At the ECSSample\HelloCube\5. SpawnFromEntity

Unity Engine
,
1

I don’t understand why create Entity pre frame? There is a function OnUpdate() create entity where in the SpawnerSystem_FromEntity. This function was executed pre frame. Why not create one time. I know the code isn’t error.It’s just I don’t know.

    protected override void OnUpdate()
    {
        //Instead of performing structural changes directly, a Job can add a command to an EntityCommandBuffer to perform such changes on the main thread after the Job has finished.
        //Command buffers allow you to perform any, potentially costly, calculations on a worker thread, while queuing up the actual insertions and deletions for later.
        var commandBuffer = m_EntityCommandBufferSystem.CreateCommandBuffer().ToConcurrent();

        // Schedule the Entities.ForEach lambda job that will add Instantiate commands to the EntityCommandBuffer.
        // Since this job only runs on the first frame, we want to ensure Burst compiles it before running to get the best performance (3rd parameter of WithBurst)
        // The actual job will be cached once it is compiled (it will only get Burst compiled once).
        Entities
            .WithName("SpawnerSystem_FromEntity")
            .WithBurst(FloatMode.Default, FloatPrecision.Standard, true)
            .ForEach((Entity entity, int entityInQueryIndex, in Spawner_FromEntity spawnerFromEntity, in LocalToWorld location) =>
        {
            for (var x = 0; x < spawnerFromEntity.CountX; x++)
            {
                for (var y = 0; y < spawnerFromEntity.CountY; y++)
                {
                    var instance = commandBuffer.Instantiate(entityInQueryIndex, spawnerFromEntity.Prefab);

                    // Place the instantiated in a grid with some noise
                    var position = math.transform(location.Value,
                        new float3(x * 1.3F, noise.cnoise(new float2(x, y) * 0.21F) * 2, y * 1.3F));
                    commandBuffer.SetComponent(entityInQueryIndex, instance, new Translation {Value = position});
                }
            }

            commandBuffer.DestroyEntity(entityInQueryIndex, entity);
        }).ScheduleParallel();

        // SpawnJob runs in parallel with no sync point until the barrier system executes.
        // When the barrier system executes we want to complete the SpawnJob and then play back the commands (Creating the entities and placing them).
        // We need to tell the barrier system which job it needs to complete before it can play back the commands.
        m_EntityCommandBufferSystem.AddJobHandleForProducer(Dependency);
2

I can’t see the example properly but from memory there is a spawner entity, the lambda only runs when it find the spawner entity and the entity is destroy. So the code will only run once

3

Thank you. I think I understand a bit. As your mean:
In the line 28:
commandBuffer.DestroyEntity(entityInQueryIndex, entity);

It’s remove entity(spawner entity), so it wouldn’t execute lambda again when OnUpdate is executed at second time or later. As it’s commond “…this job only runs on the first frame,…”