[ECS] Set data to a SharedComponent of Type RenderMesh from EntityCommandBuffer

Entities Version 0.14.0-preview.19 - September 10, 2020

One answer to this is instantiating from pre-defined pool of entity prefabs and not touching reference types after that at all. This system below, for example, will destroy & instantiate 1000 mesh entities every frame < 1ms (Burst enabled) while sourcing prefab entities from world entities (differentiated by id/hash calculated as "specific string name id".GetHashcode()).

Note: filter_prefabs_job can be removed once you figure out a way to create a persistent NativeArray<Entity> prefabs, NativeHashMap<int,Entity> prefabs_lookup or even just a single valid Entity prefab for this system to use.

PrefabPoolTestSystem.cs

 using Debug = UnityEngine.Debug;
 using Unity.Collections;
 using Unity.Entities;
 using Unity.Jobs;
 using Unity.Mathematics;
 using Unity.Transforms;
 using Unity.Rendering;
 using Random = Unity.Mathematics.Random;
 
 public class PrefabPoolTestSystem : SystemBase
 {
     int _poolId = "grey capsule".GetHashCode();
 
     protected override void OnCreate ()
     {
         Debug.Log($"{GetType().Name} will be looking for {nameof(EntityMeshPrefab)} hash: '{_poolId}'");
     }
     
     protected override void OnUpdate ()
     {
         var ecb = new EntityCommandBuffer( Allocator.TempJob );
         var ecbpw = ecb.AsParallelWriter();
         
         Entities
             .WithName("delete_existing_instances_job")
             .WithNone<Prefab>()// do not delete prefabs
             .WithAll<EntityMeshPrefab>()
             .ForEach( ( in int entityInQueryIndex , in Entity entity ) =>
             {
                 ecbpw.DestroyEntity( entityInQueryIndex , entity );
             } )
             .WithBurst().ScheduleParallel();
         
         NativeList<Entity> prefabs = new  NativeList<Entity>( 100 , Allocator.TempJob );
         NativeList<Entity>.ParallelWriter prefabs_writer = prefabs.AsParallelWriter();
         int poolId = _poolId;
         Entities
             .WithName("filter_prefabs_job")
             .WithAll<Prefab>()
             .ForEach( ( in int entityInQueryIndex , in Entity entity , in EntityMeshPrefab entityMeshPrefab ) =>
             {
                 if( entityMeshPrefab.id==poolId )
                     prefabs_writer.AddNoResize( entity );
             } )
             .WithBurst().ScheduleParallel();
 
         uint seed = (uint) math.pow(Time.ElapsedTime+1,2).GetHashCode();
         Job
             .WithName("instantiate_job")
             .WithReadOnly( prefabs )
             .WithCode( () =>
             {
                 int numPrefabs = prefabs.Length;
                 if( numPrefabs!=0 )
                 {
                     var rnd = new Random( seed );
                     for( int i=0 ; i<1000 ; i++ )
                     {
                         Entity prefab = prefabs[ rnd.NextInt(0,numPrefabs) ];
                         ecb.Instantiate( prefab );
                         ecb.SetComponent( prefab , new Translation{
                             Value = rnd.NextFloat3( new float3{ x=-10 , y=-10 , z=-10 } , new float3{ x=10 , y=10 , z=10 } )
                         } );
                     }
                 }
             } )
             .WithBurst().Schedule();
 
         Job
             .WithName("disposal_job")
             .WithCode( () =>
             {
                 prefabs.Dispose();
             })
             .WithoutBurst().Run();
 
         Job
             .WithName("playback_commands")
             .WithCode( () =>
             {
                 ecb.Playback( EntityManager );
                 ecb.Dispose();
             })
             .WithoutBurst().Run();
     }
 }
 

EntityMeshPrefab.cs

 using Unity.Entities;
 public struct EntityMeshPrefab : IComponentData
 {
     public int id;
 }

This component allows you to create entity prefabs from scene gameobjects in, so called, hybrid approach (GameObject to Entity workflow):

EntityMeshPrefabComponent.cs

 using UnityEngine;
 using Unity.Entities;
 
 [DisallowMultipleComponent]
 [RequireComponent( typeof(MeshFilter) , typeof(MeshRenderer) )]
 [RequiresEntityConversion]
 public class EntityMeshPrefabComponent : MonoBehaviour, IConvertGameObjectToEntity
 {
     [SerializeField] string _id = "grey capsule";
     void IConvertGameObjectToEntity.Convert ( Entity entity , EntityManager dstManager , GameObjectConversionSystem conversionSystem )
     {
         dstManager.AddComponent<Prefab>( entity );
         
         int hash = _id.GetHashCode();
         dstManager.AddComponentData<EntityMeshPrefab>( entity , new EntityMeshPrefab{
             id = hash
         } );
         Debug.Log($"{name} created an {nameof(EntityMeshPrefab)} with id '{hash}'");
     }
 }