public OneWayPlatform(Fixture f)
{
f.CollisionCategories = Category.Cat31;
var sensor = f.Clone(f.Body);
sensor.IsSensor = true;
CollisionUtils.RegisterOnSeparatedListener<Entity>(sensor, OnSeparation);
f.UserData = this;
CollisionUtils.RegisterOnCollidedListener<Entity>(f, OnCollision);
}