Pool onAdded / onRemoved

A Pool is a List. Lists can tell you when a member arrives or leaves. That is the cheap way to keep a second structure in sync: a spatial hash, a render index, a count on the HUD.

The hook

function Physics:init ()
  local hash = spatialhash()
  self.hash = hash

  function self.boxes:onAdded (entity)
    hash:add(entity)
  end

  function self.boxes:onRemoved (entity)
    hash:remove(entity)
  end
end

onAdded runs when the Filter accepts an Entity. onRemoved runs when the Entity no longer matches (or leaves the World). List:clear does not fire onRemoved — if you need teardown on clear, walk the List yourself first.

These callbacks run during a flush. Do not flush(), and do not emit something that would auto-flush. Mutate your side structure, not the World’s queues. Pool Flushing and Timing is the rule.

Compared to the other callbacks

This Entity matched or left this Filter: pool:onAdded / onRemoved
Any Entity entered or left the World: World:onEntityAdded / World:onEntityRemoved
This Component was torn down: Component:removed — Component:removed Guide

World-level callbacks see everyone. Pool callbacks see the question your System actually asked ({"position", "size"}). Component removed is about the data, not membership.

See also