ecb-manual-immediate

ECB Manual Immediate Playback

Intent

Create an ECB inline within a single OnUpdate, play it back immediately, and dispose it — for cases where the structural change must be visible within the same system update without waiting for an ECBSystem boundary.

Use When

One-shot initialisation systems. Cleanup after spawn (destroy the trigger entity immediately). Any scenario where a one-frame delay from an ECBSystem is semantically incorrect.

Avoid When

The structural change happens inside a ScheduleParallel job — manual ECBs are not thread-safe without AsParallelWriter. Avoid for recurring per-frame operations — manual ECB allocation has more overhead than reusing an ECBSystem's pre-allocated buffer.

Senior Pattern

• var ecb = new EntityCommandBuffer(Allocator.Temp) — cheapest allocator for within-frame use.
• Record commands during main-thread iteration (SystemAPI.Query foreach is safe for recording — structural changes happen at Playback, not during recording).
• ecb.Playback(state.EntityManager) — executes all recorded structural changes immediately.
• Allocator.Temp is automatically reclaimed at end of frame — no explicit Dispose needed, but calling it is harmless.
• Pair with state.Enabled = false for one-shot init systems that must never run again.

Code Template

[BurstCompile]
public partial struct SpawnAndCleanupSystem : ISystem
{
    public void OnCreate(ref SystemState state)
        => state.RequireForUpdate<SpawnTrigger>();

    public void OnUpdate(ref SystemState state)
    {
        var ecb = new EntityCommandBuffer(Allocator.Temp);

        foreach (var (trigger, entity) in
            SystemAPI.Query<RefRO<SpawnTrigger>>().WithEntityAccess())
        {
            for (int i = 0; i < trigger.ValueRO.Count; i++)
            {
                var spawned = ecb.Instantiate(trigger.ValueRO.Prefab);
                ecb.SetComponent(spawned, LocalTransform.FromPosition(
                    trigger.ValueRO.Origin + new float3(i, 0, 0)));
            }
            ecb.DestroyEntity(entity);  // remove the trigger
        }

        ecb.Playback(state.EntityManager);
        // state.Enabled = false;  // uncomment for true one-shot systems
    }
}