box2d之刚体，定制器笔记

刚体(Body)：
通过世界创建的对象，物理引擎里面所有东西都是刚体
创建方式：
定义一个b2BodyDef，b2World调用createBody的方法传入此参数

定制器(Fixture)：
带有形状，密度，摩擦力等属性
创建方式：
定义一个b2FixtureDef，调用刚体的createFixture方法为刚体创建定制器（相当于附加属性），每个刚体支持添加多个Fixture；
刚体的createFixture重载了两种模式，一种是b2FixtureDef作为参数，一种是shape作为参数，效果是一样的。

形状（shape）：
刚体的形状
种类：
circle（圆形），Polygon（多边形），其他等等

基本上他们的范围是这样：world，body，fixture（刚体的多个属性）

tips：b2Vec2是2dx里面类似CGPoint的东西

具体看代码

    //刚体
    b2BodyDef testBodyDef;
    testBodyDef.type = b2_staticBody;
    testBodyDef.position.Set(0,0);
    
    //底部的边缘线
    b2EdgeShape testShape;
    testShape.Set(b2Vec2(0,0), b2Vec2(s.width/PTM_RATIO,0));
    
    b2FixtureDef testFixtueDef;
    testFixtueDef.shape = &testShape;

    b2Body* testBody = world->CreateBody(&testBodyDef);
    testBody->CreateFixture(&testFixtueDef);
    
    //顶部边缘线
    testShape.Set(b2Vec2(0,s.height/PTM_RATIO), b2Vec2(s.width/PTM_RATIO,s.height/PTM_RATIO));
    testBody->CreateFixture(&testFixtueDef);
    
    //左边边缘线
    testShape.Set(b2Vec2(0,0), b2Vec2(0,s.height/PTM_RATIO));
    testBody->CreateFixture(&testFixtueDef);
    
    //右边边缘线
    testShape.Set(b2Vec2(s.width/PTM_RATIO,0), b2Vec2(s.width/PTM_RATIO,s.height/PTM_RATIO));
    testBody->CreateFixture(&testFixtueDef);

    //创建可运动的缸体
    CCPoint ballPosition = CCPointMake(200, 100);
    CCSprite* ballSprite = CCSprite::create("Ball.png");
    ballSprite->setPosition(ballPosition);
    this->addChild(ballSprite);
    
    b2BodyDef moveableBodyDef;
    moveableBodyDef.type = b2_dynamicBody;
    
    //设置刚体位置
    //刚体相对于世界的位置，坐标转换为box2d里面的坐标
    moveableBodyDef.position.Set(ballPosition.x/PTM_RATIO,ballPosition.y/PTM_RATIO);
    //把精灵放到b2BodyDef的userData里面，在update里面刷新精灵位置
    moveableBodyDef.userData = ballSprite;
    
    b2Body *moveableBody = world->CreateBody(&moveableBodyDef);
    
    //创建圆形小球
    b2CircleShape testCircleShape;
    //刚体和形状设置坐标的时候，选一个就ok
    testCircleShape.m_p.Set(0,0);
    float radius = ballSprite->getContentSize().width/(2*PTM_RATIO);
    testCircleShape.m_radius = radius;
    
    b2FixtureDef moveableFixtureDef;
    moveableFixtureDef.shape = &testCircleShape;
    moveableFixtureDef.density = 1.0f;
    moveableFixtureDef.friction = 0.3f;
    moveableBody->CreateFixture(&moveableFixtureDef);

update方法

void HelloWorld::update(float dt)
{
    int velocityIterations = 8;
    int positionIterations = 1;
    
    world->Step(dt, velocityIterations, positionIterations);
    
    //刷新精灵位置
    for (b2Body* body = world->GetBodyList(); NULL!= body; body = body->GetNext())
    {
        if ( NULL != body->GetUserData() )
        {
            CCSprite* sprite = (CCSprite*)body->GetUserData();
            
            sprite->setPosition( CCPointMake( body->GetPosition().x * PTM_RATIO, body->GetPosition().y * PTM_RATIO) );
            sprite->setRotation( -1 * CC_RADIANS_TO_DEGREES(body->GetAngle()) );
        }    
    }
}