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LeoEcsLite - Lightweight C# Entity Component System framework

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# LeoEcsLite - Lightweight C# Entity Component System framework
Performance, zero/small memory allocations/footprint, no dependencies on any game engine - are the main goals of this project.

> **Important!** Don't forget to use `DEBUG` builds for development and `RELEASE` builds in production: all internal error checks / exception throwing work only in `DEBUG` builds and removed for performance reasons in `RELEASE`.

> **Important!** LeoEcsLite API **is not thread safe** and will never be! If you need multithread-processing - you should implement it on your side as part of ecs-system.

# Table of content
* [Socials](#socials)
* [Installation](#installation)
    * [As unity module](#as-unity-module)
    * [As source](#as-source)
* [Main parts of ecs](#main-parts-of-ecs)
    * [Entity](#entity)
    * [Component](#component)
    * [System](#system)
* [Data sharing](#data-sharing)
* [Special classes](#special-classes)
    * [EcsPool](#ecspool)
    * [EcsFilter](#ecsfilter)
    * [EcsWorld](#ecsworld)
    * [EcsSystems](#ecssystems)
* [Engine integration](#engine-integration)
    * [Unity](#unity)
    * [Custom engine](#custom-engine)
* [Projects powered by LeoECS Lite](#projects-powered-by-leoecs-lite)
    * [With sources](#with-sources)
* [Extensions](#extensions)
* [License](#license)
* [FAQ](#faq)

# Socials
[![discord](https://img.shields.io/discord/404358247621853185.svg?label=enter%20to%20discord%20server&style=for-the-badge&logo=discord)](https://discord.gg/5GZVde6)

# Installation

## As unity module
This repository can be installed as unity module directly from git url. In this way new line should be added to `Packages/manifest.json`:
```
"com.leopotam.ecslite": "https://github.com/Leopotam/ecslite.git",
```
By default last released version will be used. If you need trunk / developing version then `develop` name of branch should be added after hash:
```
"com.leopotam.ecslite": "https://github.com/Leopotam/ecslite.git#develop",
```

## As source
If you can't / don't want to use unity modules, code can be cloned or downloaded as archive from `releases` page.

# Main parts of ecs

## Entity
Сontainer for components. Implemented as `int`:
```csharp
// Creates new entity in world context.
int entity = _world.NewEntity ();

// Any entity can be destroyed. All components will be removed first, then entity will be destroyed. 
world.DelEntity (entity);
```

> **Important!** Entities can't live without components and will be killed automatically after last component is removed.

## Component
Container for user data without / with small logic inside:
```csharp
struct Component1 {
    public int Id;
    public string Name;
}
```
Components can be added / requested / removed through [component pools](#ecspool).

## System
Сontainer for logic for processing filtered entities. User class should implement at least one of `IEcsInitSystem`, `IEcsDestroySystem`, `IEcsRunSystem` (or other supported) interfaces:
```csharp
class UserSystem : IEcsPreInitSystem, IEcsInitSystem, IEcsRunSystem, IEcsDestroySystem, IEcsPostDestroySystem {
    public void PreInit (EcsSystems systems) {
        // Will be called once during EcsSystems.Init() call and before IEcsInitSystem.Init().
    }
    
    public void Init (EcsSystems systems) {
        // Will be called once during EcsSystems.Init() call and after IEcsPreInitSystem.PreInit().
    }
    
    public void Run (EcsSystems systems) {
        // Will be called on each EcsSystems.Run() call.
    }

public void Destroy (EcsSystems systems) {
        // Will be called once during EcsSystems.Destroy() call and before IEcsPostDestroySystem.PostDestroy().
    }
    
    public void PostDestroy (EcsSystems systems) {
        // Will be called once during EcsSystems.Destroy() call and after IEcsDestroySystem.Destroy().
    }
}
```

# Data sharing
Instance of any custom type can be shared between all systems:
```csharp
class SharedData {
    public string PrefabsPath;
}
...
SharedData sharedData = new SharedData { PrefabsPath = "Items/{0}" };
EcsSystems systems = new EcsSystems (world, sharedData);
systems
    .Add (new TestSystem1 ())
    .Init ();
...
class TestSystem1 : IEcsInitSystem {
    public void Init(EcsSystems systems) {
        SharedData shared = systems.GetShared<SharedData> (); 
        string prefabPath = string.Format (shared.PrefabsPath, 123);
        // prefabPath = "Items/123" here.
    } 
}
```

# Special classes

## EcsPool
Container for components, provides api for adding / requesting / removing components on entity:
```csharp
int entity = world.NewEntity ();
EcsPool<Component1> pool = world.GetPool<Component1> ();

// Add() adds component to entity. If component already exists - exception will be raised in DEBUG.
ref Component1 c1 = ref pool.Add (entity);

// Get() returns exist component on entity. If component does not exists - exception will be raised in DEBUG.
ref Component1 c1 = ref pool.Get (entity);

// Del() removes component from entity. If it was last component - entity will be removed automatically too.
pool.Del (entity);
```

> **Important!** After removing component will be pooled and can be reused later. All fields will be reset to default values automatically.

## EcsFilter
Container for keeping filtered entities with specified component list:
```csharp
class WeaponSystem : IEcsInitSystem, IEcsRunSystem {
    public void Init (EcsSystems systems) {
        // We want to get default world instance...
        EcsWorld world = systems.GetWorld ();
        
        // and create test entity...
        int entity = world.NewEntity ();
        
        // with "Weapon" component on it.
        var weapons = world.GetPool<Weapon>();
        weapons.Add (entity);
    }

public void Run (EcsSystems systems) {
        EcsWorld world = systems.GetWorld ();
        // We want to get entities with "Weapon" and without "Health".
        // You can cache this filter somehow if you want.
        var filter = world.Filter<Weapon> ().Exc<Health> ().End ();
        
        // We want to get pool of "Weapon" components.
        // You can cache this pool somehow if you want.
        var weapons = world.GetPool<Weapon>();
        
        foreach (int entity in filter) {
            ref Weapon weapon = ref weapons.Get (entity);
            weapon.Ammo = System.Math.Max (0, weapon.Ammo - 1);
        }
    }
}
```

Additional constraints can be added with `Inc<>()` / `Exc<>()` methods.

> Important: Filters support any amount of components, include and exclude lists should be unique. Filters can't both include and exclude the same component.

## EcsWorld
Root level container for all entities / components, works like isolated environment.

> Important: Do not forget to call `EcsWorld.Destroy()` method if instance will not be used anymore.

## EcsSystems
Group of systems to process `EcsWorld` instance:
```csharp
class Startup : MonoBehaviour {
    EcsWorld _world;
    EcsSystems _systems;

void Start () {
        // create ecs environment.
        _world = new EcsWorld ();
        _systems = new EcsSystems (_world)
            .Add (new WeaponSystem ());
        _systems.Init ();
    }
    
    void Update () {
        // process all dependent systems.
        _systems?.Run ();
    }

void OnDestroy () {
        // destroy systems logical group.
        if (_systems != null) {
            _systems.Destroy ();
            _systems = null;
        }
        // destroy world.
        if (_world != null) {
            _world.Destroy ();
            _world = null;
        }
    }
}
```

> Important: Do not forget to call `EcsSystems.Destroy()` method if instance will not be used anymore.

# Engine integration

## Unity
> Tested on unity 2020.3 (but not dependent on it) and contains assembly definition for compiling to separate assembly file for performance reason.

[Unity editor integration](https://github.com/Leopotam/ecslite-unityeditor) contains code templates and world debug viewer.

## Custom engine
> C#7.3 or above required for this framework.

Code example - each part should be integrated in proper place of engine execution flow.
```csharp
using Leopotam.EcsLite;

class EcsStartup {
    EcsWorld _world;
    EcsSystems _systems;

// Initialization of ecs world and systems.
    void Init () {        
        _world = new EcsWorld ();
        _systems = new EcsSystems (_world);
        _systems
            // register additional worlds here.
            // .AddWorld (customWorldInstance, "events")
            // register your systems here, for example:
            // .Add (new TestSystem1 ())
            // .Add (new TestSystem2 ())
            .Init ();
    }

// Engine update loop.
    void UpdateLoop () {
        _systems?.Run ();
    }

// Cleanup.
    void Destroy () {
        if (_systems != null) {
            _systems.Destroy ();
            _systems = null;
        }
        if (_world != null) {
            _world.Destroy ();
            _world = null;
        }
    }
}
```

# Projects powered by LeoECS Lite
## With sources
* ["3D Platformer"](https://github.com/supremestranger/3D-Platformer-Lite)
  [![](https://camo.githubusercontent.com/dcd2f525130d73f4688c1f1cfb12f6e37d166dae23a1c6fac70e5b7873c3ab21/68747470733a2f2f692e6962622e636f2f686d374c726d342f506c6174666f726d65722e706e67)](https://github.com/supremestranger/3D-Platformer-Lite)

* ["SharpPhysics2D"](https://github.com/7Bpencil/sharpPhysics)
  [![](https://github.com/7Bpencil/sharpPhysics/raw/master/pictures/preview.png)](https://github.com/7Bpencil/sharpPhysics)

* ["Busy ECS - extremely nice (and most likely slow) ECS framework"](https://github.com/kkolyan/busyecs)

# Extensions
* [Dependency injection](https://github.com/Leopotam/ecslite-di)
* [Extended filters](https://github.com/Leopotam/ecslite-extendedfilters)
* [Extended systems](https://github.com/Leopotam/ecslite-extendedsystems)
* [Threads support](https://github.com/Leopotam/ecslite-threads)
* [Unity editor integration](https://github.com/Leopotam/ecslite-unityeditor)
* [Unity uGui bindings](https://github.com/Leopotam/ecslite-unity-ugui)
* [Unity jobs support](https://github.com/Leopotam/ecslite-threads-unity)
* [UniLeo - Unity scene data converter](https://github.com/voody2506/UniLeo-Lite)
* [Unity Physx events support](https://github.com/supremestranger/leoecs-lite-physics)
* [Multiple Shared injection](https://github.com/GoodCatGames/ecslite-multiple-shared)
* [Unity native collections support](https://github.com/odingamesdev/native-ecslite)

# License
The software is released under the terms of the [MIT license](./LICENSE.md).

No personal support or any guarantees.

# FAQ

### What is the difference from Ecs-full?

I prefer to name them `lite` (ecs-lite) and `classic` (ecs-full). Main differences between them (based on `lite`):
* Codebase decreased by 50% (easier to maintain and extend).
* Zero static data in core.
* No caches for components in filter (less memory consuming).
* Fast access to any component on any entity (with performance of cached filter components in `classic`).
* No limits to amount of filter contraints (filter is not generic class anymore).
* Performance is similar to `classic`, maybe slightly better in some cases (worse in some corner cases on very huge amount of data).
* Is aimed at using multiple worlds at same time (can be useful to keep memory consumption low on huge amount of short living components like "events").
* No reflection at runtime (can be used with aggressive code stripping).
* No data injection through reflection by default (you can use custom shared class between systems with required data or `ecslite-di` from extension's list).
* Entities switched back to `int` (memory consuming decreased). Saving entity as component field supported through packing to `classic` `EcsEntity`-similar struct.
* Small core, all new features can be added through extension repos.
* All new features will be added to `lite` only, `classic` looks stable and mature enough - no new features, bugfixes only.

### I want to process one system at MonoBehaviour.Update() and another - at MonoBehaviour.FixedUpdate(). How can I do it?

For splitting systems by `MonoBehaviour`-method multiple `EcsSystems` logical groups should be used:
```csharp
EcsSystems _update;
EcsSystems _fixedUpdate;

void Start () {
    EcsWorld world = new EcsWorld ();
    _update = new EcsSystems (world).Add (new UpdateSystem ());
    _update.Init ();
    _fixedUpdate = new EcsSystems (world).Add (new FixedUpdateSystem ());
    _fixedUpdate.Init ();
}

void Update () {
    _update.Run ();
}

void FixedUpdate () {
    _fixedUpdate.Run ();
}
```

### I copy&paste my reset components code again and again. How can I do it in other manner?

If you want to simplify your code and keep reset/init code at one place, you can setup custom handler to process cleanup / initialization for component:
```csharp
struct MyComponent : IEcsAutoReset<MyComponent> {
    public int Id;
    public object LinkToAnotherComponent;

public void AutoReset (ref MyComponent c) {
        c.Id = 2;
        c.LinkToAnotherComponent = null;
    }
}
```
This method will be automatically called for brand new component instance and after component removing from entity and before recycling to component pool.
> Important: With custom `AutoReset` behaviour there are no any additional checks for reference-type fields, you should provide custom correct cleanup/init behaviour to avoid possible memory leaks.

### I want to keep references to entities in components, but entity can be killed at any system and I need protection from reusing the same ID. How can I do it?

For keeping entity somewhere you should pack it to special `EcsPackedEntity` or `EcsPackedEntityWithWorld` types:
```csharp
EcsWorld world = new EcsWorld ();
int entity = world.NewEntity ();
EcsPackedEntity packed = world.PackEntity (entity);
EcsPackedEntityWithWorld packedWithWorld = world.PackEntityWithWorld (entity);
...
if (packed.Unpack (world, out int unpacked)) {
    // unpacked is valid and can be used.
}
if (packedWithWorld.Unpack (out EcsWorld unpackedWorld, out int unpackedWithWorld)) {
    // unpackedWithWorld is valid and can be used.
}
```

### I want to add some reactive behaviour on world changes, how I can do it?

You can use `LEOECSLITE_WORLD_EVENTS` definition to enable custom event listeners support on worlds:

```csharp
class TestWorldEventListener : IEcsWorldEventListener {
    public void OnEntityCreated (int entity) {
        // entity created - raises on world.NewEntity().
    }

public void OnEntityChanged (int entity) {
        // entity changed - raises on pool.Add() / pool.Del().
    }

public void OnEntityDestroyed (int entity) {
        // entity destroyed - raises on world.DelEntity() or last component removing.
    }

public void OnFilterCreated (EcsFilter filter) {
        // filter created - raises on world.Filter().End() for brand new filter.
    }

public void OnWorldResized (int newSize) {
        // world resized - raises on world/pools resizing when no room for entity at world.NewEntity() call.
    }

public void OnWorldDestroyed (EcsWorld world) {
        // world destroyed - raises on world.Destroy().
    }
}
...
var world = new EcsWorld ();
var listener = new TestWorldEventListener ();
world.AddEventListener (listener);
```

### I want to add some reactive behaviour on filter changes, how I can do it?

You can use `LEOECSLITE_FILTER_EVENTS` definition to enable custom event listeners support on filters:

```csharp
class TestFilterEventListener : IEcsFilterEventListener {
    public void OnEntityAdded (int entity) {
        // entity added to filter.
    }

public void OnEntityRemoved (int entity) {
        // entity removed from filter.
    }
}
...
var world = new EcsWorld ();
var filter = world.Filter<C1> ().End ();
var listener = new TestFilterEventListener ();
filter.AddEventListener (listener);
```

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