RootEntity purpose in megacity-metro

koirat · April 28, 2024, 12:37pm

I was investigating megacity-metro DOTS project.
And I have stumbled upon RootEntity component.

public struct RootEntity : IComponentData
{
    public Entity Entity;
}

I’m not sure what is exact purpose of this, I’m guessing it might be topmost element in some object hierarchy.
Let’s say we got ship made of some smaller parts each smaller part entity has RootEntity pointing to topmost Entity. Am I guessing correctly ?

Spy-Master · April 28, 2024, 12:52pm

github.com

Unity-Technologies/megacity-metro/blob/c41a4606fe15fb802a1df58f0760f719001b406a/Assets/Scripts/Authoring/Player/ShipRandomVisualsAuthoring.cs#L10


      
          using UnityEngine;
          using Unity.Entities;
          
          public class ShipRandomVisualsAuthoring : MonoBehaviour
          {
              public GameObject LEGEntity;
              public GameObject[] ShipVisuals;
          
              [BakingVersion("megacity-metro", 1)]
              public class Baker : Baker<ShipRandomVisualsAuthoring>
              {
                  public override void Bake(ShipRandomVisualsAuthoring authoring)
                  {
                      var entity = GetEntity(authoring.gameObject, TransformUsageFlags.Dynamic);
          
                      AddComponent(entity, new RootEntity
                      {
                          Entity = GetEntity(authoring.LEGEntity, TransformUsageFlags.Dynamic),
                      });

github.com

Unity-Technologies/megacity-metro/blob/c41a4606fe15fb802a1df58f0760f719001b406a/Assets/Scripts/Gameplay/Mix/Player/RandomShipVisualsSystem.cs#L20


      
          using UnityEngine;
          using Random = Unity.Mathematics.Random;
          
          public partial struct RandomShipVisualsSystem : ISystem
          {
              public void OnUpdate(ref SystemState state)
              {
                  uint nowMiliseconds = (uint)DateTime.Now.Millisecond;
                  ComponentLookup<PlayerName> playerNameLookupRO = SystemAPI.GetComponentLookup<PlayerName>(true);
                  EntityCommandBuffer ecb = SystemAPI.GetSingletonRW<BeginSimulationEntityCommandBufferSystem.Singleton>().ValueRW.CreateCommandBuffer(state.WorldUnmanaged);
                  foreach (var (legEntity, prefabs, entity) in SystemAPI.Query<RootEntity, DynamicBuffer<ShipRandomVisuals>>().WithEntityAccess())
                  {
                      Random random;
                      if (playerNameLookupRO.TryGetComponent(legEntity.Entity, out PlayerName playerName))
                      {
                          if (playerName.Name == "") continue;
                          random = new Random((uint)playerName.Name.GetHashCode());
                      }
                      else
                      {
                          random = Random.CreateFromIndex(nowMiliseconds);

It’s just used to track which entity to attach generated vehicle pieces / effects onto because the visuals authoring is on a separate GameObject / entity from the main one, nothing deeper than that judging by there being no other references to the component.

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RootEntity purpose in megacity-metro

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