Reusing Entities

Entity:destroy is not an object pool. After the next flush the Entity is gone. If you spawn a few hundred bullets a second, creating and destroying that often is wasted work: you can keep the Entity and reset its Components.

When reuse is worth it

Short-lived, high-churn things: bullets, particles, damage numbers. A boss you fight once should just destroy.

The idea is a flag that means “this slot is free”:

Concord.Component("inactive")

local function fireBullet (world, x, y, dx, dy)
  local bullet = world:query({ "inactive", "bullet" })[1]
  if bullet then
    bullet:remove("inactive")
  else
    bullet = world:newEntity():give("bullet")
  end

  bullet
    :give("position", x, y)
    :give("velocity", dx, dy)
    :ensure("drawable")
end

function Bullet:update (dt)
  for _, entity in ipairs(self.live) do
    -- ...
    if offscreen(entity) then
      entity:remove("velocity"):give("inactive")
    end
  end
end

live is { "bullet", "position", "!inactive" }. Free slots are { "inactive", "bullet" }. ensure keeps identity Components (bullet, maybe key) from being rebuilt; give resets the values that change every shot.

World:query is fine to find one free slot. A dedicated Pool on the System is better if you recycle every frame.

Do not assume remove("inactive") puts the bullet in live this iteration. Recycle, then wait until the next update to iterate live, or give the Components the current loop already understands. Pool Flushing and Timing is the timing.

Clone a prefab once; reuse the instances you fire. Cloning an Entity creates a new Entity from a snapshot. Reuse keeps the same object and swaps flags.

Reusing Entities

When reuse is worth it

See also