Simple entity selection system

readme.md

using UnityEngine;
using Unity.Mathematics;
using Unity.Entities;
using Unity.Transforms;
using Unity.Rendering;
public class LookAtDoubleClickedEntity : MonoBehaviour
{
    void OnEnable ()
    {
        SelectionSystem.onDoubleClick += OnDoubleClick;
    }
    void OnDisable ()
    {
        SelectionSystem.onDoubleClick -= OnDoubleClick;
    }
    void OnDoubleClick ( Entity entity , EntityManager entityManager )
    {
        var clickedTransform = entityManager.GetComponentData<LocalToWorld>( entity );
        Vector3 clickedPos = clickedTransform.Position;
        
        AABB clickedBounds = entityManager.GetComponentData<WorldRenderBounds>( entity ).Value;
        Vector3 clickedCenter = clickedBounds.Center;

        Debug.DrawLine( transform.position , clickedBounds.Center , Color.magenta , 5f );
        transform.rotation = Quaternion.LookRotation( clickedCenter-transform.position , Vector3.up );
    }
}

SelectionSystem.cs

// src: https://gist.github.com/andrew-raphael-lukasik/917046557baed6646c9fdc3b40671307
using UnityEngine;
using Unity.Entities;
using Unity.Mathematics;
using Unity.Collections;
using Unity.Rendering;

[UpdateInGroup( typeof(PresentationSystemGroup) )]
public class SelectionSystem : SystemBase
{
    public static event System.Action<Entity,EntityManager> onClick = (clicked,clickedEntityManager)=>
    {
        #if DEBUG
        Debug.Log($"{clicked} <b>clicked</b>");
        #endif
    };
    public static event System.Action<Entity,EntityManager> onDoubleClick = (clicked,clickedEntityManager)=>
    {
        #if DEBUG
        Debug.Log($"{clicked} <b>double-clicked</b>");
        #endif
    };
    const double k_double_click_threshold = 0.5f;
    
    public NativeQueue<(Entity entity,float distance)> _hits = new NativeQueue<(Entity,float)>( Allocator.Persistent );
    public NativeArray<(Entity entity,double time)> _clicks = new NativeArray<(Entity,double)>( length:2 , Allocator.Persistent );

    protected override void OnUpdate ()
    {
        if( EntityManager.Exists(_clicks[0].entity) )
        {
            var click = _clicks[0];
            var oldClick = _clicks[1];
            if( click.entity==oldClick.entity && (click.time-oldClick.time)<k_double_click_threshold )
                onDoubleClick( click.entity , EntityManager );
            else
                onClick( click.entity , EntityManager );
            _clicks[1] = click;
            _clicks[0] = ( Entity.Null , 0f );
        }

        if( Input.GetMouseButtonDown(0) )
        {
            var ray = Camera.main.ScreenPointToRay( Input.mousePosition );
            var hits = _hits;
            var hits_writer = _hits.AsParallelWriter();
            var clicks = _clicks.Slice();// alias
            double time = Time.ElapsedTime;
            
            Entities
                .WithNone<Disabled,Prefab>()
                // .WithAll<SELECTABLE>()// uncomment to enable this filter
                .ForEach( ( in Entity entity , in WorldRenderBounds wrb ) =>
                {
                    if( wrb.Value.ToBounds().IntersectRay(ray,out float distance) )
                        hits_writer.Enqueue( (entity,distance) );
                })
                .WithBurst().Schedule();
            
            Job
                .WithName("select_best_click_candidate_job")
                .WithCode( ()=>
                {
                    Entity nearest = Entity.Null;
                    float dist = float.MaxValue;
                    while( hits.Count!=0 )
                    {
                        var hit = hits.Dequeue();
                        if( hit.distance<dist )
                        {
                            nearest = hit.entity;
                            dist = hit.distance;
                        }
                    }
                    clicks[0] = ( entity:nearest , time:time );
                })
                .WithBurst().Schedule();
        }
    }

    protected override void OnDestroy ()
    {
        _hits.Dispose();
        _clicks.Dispose();
    }

}

// <summary> Tag component to flag selectable entities. </summary>
public struct SELECTABLE : IComponentData {}