SpawnRandomObjectsAuthoring.cs

using System;
using System.Collections.Generic;
using Unity.Collections;
using Unity.Entities;
using Unity.Mathematics;
using Unity.Transforms;
using UnityEngine;

class SpawnRandomObjectsAuthoring : SpawnRandomObjectsAuthoringBase<SpawnSettings>
{
}

abstract class SpawnRandomObjectsAuthoringBase<T> : MonoBehaviour, IConvertGameObjectToEntity, IDeclareReferencedPrefabs
    where T : struct, IComponentData, ISpawnSettings
{
    #pragma warning disable 649
    public GameObject prefab;
    public float3 range = new float3(10f);
    public int count;
    #pragma warning restore 649

    public void Convert(Entity entity, EntityManager dstManager, GameObjectConversionSystem conversionSystem)
    {
        var spawnSettings = new T
        {
            Prefab = conversionSystem.GetPrimaryEntity(prefab),
            Position = transform.position,
            Range = range,
            Count = count
        };
        Configure(ref spawnSettings);
        dstManager.AddComponentData(entity, spawnSettings);
    }

    internal virtual void Configure(ref T spawnSettings) { }

    public void DeclareReferencedPrefabs(List<GameObject> referencedPrefabs) => referencedPrefabs.Add(prefab);
}

interface ISpawnSettings
{
    Entity Prefab { get; set; }
    float3 Position { get; set; }
    float3 Range { get; set; }
    int Count { get; set; }
}

struct SpawnSettings : IComponentData, ISpawnSettings
{
    public Entity Prefab { get; set; }
    public float3 Position { get; set; }
    public float3 Range { get; set; }
    public int Count { get; set; }
}

class SpawnRandomObjectsSystem : SpawnRandomObjectsSystemBase<SpawnSettings>
{
}

abstract class SpawnRandomObjectsSystemBase<T> : ComponentSystem where T : struct, IComponentData, ISpawnSettings
{
    internal virtual void OnBeforeInstantiatePrefab(T spawnSettings) { }

    internal virtual void ConfigureInstance(Entity instance, T spawnSettings) { }

    protected override void OnUpdate()
    {
        Entities.ForEach((Entity entity, ref T spawnSettings) =>
        {
            var count = spawnSettings.Count;

            OnBeforeInstantiatePrefab(spawnSettings);

            var instances = new NativeArray<Entity>(count, Allocator.Temp);
            EntityManager.Instantiate(spawnSettings.Prefab, instances);

            var positions = new NativeArray<float3>(count, Allocator.Temp);
            var rotations = new NativeArray<quaternion>(count, Allocator.Temp);
            RandomPointsOnCircle(spawnSettings.Position, spawnSettings.Range, ref positions, ref rotations);

            for (int i = 0; i < count; i++)
            {
                var instance = instances[i];
                EntityManager.SetComponentData(instance, new Translation { Value = positions[i] });
                EntityManager.SetComponentData(instance, new Rotation { Value = rotations[i] });
                ConfigureInstance(instance, spawnSettings);
            }

            EntityManager.RemoveComponent<T>(entity);
        });
    }

    static void RandomPointsOnCircle(float3 center, float3 range, ref NativeArray<float3> positions, ref NativeArray<quaternion> rotations)
    {
        var count = positions.Length;
        // initialize the seed of the random number generator
        var random = new Unity.Mathematics.Random();
        random.InitState(10);
        for (int i = 0; i < count; i++)
        {
            positions[i] = center + random.NextFloat3(-range, range);
            rotations[i] = random.NextQuaternionRotation();
        }
    }
}