Systems

Systems in an ECS are typically queries. So you can still use the world.Query<Position, Velocity>() shown in the "Hello World" example.

Using Systems is optional but they have some significant advantages.

System features

• Enable chaining multiple decoupled QuerySystem classes in a SystemGroup. Each group provide a CommandBuffer.

• A system can have state - fields or properties - which can be used as parameters in OnUpdate(). The system state can be serialized to JSON.

• Systems can be enabled/disabled or removed. The order of systems in a group can be changed.

• Systems have performance monitoring build-in to measure execution times and memory allocations. If enabled systems detected as bottleneck can be optimized. A perf log (see example below) provide a clear overview of all systems their amount of entities and impact on performance.

• Multiple worlds can be added to a single SystemRoot instance. root.Update() will execute every system on all worlds.

Example

The example shows the setup of a small system hierarchy. Multiple systems can be added to a system hierarchy.

public static void HelloSystem()
{
    var world = new EntityStore();
    for (int n = 0; n < 10; n++) {
        world.CreateEntity(new Position(n, 0, 0), new Velocity(), new Scale3());
    }
    var root = new SystemRoot(world) {
        new MoveSystem(),
    //  new PulseSystem(),
    //  new ... multiple systems can be added. The execution order still remains clear.
    };
    root.Update(default);
}
        
class MoveSystem : QuerySystem<Position, Velocity>
{
    protected override void OnUpdate() {
        Query.ForEachEntity((ref Position position, ref Velocity velocity, Entity entity) => {
            position.value += velocity.value;
        });
    }
}

A valuable strength of an ECS is establishing a clear and decoupled code structure. Adding the PulseSystem below to the SystemRoot above is trivial. This system uses a foreach (var entity in Query.Entities) as an alternative to Query.ForEachEntity((...) => {...}) to iterate the query result.

The example also shows how to set a Filter in a system constructor to limit the query result to entities matching this Filter. See all available filters at the QueryFilter - API.

struct Pulsating : ITag { }

class PulseSystem : QuerySystem<Scale3>
{
    float frequency = 4f;
    
    public PulseSystem() => Filter.AnyTags(Tags.Get<Pulsating>());
    
    protected override void OnUpdate() {
        foreach (var entity in Query.Entities) {
            ref var scale = ref entity.GetComponent<Scale3>().value;
            scale = Vector3.One * (1 + 0.2f * MathF.Sin(frequency * Tick.time));
        }
    }
}

The system hierarchy can be modified at runtime. New systems can be added, inserted or removed.

    var root = new SystemRoot();
    var mySystem1 = new MySystem1();
    var mySystem2 = new MySystem2();
    root.Add(mySystem1);
    root.Insert(1, mySystem2);
    root.Remove(mySystem1);

To prevent running a system when executing Update() without changing the hierarchy a system can be disabled with

    system.Enabled = false;  

System monitoring

System performance monitoring is disabled by default. To enable monitoring call:

root.SetMonitorPerf(true);

When enabled system monitoring captures

• Number of system executions.

• System execution duration in ms.

• Memory heap allocations per system in bytes.

• The number of entities matching a query system.

Realtime monitoring

In a game editor like Unity system monitoring is available in the ECS System Set component.

Screenshot: ECS System Set component in Play mode

Log monitoring

The performance statistics available at SystemPerf. To get performance statistics on console use:

root.Update(default);
Console.WriteLine(root.GetPerfLog());

The log result will look like:

stores: 1                  on   last ms    sum ms   updates  last mem   sum mem  entities
---------------------      --  --------  --------  --------  --------  --------  --------
Systems [2]                 +     0.076     3.322        10       128      1392
| ScaleSystem               +     0.038     2.088        10        64       696     10000
| PositionSystem            +     0.038     1.222        10        64       696     10000

on                  + enabled  - disabled
last ms, sum ms     last/sum system execution time in ms
updates             number of executions
last mem, sum mem   last/sum allocated bytes
entities            number of entities matching a QuerySystem

SystemRoot

The SystemRoot is a container of systems forming a hierarchy of systems. These systems are executed in their specified order.

Typically a SystemRoot operates on single EntityStore passed to its constructor. A system hierarchy can also operate on multiple EntityStore's. Additional stores are added with root.AddStore().

If needed a system hierarchy can be setup without any EntityStore. This enables creating the hierarchy without the need of an EntityStore at initialization phase.

Update Execution

Execution of systems start always with the SystemRoot. Info - a SystemRoot is a SystemGroup. Its child systems are executed when calling root.Update(). The child systems of a SystemGroup are executed in the order added to the group.

    var root = new SystemRoot(store) {
        new MoveSystem(),
        new PulseSystem()
    };
    root.Update(default);

Each QuerySystem can override the following methods.

    protected   override void   OnUpdateGroupBegin() { } // called once per Update()
    protected   override void   OnUpdate()           { } // called for every store
    protected   override void   OnUpdateGroupEnd()   { } // called once per Update()

The execution of these methods of the group children is shown in the pseudo below. Each group has a single CommandBuffer per EntityStore. CommandBuffer.Playback() is called after execution of all OnUpdate() methods.

Execution order using a single EntityStore.

    foreach (var child in children) child.OnUpdateGroupBegin();

    foreach (var child in children) child.OnUpdate();

    store.CommandBuffer.Playback();

    foreach (var child in children) child.OnUpdateGroupEnd();

Execution order when using multiple EntityStore's.

    foreach (var child in children) child.OnUpdateGroupBegin();

    foreach (var store in stores) {
        foreach (var child in children) child.OnUpdate();
    }
    foreach (var store in stores) {
        store.CommandBuffer.Playback();
    }
    foreach (var child in children) child.OnUpdateGroupEnd();

Customize Systems

In cases a system requires code which goes beyond common Query execution a system can be customized. Therefor a system can override OnAddStore()

protected override void OnAddStore(EntityStore store)

Use cases for custom systems are:

• Handle user input.

• Moving the Camera. E.g. based on user input.

• Execute multiple / nested queries in a single system. E.g. to detect collisions between two different entity sets and iterating both sets in nested loops.

• Need to make structural changes via the parent group CommandBuffer.

• Want direct access to an EntityStore.

Extend QuerySystem

Example show how to extend a QuerySystem and execute a customQuery. A QuerySystem enable access to the CommandBuffer.

public static void CustomizeQuerySystem()
{
    var world = new EntityStore();
    world.CreateEntity(new Position(0, 0, 0));
    var root = new SystemRoot(world) {
        new CustomQuerySystem()
    };
    root.Update(default);
}

// The example shows how to create a custom QuerySystem that:
// - creates a customQuery and
// - make structural changes via the parent group CommandBuffer.
class CustomQuerySystem : QuerySystem
{
    private ArchetypeQuery<Position> customQuery;
    
    protected override void OnAddStore(EntityStore store) {
        customQuery = store.Query<Position>();
    }
    
    protected override void OnUpdate() {
        var buffer = CommandBuffer;
        // Executes the customQuery instead of the base class Query.
        customQuery.ForEachEntity((ref Position component1, Entity entity) => {
            buffer.AddComponent(entity.Id, new Velocity());
        });
    }
}

Extend BaseSystem

Example shows how to extend a BaseSystem to call arbitrary methods on the EntityStore. In this case to deal with UniqueEntity's. Note: A BaseSystem has no access to the CommandBuffer.

public static void CustomizeBaseSystem()
{
    var world = new EntityStore();
    world.CreateEntity(new UniqueEntity("Camera"), new Position(0, 0, 0));
    var root = new SystemRoot(world) {
        new CameraSystem()
    };
    root.Update(default);
}

// Example of a system that does not require a Query.
// E.g. find and access a UniqueEntity as shown below. 
class CameraSystem : BaseSystem
{
    private Entity camera;
    
    protected override void OnAddStore(EntityStore store) {
        camera = store.GetUniqueEntity("Camera");
    }
    
    protected override void OnUpdateGroup() {
        ref var position = ref camera.GetComponent<Position>();
        // Update camera position based on user input
    }
}