前言

相比Ios UiKit原生支持物理引擎，Android确实麻烦的不要不要。

为什么用 libgdx

• Android上最方便的方案是jbox2D，缺点是在java层实现，物理多了之后性能很卡。笔者近期没有测试，11年左右在里程碑1上使用的时候那是巨卡无比。
• libgdx的物理引擎其实是封装的native版本box2D，在满足性能需求的同时，避免了开发JNI的烦恼，对于java程序员来说目前是最便捷的方案。

使用libgdx-box2d

STEP1: build.gradle中添加依赖

dependencies {

    configurations { natives }

    implementation "com.badlogicgames.gdx:gdx-box2d:$box2dVersion"
    natives "com.badlogicgames.gdx:gdx-box2d-platform:$box2dVersion:natives-armeabi"
    natives "com.badlogicgames.gdx:gdx-box2d-platform:$box2dVersion:natives-armeabi-v7a"
    natives "com.badlogicgames.gdx:gdx-box2d-platform:$box2dVersion:natives-arm64-v8a"
}

task copyAndroidNatives() {
    file("libs/armeabi/").mkdirs();
    file("libs/armeabi-v7a/").mkdirs();
    file("libs/arm64-v8a/").mkdirs();

    configurations.natives.files.each { jar ->
        def outputDir = null
        if(jar.name.endsWith("natives-arm64-v8a.jar")) outputDir = file("libs/arm64-v8a")
        if(jar.name.endsWith("natives-armeabi-v7a.jar")) outputDir = file("libs/armeabi-v7a")
        if(jar.name.endsWith("natives-armeabi.jar")) outputDir = file("libs/armeabi")
        if(outputDir != null) {
            copy {
                from zipTree(jar)
                into outputDir
                include "*.so"
            }
        }
    }
}

STEP2: 渲染层实现

• libgdx本身是个游戏引擎，提供基于openGLES的渲染引擎，但是对于大多数APPer来说使用成本略高。这里我们通过实现一个自定义view,并通过Canvas#draw()的方式进行渲染。

• 2.1 初始化box2D

    private void setupBox2D() {
        Box2D.init();
        //第一个参数为x、y两个方向的重力值，(0,0)点在左上y轴重力用10
        mWorld = new World(new Vector2(0, 10), true);
    }

• 2.2 在组件四周添加静态刚体

    private void createTopAndBottomBounds() {
        BodyDef bodyDef = new BodyDef();
        bodyDef.type = BodyDef.BodyType.StaticBody;

        PolygonShape box = new PolygonShape();
        float boxWidth = pixelsToMeters(mWidth);
        float boxHeight = pixelsToMeters(mRatio);
        box.setAsBox(boxWidth, boxHeight);

        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = box;
        fixtureDef.density = 0.5f;
        fixtureDef.friction = 0.3f;
        fixtureDef.restitution = 0.5f;

        bodyDef.position.set(0, -boxHeight * .5f);
        Body topBody = mWorld.createBody(bodyDef);
        topBody.createFixture(fixtureDef);

        bodyDef.position.set(0, pixelsToMeters(mHeight) + boxHeight * .5f);
        Body bottomBody = mWorld.createBody(bodyDef);
        bottomBody.createFixture(fixtureDef);
    }

    private void createLeftAndRightBounds() {
        float boxWidth = pixelsToMeters(mRatio);
        float boxHeight = pixelsToMeters(mHeight);

        BodyDef bodyDef = new BodyDef();
        bodyDef.type = BodyDef.BodyType.StaticBody;

        PolygonShape box = new PolygonShape();
        box.setAsBox(boxWidth, boxHeight);
        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = box;
        fixtureDef.density = 0.5f;
        fixtureDef.friction = 0.3f;
        fixtureDef.restitution = 0.5f;

        bodyDef.position.set(-boxWidth * .5f, boxHeight);
        Body leftBody = mWorld.createBody(bodyDef);
        leftBody.createFixture(fixtureDef);


        bodyDef.position.set(pixelsToMeters(mWidth) + boxWidth * .5f, 0);
        Body rightBody = mWorld.createBody(bodyDef);
        rightBody.createFixture(fixtureDef);
    }

• 2.3 添加一个动态刚体，从(0,0)点下落

    public void addGiftBody() {
        // First we create a body definition
        BodyDef bodyDef = new BodyDef();
        // We set our body to dynamic, for something like ground which doesn't move we would set it to StaticBody
        bodyDef.type = BodyDef.BodyType.DynamicBody;
        // Set our body's starting position in the world

        bodyDef.position.set(pixelsToMeters(3), pixelsToMeters(3));

        bodyDef.linearVelocity.set((float) Math.random(), (float) Math.random() * 100);

        // Create our body in the world using our body definition
        Body body = mWorld.createBody(bodyDef);
        //TODO 随机
        Bitmap bitmap = mBitmaps[0];
        GiftInfo giftInfo = new GiftInfo(bitmap);
        body.setUserData(giftInfo);
        body.setFixedRotation(false);

        PolygonShape box = new PolygonShape();
        float boxWidth = pixelsToMeters(pixelsToMeters(bitmap.getWidth()));
        float boxHeight = pixelsToMeters(pixelsToMeters(bitmap.getHeight()));
        box.setAsBox(boxWidth, boxHeight);

        // Create a fixture definition to apply our shape to
        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = box;
        fixtureDef.density = 1.5f;
        fixtureDef.friction = 0.4f;
        fixtureDef.restitution = .6f; // Make it bounce a little bit

        // Create our fixture and attach it to the body
        Fixture fixture = body.createFixture(fixtureDef);

        mBodyList.add(body);

        box.dispose();
    }

2.4 渲染、更新物体坐标、以此循环

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        drawGift(canvas);
        long time = System.currentTimeMillis() - mLastRenderTime;
        mLastRenderTime = time;
        doPhysicsStep(time);
        postInvalidate();
    }
    
    
    private void drawGift(Canvas canvas) {
        int size = mBodyList.size();
        for (int i = 0; i < size; i++) {
            Body body = mBodyList.get(i);
            GiftInfo giftInfo = (GiftInfo) body.getUserData();
            Bitmap bitmap = giftInfo.getBitmap();

            canvas.drawBitmap(bitmap, metersToPixels(mBodyList.get(i).getPosition().x) - bitmap.getWidth() * .5f,
                    metersToPixels(mBodyList.get(i).getPosition().y) - bitmap.getHeight() * .5f, mPaint);
        }
    }
    
    private void doPhysicsStep(float deltaTime) {
        // fixed time step
        // max frame time to avoid spiral of death (on slow devices)
        float frameTime = Math.min(deltaTime, 0.25f);
        mAccumulator += frameTime;
        while (mAccumulator >= Constants.TIME_STEP) {
            mWorld.step(Constants.TIME_STEP, Constants.VELOCITY_ITERATIONS, Constants.POSITION_ITERATIONS);
            mAccumulator -= Constants.TIME_STEP;
        }
    }

结尾

• 希望能够通过这个简单例子帮助大家使用libgdx-box2d，先水一波。