How do I resize a b2CircleShape on cocos2d iPhone with Box2D

Question

I have a 2D physics sandbox with a bunch of circles which resize on contact (the bigger one gets bigger, the smaller one gets smaller). I can resize the sprite fine, and I understand that you can't scale a B2Body - you need to destroy it and recreate it, but I'm not familiar enough with Box2D to do it yet.

Here's what I'm doing to resize the sprites:

std::vector<MyContact>::iterator pos;
for(pos = _contactListener->_contacts.begin(); 
    pos != _contactListener->_contacts.end(); ++pos) {
    MyContact contact = *pos;

    b2Body *bodyA = contact.fixtureA->GetBody();
    b2Body *bodyB = contact.fixtureB->GetBody();
    if (bodyA->GetUserData() != NULL && bodyB->GetUserData() != NULL) {
        PaintBlob *spriteA = (PaintBlob *) bodyA->GetUserData();
        PaintBlob *spriteB = (PaintBlob *) bodyB->GetUserData();

        NSLog(@"spriteA: %@ is touching spriteB: %@", spriteA, spriteB);


        if((spriteA.scale > spriteB.scale) && (spriteB.scale > 0)){
            spriteA.scale = spriteA.scale + kSCALE_INCREMENT;
            spriteB.scale = spriteB.scale - kSCALE_INCREMENT;
        }else if (spriteA.scale >0) {
            spriteB.scale = spriteB.scale + kSCALE_INCREMENT;
            spriteA.scale = spriteA.scale - kSCALE_INCREMENT;
        }

    }        
} 

How do I resize (destroy/recreate) the Box2D body (a b2CircleShape?).

I think this is how you do it in C - from emanueleferonato.com (I am not enlightened enough to understand C):

// if I selected a body...
if (body) {
                // I know it's a circle, so I am creating a b2CircleShape variable
                var circle:b2CircleShape=body.GetShapeList() as b2CircleShape;
                // getting the radius..
                var r=circle.GetRadius();
                // removing the circle shape from the body
                body.DestroyShape(circle);
                // creating a new circle shape
                var circleDef:b2CircleDef;
                circleDef = new b2CircleDef();
                // calculating new radius
                circleDef.radius=r*0.9;
                circleDef.density=1.0;
                circleDef.friction=0.5;
                circleDef.restitution=0.2;
                // attach the shape to the body
                body.CreateShape(circleDef);
                // determine new body mass
                body.SetMassFromShapes();
            }
            return body;

Answer 1

Hi there you handsome devil.

Here's how:

    //Radius is worked out by scale * kBLOBDIAMETER /2 
    contact.fixtureA->GetShape()->m_radius = (spriteA.scale * kBLOBLDIAMETER / 2) /PTM_RATIO;
    contact.fixtureB->GetShape()->m_radius = (spriteB.scale * kBLOBLDIAMETER / 2) /PTM_RATIO;
    bodyA->ResetMassData();
    bodyB->ResetMassData();