Live Coding #2: Building Powerful Games with DOTS by Unity

In this live coding session, Alexandre Nicaise, a Unity 3D developer working at Phaethon Games, explains how to use the Data-Oriented Tech Stack (DOTS) in Unity as a way of dealing with performance issues in games. To do this, he will show how to build a ground of 10,000 cubes using DOTS.

What is DOTS?

DOTS is a Data-Oriented Tech Stack of Unity composed of three elements: The entity-component-system (ECS), the jobs system, and the burst compiler.

This improves memory management when compared to object-oriented programming. For example, displaying a ground of 10,000 cubes takes 277 milliseconds normally on Unity, but only 41 milliseconds with DOTS.

Demonstration steps to build a ground of 10,000 cubes

1- Send a game object to a prefab

2- Create an empty game and convert it to an entity

3- Create a component to store the prefab

using System;using Unity.Collections;using Unity.Entities;using Unity.Mathematics;[Serializable]public struct SpawnerDemoData : IComponentData{    // Add fields to your component here. Remember that:    //    // * A component itself is for storing data and doesn't 'do' anything.    //    // * To act on the data, you will need a System.    //    // * Data in a component must be blittable, which means a component can    //   only contain fields which are primitive types or other blittable    //   structs; they cannot contain references to classes.    //    // * You should focus on the data structure that makes the most sense    //   for runtime use here. Authoring Components will be used for     //   authoring the data in the Editor.    public Entity prefab;    public int Ecol;    public int Erow;}

4- Create a proxy to add the component to the entity

using System.Collections.Generic;using Unity.Entities;using Unity.Mathematics;using UnityEngine;[DisallowMultipleComponent][RequiresEntityConversion]public class SpawnerDemoProxy : MonoBehaviour, IConvertGameObjectToEntity, IDeclareReferencedPrefabs{    // Add fields to your component here. Remember that:    //    // * The purpose of this class is to store data for authoring purposes - it is not for use while the game is    //   running.    //     // * Traditional Unity serialization rules apply: fields must be public or marked with [SerializeField], and    //   must be one of the supported types.    public GameObject prefab;    public int column;    public int row;    // Lets you convert the editor data representation to the entity optimal runtime representation    public void Convert(Entity entity, EntityManager dstManager, GameObjectConversionSystem conversionSystem)    {        SpawnerDemoData data = new SpawnerDemoData        {            // The referenced prefab will be converted due to DeclareReferencedPrefabs.            // So here we simply map the game object to an entity reference to that prefab.            prefab = conversionSystem.GetPrimaryEntity(prefab),            Ecol = column,            Erow = row        };        dstManager.AddComponentData(entity, data);    }    // Referenced prefabs have to be declared so that the conversion system knows about them ahead of time    public void DeclareReferencedPrefabs(List<GameObject> referencedPrefabs)    {        referencedPrefabs.Add(prefab);    }}

5- Create a system to spawn the entity

using Unity.Burst;using Unity.Collections;using Unity.Entities;using Unity.Jobs;using Unity.Mathematics;using Unity.Transforms;using static Unity.Mathematics.math;// JobComponentSystems can run on worker threads.// However, creating and removing Entities can only be done on the main thread to prevent race conditions.// The system uses an EntityCommandBuffer to defer tasks that can't be done inside the Job.public class SpawnerDemoSystem : JobComponentSystem{    // EndSimulationEntityCommandBufferSystem is used to create a command buffer which will then be played back    // when that barrier system executes.    // Though the instantiation command is recorded in the SpawnJob, it's not actually processed (or "played back")    // until the corresponding EntityCommandBufferSystem is updated.     EndSimulationEntityCommandBufferSystem _endSimulationEntityCommandBufferSystem;    protected override void OnCreateManager()    {        // Cache the EndSimulationEntityCommandBufferSystem in a field, so we don't have to create it every frame        _endSimulationEntityCommandBufferSystem = World.GetOrCreateSystem<EndSimulationEntityCommandBufferSystem>();    }    // This declares a new kind of job, which is a unit of work to do.    // The job is declared as an IJobForEachWithEntity<LocalToWorld, SpawnerDemoData>,    // meaning it will process all entities in the world that have both    // LocalToWorld and SpawnerDemoData components. Change it to process the component    // types you want.    struct SpawnerDemoSystemJob : IJobForEachWithEntity<LocalToWorld, SpawnerDemoData>    {        // Add fields here that your job needs to do its work.        //A thread-safe command buffer that can buffer commands that affect entities and components for later playback.        public EntityCommandBuffer commandBuffer;         public void Execute(Entity entity, int index, ref LocalToWorld location, ref SpawnerDemoData data)        {            for (int x = 0; x < data.Ecol; x++)            {                for (int z = 0; z < data.Erow; z++)                {                    //Create your entity from the prefab                    Entity instance = commandBuffer.Instantiate(data.prefab);                    //define the position                    float3 pos = math.transform(location.Value, new float3(x, noise.cnoise(new float2(x,z) *0.21f), z));                    //set the position in the world                    commandBuffer.SetComponent(instance, new Translation() { Value = pos});                }            }            //Destroy the spawner entity            commandBuffer.DestroyEntity(entity);        }    }        protected override JobHandle OnUpdate(JobHandle inputDependencies)    {        //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.        // Schedule the job that will add Instantiate commands to the EntityCommandBuffer.        var job = new SpawnerDemoSystemJob()        {            commandBuffer = _endSimulationEntityCommandBufferSystem.CreateCommandBuffer() // instantiate the Buffer        }.ScheduleSingle(this, inputDependencies) ;        // 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.        _endSimulationEntityCommandBufferSystem.AddJobHandleForProducer(job);        return job;    }}

The full project is available on GitHub.

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Illustrations by WTTJ