struct WeaponComponent {
    public int Ammo;
    public string GunName;
}

// Creates new entity in world context.
EcsEntity entity = _world.NewEntity ();
// Get() returns component on entity. If component not exists - it will be added.
ref Component1 c1 = ref entity.Get<Component1> ();
ref Component2 c2 = ref entity.Get<Component2> ();
// Del() removes component from entity.
entity.Del<Component2> ();
// Component can be replaced with new instance of component. If component not exist - it will be added.
var weapon = new WeaponComponent () { Ammo = 10, GunName = "Handgun" };
entity.Replace (weapon);
// With Replace() you can chain component's creation:
var entity2 = world.NewEntity ();
entity2.Replace (new Component1 { Id = 10 }).Replace (new Component2 { Name = "Username" });
// Any entity can be copied with all components:
var entity2Copy = entity2.Copy ();
// Any entity can be merged / "moved" to another entity (source will be destroyed):
var newEntity = world.NewEntity ();
entity2Copy.MoveTo (newEntity); // all components from entity2Copy moved to newEntity, entity2Copy destroyed.
// any entity can be destroyed. 
entity.Destroy ();

class WeaponSystem : IEcsPreInitSystem, IEcsInitSystem, IEcsDestroySystem, IEcsPostDestroySystem {
    public void PreInit () {
        // Will be called once during EcsSystems.Init() call and before IEcsInitSystem.Init.
    }

    public void Init () {
        // Will be called once during EcsSystems.Init() call.
    }

    public void Destroy () {
        // Will be called once during EcsSystems.Destroy() call.
    }

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

class HealthSystem : IEcsRunSystem {
    public void Run () {
        // Will be called on each EcsSystems.Run() call.
    }
}

class HealthSystem : IEcsSystem {
    // auto-injected fields.
    EcsWorld _world = null;
    EcsFilter<WeaponComponent> _weaponFilter = null;
}

var systems = new EcsSystems (world)
    .Add (new TestSystem1 ())
    .Add (new TestSystem2 ())
    .Add (new TestSystem3 ())
    .Inject (a)
    .Inject (b)
    .Inject (c)
    .Inject (d);
systems.Init ();

struct Component1 { }

class System1 : IEcsInitSystem {
    EcsWorld _world = null;

    public void Init () {
        _world.NewEntity ().Get<Component1> ();
    } 
}

class System2 : IEcsInitSystem {
    EcsFilter<Component1> _filter = null;

    public void Init () {
        Debug.Log (_filter.GetEntitiesCount ());
    }
}

var systems1 = new EcsSystems (world);
var systems2 = new EcsSystems (world);
systems1.Add (new System1 ());
systems2.Add (new System2 ());
systems1.Init ();
systems2.Init ();

var systems1 = new EcsSystems (world);
var systems2 = new EcsSystems (world);
systems1.Add (new System1 ());
systems2.Add (new System2 ());
systems1.ProcessInjects ();
systems2.ProcessInjects ();
systems1.Init ();
systems2.Init ();

class WeaponSystem : IEcsInitSystem, IEcsRunSystem {
    // auto-injected fields: EcsWorld instance and EcsFilter.
    EcsWorld _world = null;
    // We wants to get entities with "WeaponComponent" and without "HealthComponent".
    EcsFilter<WeaponComponent>.Exclude<HealthComponent> _filter = null;

    public void Init () {
        _world.NewEntity ().Get<WeaponComponent> ();
    }

    public void Run () {
        foreach (var i in _filter) {
            // entity that contains WeaponComponent.
            ref var entity = ref _filter.GetEntity (i);

            // Get1 will return link to attached "WeaponComponent".
            ref var weapon = ref _filter.Get1 (i);
            weapon.Ammo = System.Math.Max (0, weapon.Ammo - 1);
        }
    }
}

struct Component1 { }

struct Component2 : IEcsIgnoreInFilter { }

class TestSystem : IEcsRunSystem {
    EcsFilter<Component1, Component2> _filter = null;

    public void Run () {
        foreach (var i in _filter) {
            // its valid code.
            ref var component1 = ref _filter.Get1 (i);

            // its invalid code due to cache for _filter.Get2() is null for memory / performance reasons.
            ref var component2 = ref _filter.Get2 (i);
        }
    }
}

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.
        _systems.Destroy ();
        // destroy world.
        _world.Destroy ();
    }
}

// initialization.
var nestedSystems = new EcsSystems (_world).Add (new NestedSystem ());
// don't call nestedSystems.Init() here, rootSystems will do it automatically.

var rootSystems = new EcsSystems (_world).Add (nestedSystems);
rootSystems.Init ();

// update loop.
// don't call nestedSystems.Run() here, rootSystems will do it automatically.
rootSystems.Run ();

// destroying.
// don't call nestedSystems.Destroy() here, rootSystems will do it automatically.
rootSystems.Destroy ();

class TestSystem : IEcsRunSystem {
    public void Run () { }
}
var systems = new EcsSystems (_world);
systems.Add (new TestSystem (), "my special system");
systems.Init ();
var idx = systems.GetNamedRunSystem ("my special system");

// state will be true here, all systems are active by default.
var state = systems.GetRunSystemState (idx);

// disable system from execution.
systems.SetRunSystemState (idx, false);

using Leopotam.Ecs;

class EcsStartup {
    EcsWorld _world;
    EcsSystems _systems;

    // Initialization of ecs world and systems.
    void Init () {        
        _world = new EcsWorld ();
        _systems = new EcsSystems (_world);
        _systems
            // register your systems here, for example:
            // .Add (new TestSystem1 ())
            // .Add (new TestSystem2 ())
            
            // register one-frame components (order is important), for example:
            // .OneFrame<TestComponent1> ()
            // .OneFrame<TestComponent2> ()
            
            // inject service instances here (order doesn't important), for example:
            // .Inject (new CameraService ())
            // .Inject (new NavMeshSupport ())
            .Init ();
    }

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

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

EcsSystems _update;
EcsSystems _fixedUpdate;

void Start () {
    var 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 ();
}

...
// will be injected.
EcsFilter<C1> _filter1 = null;

// will be skipped.
[EcsIgnoreInject]
EcsFilter<C2> _filter2 = null;

struct MyComponent : IEcsAutoReset<MyComponent> {
    public int Id;
    public object LinkToAnotherComponent;

    public void AutoReset (ref MyComponent c) {
        c.Id = 2;
        c.LinkToAnotherComponent = null;
    }
}

struct MyOneFrameComponent { }

EcsSystems _update;

void Start () {
    var world = new EcsWorld ();
    _update = new EcsSystems (world);
    _update
        .Add (new CalculateSystem ())
        .Add (new UpdateSystem ())
        .OneFrame<MyOneFrameComponent> ()
        .Init ();
}

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

var config = new EcsWorldConfig() {
    // World.Entities default cache size.
    WorldEntitiesCacheSize = 1024,
    // World.Filters default cache size.
    WorldFiltersCacheSize = 128,
    // World.ComponentPools default cache size.
    WorldComponentPoolsCacheSize = 512,
    // Entity.Components default cache size (not doubled).
    EntityComponentsCacheSize = 8,
    // Filter.Entities default cache size.
    FilterEntitiesCacheSize = 256,
};
var world = new EcsWorld(config);
...