Some systems must be notified of entities that match or no longer match a set of query terms. This is necessary when implementing side-effects like spawning new entities when other entities are created or destroyed, updating a third-party library, or displaying information in a UI.

Systems can react to changes in entity composition using monitors. Below a simple system that utilizes monitors to log all created and deleted entities:

let logEntitySystem = (world: j.World) => {
    .eachIncluded(entity => {
      console.log("created", entity)
    .eachExcluded(entity => {
      console.log("deleted", entity)

The entities a monitor recieves can be narrowed using query terms.

world.monitor(Client).eachIncluded((client) => {
  let clientSocket = world.get(client, Socket)

In plain terms, monitors execute the callback provided to eachIncluded for each entity that began to match the provided query terms at the end of the last step. It executes the eachExcluded callback for entities that no longer match the terms. They can be used to:

  • mutate the world in response to entity changes
  • update list or aggregate views in a UI
  • broadcast events to a third-party library, service, or client

Data Transience

Because entity operations are deferred to the end of a step, component values are no longer available to a system by the time a monitor executes it’s eachExcluded callback.

If you wish to access component values of an entity that no longer matches a monitor’s terms, you can use a world’s monitorImmediate method. monitorImmediate returns a monitor that is configured to run within the current step.

world.monitorImmediate(Enemy).eachExcluded((enemy) => {
  let enemyPos = world.get(enemy, Position)
  world.create(LootBag, enemyPos)

An immediate monitor must be executed downstream of its causal systems in the app’s system execution pipeline. In the above example, the system should be configured using ordering constraints to occur after the system that deletes Enemy entities.

    world => world.monitorImmediate(Enemy).eachExcluded(/* ... */),