02 — Spawn & Destroy

Spheres spawn at random positions, live for a set duration, then are removed. GameObject lifetime is managed by a sample-side helper, not by the lifetime system itself.

Source: com.trecs.core/Samples~/Tutorials/02_SpawnAndDestroy/

What it does

Coloured spheres appear at random positions around the origin. Each has a countdown timer — when it expires, the entity is removed. The companion GameObject is destroyed automatically via the shared RenderableGameObjectManager (in Common/) that watches for entities with PrefabId + GameObjectId.

Schema

[Unwrap]
public partial struct Lifetime : IEntityComponent
{
    public float Value;
}

public struct Sphere : ITag { }

public partial class SphereEntity
    : ITemplate,
        IExtends<CommonTemplates.RenderableGameObject>,
        ITagged<SampleTags.Sphere>
{
    Position Position = default;
    Lifetime Lifetime;
    ColorComponent Color = new(UnityEngine.Color.white);
    PrefabId PrefabId = new(SpawnAndDestroyPrefabs.Sphere);
}

RenderableGameObject is an abstract base template from Common/ that adds the PrefabId and GameObjectId component fields. Concrete templates extend it and set PrefabId to a constant identifying the factory they registered with the manager.

Systems

SpawnSystem

Spawns a new sphere every 0.5 seconds with a random position and colour:

World.AddEntity<SampleTags.Sphere>()
    .Set(new Position(position))
    .Set(new Lifetime(_lifetime))
    .Set(new ColorComponent(color));

Uses World.Rng for deterministic random placement. Notice the spawner never touches a GameObject — that's the manager's job once the entity submits.

LifetimeSystem

Counts down the lifetime and removes expired entities:

public partial class LifetimeSystem : ISystem
{
    [ForEachEntity(typeof(SampleTags.Sphere))]
    void Execute(ref Lifetime lifetime, EntityHandle entity)
    {
        lifetime.Value -= World.DeltaTime;

        if (lifetime.Value <= 0)
            entity.Remove(World);
    }
}

GameObject destruction is decoupled — the manager's OnRemoved observer (registered in Common/RenderableGameObjectManager.cs) tears the GO down at submission time.

SpherePresenter (variable update)

Syncs Position and ColorComponent onto the GameObject each visual frame.

Concepts introduced

• Dynamic entity creation with AddEntity and component initialization
• Entity removal with entity.Remove() (deferred until submission)
• World.Rng for deterministic random numbers — see Time & RNG
• PrefabId + GameObjectId template pattern — sample-side RenderableGameObjectManager handles GameObject lifecycle reactively via Entity Events. For an explicit OnRemoved-handler example doing the same thing in user code, see Predator Prey.
• Individual component parameters — [ForEachEntity] receives components directly. See Queries & Iteration.