Cocos物理世界（box2）

Box2D

让游戏看起更加真实，比如东西掉下来等等，不用去Moveto模拟

#define PTM_RATIO 32.0f

bool T06Box2D::init()
{
	Layer::init();

	// 创建世界
	// b2Vec2 gravity(0,-9.8f);
	b2Vec2 gravity(0,0.0f);//x,y方向得到力

	_world = new b2World(gravity);
	{
		b2BodyDef def;
		def.type = b2_dynamicBody; //类型设为动态的
		//重力系统的位置跟我们屏幕位置不一样，所以我们要设置一个比例关系
		def.position.Set(winSize.width / 2/PTM_RATIO,winSize.height / 2/PTM_RATIO);

		//创建物体，创建一个物体都是在世界里面创建的
		b2Body* body = _world->CreateBody(&def);

		// 让body受力
		body->SetLinearVelocity(b2Vec2(10,20));
		
		Sprite* sprite = Sprite::create("CloseNormal.png");
		addChild(sprite);
		sprite->setPosition(body->GetPosition().x*PTM_RATIO,body->GetPosition().y*PTM_RATIO);

		// 设置body的形状，让它和sprite相一致
		b2CircleShape shape;
		shape.m_radius = sprite->getContentSize().width / 2 / PTM_RATIO;//圆的半径

		b2Fixture* fixture = body->CreateFixture(&shape,1.0f /*密度系数*/);//密度系数0---1.
		fixture->SetFriction(0.0f);  // 摩擦系数 0--1， 0代表没有摩擦
		fixture->SetRestitution(1.0f); // 弹性系数  

		// 关联body和精灵
		body->SetUserData(sprite);
	}

	// 加个地板
	{
		b2BodyDef def;
		// def.position.Set(0,0);

		b2Body* body = _world->CreateBody(&def);
		b2EdgeShape shape;//边界形状
		shape.Set(b2Vec2(0,0),b2Vec2(winSize.width / PTM_RATIO,0));

		b2Fixture* fixture = body->CreateFixture(&shape,1.0f);

		fixture->SetFriction(0.0f);  // 摩擦系数
		fixture->SetRestitution(1.0f); // 弹性系数， 1没有消耗，每次弹起高度都一样，越小越有消耗。
	}

	// 加个天花板
	{
		b2BodyDef def;
		def.position.Set(0,winSize.height/PTM_RATIO);

		b2Body* body = _world->CreateBody(&def);
		b2EdgeShape shape;
		shape.Set(b2Vec2(0,1.0f);

		fixture->SetFriction(0.0f);  // 摩擦系数
		fixture->SetRestitution(1.0f); // 弹性系数
	}

	// 左挡板
	{
		b2BodyDef def;
		//def.position.Set(0,winSize.height / PTM_RATIO);

		b2Body* body = _world->CreateBody(&def);
		b2EdgeShape shape;
		shape.Set(b2Vec2(0,b2Vec2(0,winSize.height/PTM_RATIO));

		b2Fixture* fixture = body->CreateFixture(&shape,1.0f);

		fixture->SetFriction(0.0f);  // 摩擦系数
		fixture->SetRestitution(1.0f); // 弹性系数
	}

	// 右挡板
	{
		b2BodyDef def;
		def.position.Set(winSize.width / PTM_RATIO,0);

		b2Body* body = _world->CreateBody(&def);
		b2EdgeShape shape;
		shape.Set(b2Vec2(0,winSize.height / PTM_RATIO));

		b2Fixture* fixture = body->CreateFixture(&shape,1.0f);
		fixture->SetFriction(0.0f);  // 摩擦系数
		fixture->SetRestitution(1.0f); // 弹性系数
	}

	// 球拍
	{
		b2BodyDef def;
		def.position.Set(winSize.width / 2 / PTM_RATIO,winSize.height / 4 / PTM_RATIO);
		
		b2Body* body = _world->CreateBody(&def);
		_bat = body;

		Sprite* sprite = Sprite::create("bat.png");
		body->SetUserData(sprite);
		addChild(sprite);
		sprite->setPosition(body->GetPosition().x*PTM_RATIO,body->GetPosition().y*PTM_RATIO);

		Size batSize = Size(100,30);
		Size content = sprite->getContentSize();
		sprite->setScale(batSize.width / content.width,batSize.height / content.height);

		b2PolygonShape shape;//多边形
		shape.SetAsBox(batSize.width/2/PTM_RATIO,batSize.height/2/PTM_RATIO);

		b2Fixture* fixture = body->CreateFixture(&shape,1.0f);
		fixture->SetRestitution(1.0f);
		fixture->SetFriction(0.0f);  // 摩擦系数
		fixture->SetRestitution(1.0f); // 弹性系数

		// touch
		EventListenerTouchOneByOne* ev = EventListenerTouchOneByOne::create();
		ev->onTouchBegan = [](Touch*,Event*){return true; };
		ev->onTouchMoved = [&](Touch* touch,Event*){  //注意这里要加&哦，不然不能访问_bat.
			float dx = touch->getDelta().x / PTM_RATIO;

			b2Vec2 pos = _bat->GetPosition();
			pos.x += dx;

			_bat->SetTransform(pos,0);
		};

		_eventDispatcher->addEventListenerWithSceneGraphPriority(ev,this);
		
	}

	scheduleUpdate();

	return true;
}

void T06Box2D::update(float dt)
{
	// 时间在流逝
	_world->Step(dt,8,1);

	// 遍历这个世界的body
	b2Body* body = _world->GetBodyList();
	while (body)
	{
		// 设置body相关的精灵的位置
		Sprite* sprite = (Sprite*)body->GetUserData();
		if (sprite)
		{
			sprite->setPosition(body->GetPosition().x*PTM_RATIO,body->GetPosition().y*PTM_RATIO);
			//设置旋转，比如有时物体碰撞要旋转，精灵跟着旋转
			sprite->setRotation(body->GetAngle()*180.0/M_PI);//弧度和角度的转换
		}

		body = body->GetNext();
	}
}

（编辑：李大同）

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