android - jBox2d android 绘制动态物体

Question

我从教程 here 开始并为 View 添加了绘图代码(覆盖整个屏幕)。即使我正在调用 world.step(,)，场景仍然是静态的而不是动态的。如何使场景动态化？我的绘图是正确的实现还是有更好的方法(使用 jBox2d 函数)？

private class PhysicsView extends View {

        Paint mPaint;

        public PhysicsView(Context context) {
            super(context);
            mPaint = new Paint();
            mPaint.setColor(Color.BLUE);
            mPaint.setStyle(Style.FILL_AND_STROKE);
        }

        @Override
        public void onDraw(Canvas canvas) {
            super.onDraw(canvas);

            synchronized (canvas) {
                Body[] b = mWorld.bodies;
                for (int i = 0; i < b.length; i++) {
                    if (b[i] != null) {

                        Float mass = b[i].getMass();
                        Vec2 v = b[i].getPosition();
                        canvas.drawCircle(v.x, v.y, 15, mPaint);
                    }
                }
                Paint p2 = new Paint(mPaint);
                p2.setColor(Color.GREEN);
                Vec2 base = mWorld.groundBody.getPosition();
                canvas.drawRect(new RectF((float) base.x - 25.0f,
                        (float) base.y - 10.0f, (float) base.x + 25.0f,
                        (float) base.y + 10.0f), p2);

                canvas.drawLine(0.0f, 0.0f,
                        mWorld.world.getWorldAABB().upperBound.x, mWorld.world
                                .getWorldAABB().upperBound.y, p2);
            }
        }
    }


public class PhysicsWorld {

    public int targetFPS = 40;
    public int timeStep = (1000 / targetFPS);
    public int iterations = 5;

    public Body[] bodies = new Body[50];
    public Body groundBody;
    private int count = 0;

    private AABB worldAABB;
    public World world;
    private BodyDef groundBodyDef;
    private PolygonDef groundShapeDef;

    private Vec2 screenDimensions;

    public void create(Vec2 dimens) {

        screenDimensions = dimens;

        worldAABB = new AABB();
        worldAABB.lowerBound.set(new Vec2((float) -1*dimens.x, (float) -1*dimens.y));
        worldAABB.upperBound.set(new Vec2((float) dimens.x, (float) dimens.y));

        Vec2 gravity = new Vec2((float) 0.0, (float) 0.0);
        boolean doSleep = true;
        world = new World(worldAABB, gravity, doSleep);

        groundBodyDef = new BodyDef();
        groundBodyDef.position.set(new Vec2((float) dimens.x/2, (float) dimens.y - 10.0f));
        groundBody = world.createBody(groundBodyDef);
        groundBody.m_mass = 200.0f;

        groundShapeDef = new PolygonDef();
        groundShapeDef.setAsBox((float) 50.0, (float) 10.0);
        groundShapeDef.density = 1.0f;
        groundShapeDef.friction = 0.3f;
        groundBody.createShape(groundShapeDef);

    }

    public void addBall() {

        float x = (float) Math.random() * screenDimensions.x;
        float y = (float) Math.random() * screenDimensions.y;

        BodyDef bodyDef = new BodyDef();
        bodyDef.position.set(new Vec2((float) x, (float) y));
        bodyDef.massData.mass = 20.0f;
        bodies[count] = world.createBody(bodyDef);
        bodies[count].m_type = Body.e_dynamicType;
        bodies[count].m_linearVelocity = new Vec2(1.0f, 2.0f);

        CircleDef circle = new CircleDef();
        circle.radius = (float) 1.8;
        circle.density = (float) 1.0;
        circle.type = ShapeType.CIRCLE_SHAPE;

        bodies[count].createShape(circle);

        count++;
    }

    public void update() {
        world.step(timeStep, iterations);

    }

    public void changeGravity(Vec2 gravity) {
        world.setGravity(gravity);
    }
}

Answer 1

您的代码存在许多问题....但这个答案可能为时已晚，无论如何都无法提供任何帮助。

首先，world.createBody() 似乎已被弃用。现在你必须在我拥有的最新版本的 Android Box2D jar 中显式调用 world.createDynamicBody()。

无论如何……我的版本是这样的……不完美，但至少它是动态的。

package com.box2D;

import java.util.ArrayList;

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.graphics.Canvas;
import android.graphics.Paint;
import android.util.Log;

public class PhysicsWorld 
{
    public float timeStep = 1;
    public int iterations = 1;

    private ArrayList<Body> bodies;
    private int count = 0;

    private AABB worldAABB;
    private World world;
    private BodyDef groundBodyDef;
    private PolygonDef boxShapeDef;

    //a paint
    private Paint paint;

    public void create() 
    {
        //create the paint
        paint           = new Paint();
        paint.setColor(0xFFFFFF);

        // Step 1: Create Physics World Boundaries
        worldAABB       = new AABB();
        worldAABB.lowerBound.set(new Vec2((float) -100.0, (float) -100.0));
        worldAABB.upperBound.set(new Vec2((float) 100.0, (float) 300.0));

        // Step 2: Create Physics World with Gravity
        Vec2 gravity    = new Vec2((float) 0.0, (float)10.0);
        boolean doSleep = true;
        world           = new World(worldAABB, gravity, doSleep);
        bodies          = new ArrayList<Body>();

        // Step 3: Create Ground Box
        groundBodyDef   = new BodyDef();
        groundBodyDef.position.set(new Vec2((float) 0.0, (float) 300.0));
        Body groundBody = world.createDynamicBody(groundBodyDef);
        boxShapeDef  = new PolygonDef();
        boxShapeDef.setAsBox((float) 50.0, (float) 10.0);
        groundBody.createShape(boxShapeDef);
    }

    public void addBall() 
    {
        // Create Dynamic Body
        BodyDef bodyDef  = new BodyDef();
        bodyDef.position.set((float)16.0+(count * 10), (float)24.0);        
        Body newBody     = world.createDynamicBody(bodyDef);        

        // Create Shape with Properties
        CircleDef circle = new CircleDef();
        circle.radius    = (float)5;
        circle.density   = (float)1.0;

        // Assign shape to Body
        newBody.createShape(circle);
        newBody.setMassFromShapes();  
        newBody.m_userData = "Circle";
        bodies.add(newBody);

        // Increase Counter
        count += 1;
    }    

    public void addBox() 
    {
        // Create Dynamic Body
        BodyDef bodyDef  = new BodyDef();
        bodyDef.position.set((float)36.0+(count * 10), (float)24.0);
        Body newBody     = world.createDynamicBody(bodyDef);        

        // Create Shape with Properties
        boxShapeDef  = new PolygonDef();
        boxShapeDef.setAsBox((float) 50.0, (float) 10.0);

        // Assign shape to Body
        newBody.createShape(boxShapeDef);
        newBody.setMassFromShapes();  
        newBody.m_userData = "Box";
        bodies.add(newBody);

        // Increase Counter
        count += 1;
    }

    /*
     * 
     * 
     * Physics Update
     */

    private Vec2 position;
    private float angle;

    public void Update(Canvas canvas) 
    {
        // Update Physics World
        world.step(timeStep/5, 1);

        // Print info of latest body
        for (Body i : bodies) 
        {
            position = i.getPosition();
            angle    = i.getAngle(); 

            if(i.m_userData == "Circle")
            canvas.drawCircle(position.x, position.y, 5,paint );

            if(i.m_userData == "Box")
            canvas.drawRect((float)(position.x -5), (float)(position.y - 5), (float)(position.x + 5), (float)(position.y + 5), paint );

            Log.v("Physics Test", "Pos: (" + position.x + ", " + position.y + "), Angle: " + angle);
        }
    }
}