Changing the polygon vertices in a dynamic polygon in cocos2d - cocos2d-iphone

Question

I am new to cocos2d and first I learned how to make a circle then a square and now I learned how to create a polygon with an amount of vertices that I pick with a "b2polygonshape polygon" and here is my code for that
-(void) createDynamicPoly:(CGPoint)p;
{
b2BodyDef bodyDefPoly;
bodyDefPoly.type = b2_dynamicBody;
bodyDefPoly.position.Set(p.x/PTM_RATIO, p.y/PTM_RATIO);
b2Body *polyBody = world->CreateBody(&bodyDefPoly);
int count = 8;
b2Vec2 vertices[8];
vertices[0].Set(10.0f/PTM_RATIO,0.0/PTM_RATIO);
vertices[1].Set(20.0f/PTM_RATIO,0.0f/PTM_RATIO);
vertices[2].Set(30.0f/PTM_RATIO,10.0f/PTM_RATIO);
vertices[3].Set(30.0f/PTM_RATIO,20.0f/PTM_RATIO);
vertices[4].Set(20.0f/PTM_RATIO,30.0f/PTM_RATIO);
vertices[5].Set(10.0f/PTM_RATIO,30.0f/PTM_RATIO);
vertices[6].Set(00.0f/PTM_RATIO,20.0f/PTM_RATIO);
vertices[7].Set(0.0f/PTM_RATIO,10.0f/PTM_RATIO);
b2PolygonShape polygon;
polygon.Set(vertices, count);
b2FixtureDef fixtureDefPoly;
fixtureDefPoly.shape = &polygon;
fixtureDefPoly.density = 1.0f;
fixtureDefPoly.friction = 0.3f;
polyBody->CreateFixture(&fixtureDefPoly);
}
My question is how can I actively change the vertices of this polygon and it change the shape on my screen, without drawing a new shape. My over all goal is to create a free flowing blob.
Thankyou

Answer 1

Modify your last statement to return a pointer to the resulting b2Fixture object. This can be kept as a class variable (ie. b2Fixture* fixture in your class interface).
fixture = polyBody->CreateFixture(&fixtureDefPoly);
Then, wherever you want to change the vertices of the polygon shape, grab a pointer to the shape object associated with your fixture:
b2PolygonShape* shape = (b2PolygonShape*) fixture->GetShape();
And modify the vertices as appropriate:
shape->m_vertices[0].Set(new_x0/PTM_RATIO,new_y0/PTM_RATIO);
shape->m_vertices[1].Set(new_x1/PTM_RATIO,new_y1/PTM_RATIO);
shape->m_vertices[2].Set(new_x2/PTM_RATIO,new_y2/PTM_RATIO);
...
shape->m_vertices[7].Set(new_x7/PTM_RATIO,new_y7/PTM_RATIO);
Good luck!