问获取b2body Box2d的b2Fixture？

EN

-(void) addNewSprite:(NSString *)spriteName AtPosition:(CGPoint)p isDynamic:(BOOL)dynamic
{
    //CCNode *parent = [self getChildByTag:kTagParentNode];

PhysicsSprite *sprite = [PhysicsSprite spriteWithFile:spriteName];

[self addChild:sprite];

sprite.position = ccp( p.x, p.y);

sprite.tag = check;

// Define the dynamic body.
//Set up a 1m squared box in the physics world
b2BodyDef bodyDef;
bodyDef.type = b2_dynamicBody;  
bodyDef.position.Set(sprite.position.x/PTM_RATIO, sprite.position.y/PTM_RATIO);
bodyDef.userData = sprite;
b2Body *body = world->CreateBody(&bodyDef);

// Define another box shape for our dynamic body.
b2PolygonShape dynamicBox;
dynamicBox.SetAsBox((sprite.contentSize.width/PTM_RATIO/2)*(sprite.scaleX),
                    (sprite.contentSize.height/PTM_RATIO/2)*(sprite.scaleY));//These are mid points for our 1m box

// Define the dynamic body fixture.
b2FixtureDef fixtureDef;
fixtureDef.shape = &dynamicBox; 
fixtureDef.density = 1.0f;
fixtureDef.friction = 1.0f;
fixtureDef.userData = sprite;

switch (check)
{
    case 1:
        fixtureDef.filter.categoryBits = 0x0002;

        fixtureDef.filter.maskBits = 0x0004;
        break;
    case 2:
        fixtureDef.filter.categoryBits = 0x0004;

        fixtureDef.filter.maskBits = 0x0002;
        break;
}

body->CreateFixture(&fixtureDef);

[sprite setPhysicsBody:body];

check++;
}

-(void) update: (ccTime) dt
{
    //It is recommended that a fixed time step is used with Box2D for stability
    //of the simulation, however, we are using a variable time step here.
    //You need to make an informed choice, the following URL is useful
    //http://gafferongames.com/game-physics/fix-your-timestep/

int32 velocityIterations = 8;
int32 positionIterations = 1;

// Instruct the world to perform a single step of simulation. It is
// generally best to keep the time step and iterations fixed.
world->Step(dt, velocityIterations, positionIterations);    

for(b2Body *b = world->GetBodyList(); b; b=b->GetNext()) {    

    if (b->GetUserData() != NULL) 
    {

        //b2Fixture fixtureDef = *b->GetFixtureList();
        //b2Filter filter = fixtureDef.GetFilterData();

        // how to get b2FixtureDef from b

    }
}