CGRect rect;
CGRectGetMidX(rect) == rect.origin.x + rect.size.width/2.0
CGRect rect;
CGRectGetMidY(rect) == rect.origin.y + rect.size.height/2.0
CGRect rect;
CGRectGetMidX(rect) == rect.origin.x + rect.size.width/2.0
CGRect rect;
CGRectGetMidY(rect) == rect.origin.y + rect.size.height/2.0