Events

A CombatSystem deals damage and needs to tell an AudioSystem, a floating-damage-number UI, and an achievement tracker, without knowing any of them exist. That’s what events are for: emit once, let anyone interested read it.

Emitting

public struct DamageDealt : IEvent
{
    public Entity Target;
    public int Amount;
}

world.Emit(new DamageDealt { Target = goblin, Amount = 5 });

IEvent is a marker interface, same shape as IComponent. Emit is immediate, no CommandBuffer involved, and safe to call concurrently from several systems in the same parallel stage.

Reading

public sealed partial class DamageNumbersSystem(World world) : QuerySystem
{
    private readonly EventReader<DamageDealt> _damageEvents = world.CreateEventReader<DamageDealt>();

    protected override IQuery DefineQuery(Query query) => query.With<Position>();

    public void Update(Time time)
    {
        foreach (var damage in _damageEvents.Read())
        {
            // spawn a floating number at damage.Target's position
        }
    }
}

Create one EventReader<T> per system that cares, store it, call Read() once every tick. Each reader tracks its own position independently, one system reading DamageDealt doesn’t affect what another sees.

Caution

An event stays readable for the Update call it was emitted in, plus the one after that, then it’s gone. A reader that calls Read() less often than every Update can miss whatever fell outside that window.

Tip

Need to know what a component’s value actually became, not just that something happened, or need it to persist across more than two ticks? See Change Tracking.

Next

Events tell you something happened. For inspecting live world state directly, see Debugging.