodyDef();
102. bodyDef.position.set(x/RATE, y/RATE);
103. Body body1= world.createBody(bodyDef);
104. body1.createShape(shape);
105. body1.setMassFromShapes();
106. }
108. public void createCircle(float x, float y, float radius)
109. {
110. CircleDef shape = new CircleDef();
111. shape.density = 7;
112. shape.friction = 0.2f;
113. shape.radius = radius/RATE;
115. BodyDef bodyDef = new BodyDef();
116. bodyDef.position.set(x/RATE, y/RATE);
117. body2 = world.createBody(bodyDef);
118. body2.createShape(shape);
119. body2.setMassFromShapes();
120. }
122. public void createCircle1(float x, float y, float radius)
123. {
124. CircleDef shape = new CircleDef();
125. shape.density = 7;
126. shape.friction = 0.2f;
127. shape.radius = radius/RATE;
129. BodyDef bodyDef = new BodyDef();
130. bodyDef.position.set(x/RATE, y/RATE);
131. body3 = world.createBody(bodyDef);
132. body3.createShape(shape);
133. body3.setMassFromShapes();
134. }
136. public void createBox1(float x, float y, float half_width, float half_height, boolean isStatic)
137. {
138. PolygonDef shape = new PolygonDef();
139. if(isStatic)
140. {
141. shape.density = 0;
142. }
143. else
144. {
145. shape.density = 2.0f;
146. }
147. shape.friction = 0.3f;
148. shape.setAsBox(half_width/RATE, half_height/RATE);
150. BodyDef bodyDef = new BodyDef();
151. bodyDef.position.set(x/RATE, y/RATE);
152. body= world.createBody(bodyDef);
153. body.createShape(shape);
154. body.setMassFromShapes();
155. }
157. class MyView extends View
158. {
159. Canvas canvas;
160. public float x=160,y=150;
161. public float x1=160,y1=100;
162. public float x2=150,y2=60;
163. public MyView(Context context)
164. {
165. super(context);
166. }
168. public void drawBox(float x, float y)
169. {
170. Paint mPaint = new Paint();
171. mPaint.setAntiAlias(true);
172. mPaint.setColor(Color.RED);
173. canvas.drawRect(x-160, y-10, x+160, y+10, mPaint);
174. }
176. public void drawGround()
177. {
178. Paint mPaint = new Paint();
179. mPaint.setAntiAlias(true);
180. mPaint.setColor(Color.BLUE);
181. canvas.drawRect(0, 460, 320, 480, mPaint);
182. }
184. public void drawCircle(float x1, float y1)
185. {
186. Paint mPaint = new Paint();
187. mPaint.setAntiAlias(true);
188. mPaint.setColor(Color.GREEN);
189. canvas.drawCircle(x1, y1, 10, mPaint);
190. }
192. public void update()
193. {
194. postInvalidate();
195. }
197. protected void onDraw(Canvas canvas)
198. {
199. super.onDraw(canvas);
200. this.canvas = canvas;
201. drawGround();
202. drawBox(x, y);
203. drawCircle(x1, y1);
204. drawCircle(x2, y2);
205. }
206. }
207. }