Svelto.ECS C# Entity Component System framework

Real ECS framework for c#. Enables to write encapsulated, decoupled, maintainable, highly efficient, data oriented, cache friendly, code without pain. Although the framework is platform agnostic (compatible with c# 7 and above and .net standard 2.0 and above), it comes with several Unity extensions.

Svelto.ECS in pills

Svelto.ECS is easy to start with, but full of tricks for expert users. The hardest problem to overcome is usually to shift mentality from OOP programming to ECS programming rather than using the framework itself. If you would like to read an ECS faq, you can check this article: https://github.com/SanderMertens/ecs-faq

Svelto.ECS at glance

simplest setup:

using System;
using Svelto.ECS;
using Svelto.ECS.Schedulers;
using Svelto.ECS.Vanilla.Example.SimpleEntityEngine;

/// <summary>
///     This is the common pattern to declare Svelto Exclusive Groups (usually split by composition root)
/// </summary>
public static class ExclusiveGroups
{
    public static ExclusiveGroup group0 = new ExclusiveGroup();
    public static ExclusiveGroup group1 = new ExclusiveGroup();
}

namespace Svelto.ECS.Vanilla.Example
{
    /// <summary>
    ///     The Context is the application starting point.
    ///     As a Composition root, it gives to the coder the responsibility to create, initialize and
    ///     inject dependencies.
    ///     Every application can have more than one context and every context can have one
    ///     or more composition roots (a facade, but even a factory, can be a composition root)
    /// </summary>
    public class SimpleContext
    {
        public SimpleContext()
        {
            //an entity submission scheduler is needed to submit entities to the Svelto database, Svelto is not 
            //responsible to decide when to submit entities, it's the user's responsibility to do so.
            var entitySubmissionScheduler = new SimpleEntitiesSubmissionScheduler();
            //An EnginesRoot holds all the engines and entities created. it needs a EntitySubmissionScheduler to know when to
            //add previously built entities to the Svelto database. Using the SimpleEntitiesSubmissionScheduler
            //is expected as it gives complete control to the user about when the submission happens
            _enginesRoot = new EnginesRoot(entitySubmissionScheduler);

            //an entity factory allows to build entities inside engines
            var entityFactory = _enginesRoot.GenerateEntityFactory();
            //the entity functions allows other operations on entities, like remove and swap
            var entityFunctions = _enginesRoot.GenerateEntityFunctions();

            //Add the Engine to manage the SimpleEntities
            var behaviourForEntityClassEngine = new BehaviourForEntityClassEngine(entityFunctions);
            _enginesRoot.AddEngine(behaviourForEntityClassEngine);

            //build Entity with ID 0 in group0
            entityFactory.BuildEntity<SimpleEntityDescriptor>(new EGID(0, ExclusiveGroups.group0));

            //submit the previously built entities to the Svelto database
            entitySubmissionScheduler.SubmitEntities();

            //as Svelto doesn't provide an engine/system ticking system, it's the user's responsibility to
            //update engines  
            behaviourForEntityClassEngine.Update();

            Console.Log("Done - click any button to quit");

            System.Console.ReadKey();

            Environment.Exit(0);
        }

        readonly EnginesRoot _enginesRoot;
    }

    //An EntityComponent must always implement the IEntityComponent interface
    //don't worry, boxing/unboxing will never happen.
    public struct EntityComponent : IEntityComponent
    {
        public int counter;
    }

    /// <summary>
    ///     The EntityDescriptor identifies your Entity. It's essential to identify
    ///     your entities with a name that comes from the Game Design domain.
    /// </summary>
    class SimpleEntityDescriptor : GenericEntityDescriptor<EntityComponent> { }

    namespace SimpleEntityEngine
    {
        public class BehaviourForEntityClassEngine :
                //this interface makes the engine reactive, it's absolutely optional, you need to read my articles
                //and wiki about it.
                IReactOnAddEx<EntityComponent>, IReactOnSwapEx<EntityComponent>, IReactOnRemoveEx<EntityComponent>,
                //while this interface is optional too, it's almost always used as it gives access to the entitiesDB
                IQueryingEntitiesEngine
        {
            //extra entity functions
            readonly IEntityFunctions _entityFunctions;

            public BehaviourForEntityClassEngine(IEntityFunctions entityFunctions)
            {
                _entityFunctions = entityFunctions;
            }

            public EntitiesDB entitiesDB { get; set; }

            public void Ready() { }

            public void Update()
            {
                //Simple query to get all the entities with EntityComponent in group1
                var (components, entityIDs, count) = entitiesDB.QueryEntities<EntityComponent>(ExclusiveGroups.group1);

                uint entityID;
                for (var i = 0; i < count; i++)
                {
                    components[i].counter++;
                    entityID = entityIDs[i];
                }

                Console.Log("Entity Struct engine executed");
            }

            //the following methods are called by Svelto.ECS when an entity is added to a group
            public void Add((uint start, uint end) rangeOfEntities, in EntityCollection<EntityComponent> entities
              , ExclusiveGroupStruct groupID)
            {
                var (_, entityIDs, _) = entities;

                for (uint index = rangeOfEntities.start; index < rangeOfEntities.end; index++)
                    //Swap entities between groups is a very common operation and it's necessary to
                    //move entities between groups/sets. A Group represent a state/adjective of an entity, so changing
                    //group means change state/behaviour as different engines will process different groups.
                    //it's the Svelto equivalent of adding/remove components to an entity at run time
                    _entityFunctions.SwapEntityGroup<SimpleEntityDescriptor>(new EGID(entityIDs[index], groupID), ExclusiveGroups.group1);
            }

            //the following methods are called by Svelto.ECS when an entity is swapped from a group to another
            public void MovedTo((uint start, uint end) rangeOfEntities, in EntityCollection<EntityComponent> entities
              , ExclusiveGroupStruct fromGroup, ExclusiveGroupStruct toGroup)
            {
                var (_, entityIDs, _) = entities;
                
                for (var index = rangeOfEntities.start; index < rangeOfEntities.end; index++)
                {
                    Console.Log($"entity {entityIDs[index]} moved from {fromGroup} to {toGroup}");
                    //like for the swap operation, removing entities from the Svelto database is a very common operation.
                    //For both operations is necessary to specify the EntityDescriptor to use. This has also a philosophical
                    //reason to happen, it's to always remind which entity type we are operating with. 
                    _entityFunctions.RemoveEntity<SimpleEntityDescriptor>(new EGID(entityIDs[index], toGroup));
                }
            }

            //the following methods are called by Svelto.ECS when an entity is removed from a group
            public void Remove((uint start, uint end) rangeOfEntities, in EntityCollection<EntityComponent> entities, ExclusiveGroupStruct groupID)
            {
                var (_, entityIDs, _) = entities;

                for (uint index = rangeOfEntities.start; index < rangeOfEntities.end; index++)
                    Console.Log($"entity {entityIDs[index]} removed from {groupID.ToString()}");
            }
        }
    }
}

learn more about svelto on the Wiki page: https://github.com/sebas77/Svelto.ECS/wiki

Svelto.ECS philosophy

Svelto.ECS wasn’t born just from the needs of a large team, but also as a result of years of reasoning behind software engineering applied to game development. Svelto.ECS hasn’t been created just to write faster code, it has been designed to help developing better code. Performance gains is just one of the benefits in using Svelto.ECS, as ECS is a great way to write cache-friendly code. Svelto.ECS has been developed with the idea of ECS being a paradigm and not just a pattern, helping the user to shift away from Object Oriented Programming with consequent improvements on the code design and code maintainability. Svelto.ECS is the result of years of iteration of the ECS paradigm applied to real game development with the intent to be as foolproof as possible. Svelto.ECS has been designed to be used by a medium-size/large team working on long term projects where the cost of maintainability is relevant.

Is it Archetype? Is it Sparse set? No it’s something else :O

Svelto.ECS is loosely based on the Archetype idea. The main difference compared to any other Archetype-based model is that Svelto Archetypes are static, meaning that users cannot add or remove components at runtime. There are many design reasons behind this decision, including the fact that users are often not aware of the costs of structural changes.

While other frameworks typically limit user freedom to avoid exposing flaws in the archetype-based concept, Svelto.ECS introduces the concept of groups. This is simply an explicit way for the user to define sets of entities and iterate through them as quickly as possible.

GroupCompounds build on this idea by allowing users to change the “state”/”set”/”group” according to tags that serve effectively as adjective or state identifiers.

Entities can change state moving between sets swapping them in groups explcitly, rather than changing archetype.

Why using Svelto.ECS with Unity?

Svelto.ECS doens’t use a traditional archetype model like DOTS ECS does. The novel hybrid approach based on groups instead than archetypes has been designed to allow the user to take the right decision when it comes to states management. Svelto.ECS doesn’t allow archetypes to change dynamically, the user cannot add or remove components after the entity is created. Handling entities states with components can quickly lead to very intensive structural changes operations, so groups have been introduced to avoid the wild explosion of states permutations and let the user see explicitly the cost of their decisions.

Filters have been added to make the handling of multiple states even more flexible by adding a new dimension to subset creation. Using filters it’s possible to subset groups while avoiding structural changes (that happens when changing groups in Svelto.ECS too). DOTS engineers also realised this and consequentially introduced the new Enableable components which are less flexible than the Svelto.ECS filters as they implicitly tie the subsets to enabled/disabled components, while Filters in Svelto.ECS don’t.

Thanks to the explicit use of Groups and Filters, the Svelto user is able to find the right trade off to handle entities states.

Svelto.ECS is lean. It hasn’t been designed to move a whole engine such as Unity from OOP to ECS, therefore it doesn’t suffer from unjustifiable complexity overhead to try to solve problems that often are not linked to gameplay development. Svelto.ECS is fundamentally feature complete at this point of writing and new features in new versions are more nice to have than fundamental.

Unity Compatibility

Svelto.ECS is partially compatible with Unity 2019.3.x cycle as long as it’s not used with any DOTS package (including collections). It is compatible with all the versions of Unity from 2020 and above.

Svelto.ECS is fully compatible with DOTS Burst and Jobs.

Svelto.ECS is designed to take full advantange of the DOTS modules and to use specifically DOTS ECS as an engine library, through the (optional) SveltoOnDOTS wrapper. Svelto.ECS native components and interfaces are fully compatible with Burst.

Why using Svelto.ECS without Unity?

There are so many c# game engines out there (Stride, Flax, Monogame, FlatRedBall, Evergine, Godot, UniEngine just to mention some) and Svelto.ECS is compatible with all of them!

Performance considerations

Aside from resizing the database absolutely when necessary, all the Svelto operations are memory allocation free. Some containers may need to be preallocated (and then disposed) but those are already advanced scenarios. When using pure ECS (no EntityViewComponents) components are stored in native collections across all the platforms, which means gaining some performance from losing the managed memory checks. With pure ECS, iterating components is automatically cache-friendly.

Note: Svelto.ECS has a ton of allocating run-time checks in debug, so if you want to profile you need to profile a release version or use PROFILE_SVELTO define

If you decide to use Svelto.ECS

Svelto.ECS is an Open Source Project provided as it is, no support is guaranteed other than the help given on the Svelto Discord channel. Issues will be fixed when possible. If you decide to adopt Svelto.ECS, it’s assumed you are willing to partecipate to the development of the product if necessary.

Official Examples (A.K.A. where is the documentation?)

Documentation is costly to mantain so check the highly documented and simple mini-examples. Please study them all regardless the platform you intend to use Svelto with.

First of all please check the wiki page:

https://github.com/sebas77/Svelto.ECS/wiki

After that, you can get all the help you need from the official chat:

Official Discord Server (join to get help from me for free!)

https://discord.gg/3qAdjDb

but don’t forget to check the FAQ AKA Discussions first (all the FAQ like questions will be redirected to Discussions)

https://github.com/sebas77/Svelto.ECS/discussions

Official Articles

Theory related articles (from the most recent to the oldest, read from the oldest if you are new to it):

Practical articles

Note: I included the IoC articles just to show how I shifted over the years from using an IoC container to use an ECS framework and the rationale behind its adoption.

Users Generated Content (I removed all the outdated articles, so this is a call for new ones!)

A Beginner’s Guide to Svelto.ECS (3.0) with Unity by Jiheh Ritterling

Svelto Community Extensions

Svelto.ECS inspector
Svelto.ECS.Schema - Schema and State Machine extensions for Svelto.ECS
Automatic way to control svelto engines order without having to pass in a string using attributes
[***Svelto Doofuses example on ‘Roids] (https://github.com/svermeulen/svelto-doofus-sample)

How to clone the repository:

The folders Svelto.ECS and Svelto.Common, where present, are submodules pointing to the relative repositories. If you find them empty, you need to update them through the submodule command. Check some instructions here: https://github.com/sebas77/Svelto.ECS.Vanilla.Example/wiki

Svelto distributed as Unity Package through OpenUPM

or just install the package that comes from the link https://package-installer.glitch.me/v1/installer/OpenUPM/com.sebaslab.svelto.ecs?registry=https%3A%2F%2Fpackage.openupm.com

this is shown in this example too: https://github.com/sebas77/Svelto.MiniExamples/tree/master/UPM-Integration/UPM

Svelto distributed as Nuget

I am not a Nuget expert, but thanks to our contributors, Svelto.ECS can be found at https://www.nuget.org/packages/Svelto.ECS/

the Hello World example uses the nuget package directly: https://github.com/sebas77/Svelto.MiniExamples/tree/master/Example5-Net-HelloWorld

In case of bugs

Best option is to fork and clone https://github.com/sebas77/Svelto.ECS.Tests, add new tests to reproduce the problem and request a pull. I will then fix the issue. Also feel free to contact me on Discord.

Unity Installation Note:

if you are installing Svelto.ECS manually and not through OUPM, you need to copy the projecs folders under the Package folder like:

and add this into your manifest:

,
  "scopedRegistries": [
    {
      "name": "OpenUPM",
      "url": "https://package.openupm.com",
      "scopes": [
        "org.nuget.system.runtime.compilerservices.unsafe"
      ]
    }
  ]

looking like:

{
  "dependencies": {
...
  },
  "scopedRegistries": [
    {
      "name": "OpenUPM",
      "url": "https://package.openupm.com",
      "scopes": [
        "org.nuget.system.runtime.compilerservices.unsafe"
      ]
    }
  ]
}

I like the project, how can I help?

Hey, thanks a lot for considering this. You can help in several ways. The simplest is to talk about Svelto.ECS and spread the word, the more we are, the better it is for the community. Then you can help with the documentation, updating the wiki or writing your own articles. Svelto.ECS has all the features needed to make a game with the ECS pattern, but some areas are lacking: A visual debugger and more unit tests are needed. Other platforms other than Unity could get some love too: Stride Game, Godot, monogame, FNA or whatever supports c#. Porting to other languages, especially c++, would be awesome but probably pointless. Please check the lane dedicated to the community tasks list here: https://github.com/users/sebas77/projects/3 and let me know if you want to take something on!

Svelto Framework is used to develop the following products(*):

Toy Trains Beyond These Stars Robocraft Techblox Gamecraft Robocraft Infinity Cardlife HoleIo

*If you want your products made with Svelto here, just send me an email or whatever, I’ll be super happy to add them.

Note: Dear Svelto Users : Although I am committed to help you and write articles as much as I can, I will never be able to keep all the documentation up to date. If you are a happy svelto user and you want to contribute, please feel free to update the github wiki! 🙏👊