A Golang Entity Component System

A Golang Entity Component System implementation

tl;dr

If you really want to just see how to use this package, skip ahead to the Examples section.

This ECS implementation provides at-runtime entityy/component association without heavy use of reflection.

What is ECS?

Entity Component System is a design pattern.

In ECS, an entity represents anything, and is defined through composition
as opposed to inheritance. What this means is that an entity is composed of pieces, instead of defined with a class hierarchy. More concretely, this means that an entity ends up being an identifier that points to various "aspects" about the entity. All "aspects" of an entity are expressed as components.

In practice, components store data, and hardly ever any logic. Components express the various aspects of an entity; position, velocity, rotation, scale, etc.

Systems end up being where most logic is located. Most systems will iterate over entities and use the entity's components for performing whatever logic the system requires. Consider a MovementSystem, which will operate upon all entities that have a Position and Velocity component.

Peculiarities about Akara

There are several parts of Akara's API that are peculiar only to Akara.

These things include the following:

The ECS World
Component registration
Component Factories
Component Subscription & Component Filters
System tick rates
there's likely a lot more to list here, this doc needs editing...

Here are some big-picture ideas to keep in mind when working with Akara:

Entities are literally just numbers (uint64's)
Components must be registered in the ECS world
Every entity has a BitSet that describes which components it has
There is only one component factory for any given component type
Systems are in charge of their own tick rate

The ECS `World`

The World is the single place where systems, components, and entities are stored and managed. The world is in charge of authoring entity ID's, registering components, creating and updating component subscriptions, etc.

Here is a very simple example of creating a World and invoking the update loop:

package main

import (
	"github.com/gravestench/akara"

	"github.com/example/project/systems/magick"
	"github.com/example/project/systems/monster"
	"github.com/example/project/systems/itemspawn"
)

func main() {
	cfg := akara.NewWorldConfig()

	cfg.With(&monster.System{}).
		With(&itemspawn.System{}).
		With(&magick.System{})

	world := akara.NewWorld(cfg)

	for {
		if err := world.Update(); err != nil {
			panic(err)
		}
	}
}

Component registration

Creating components in Akara requires registering the component withing a World. The World will only ever have a single component factory for any given component.

Registering a component will do two things:

associate a unique ID for the component type
create an abstract component factory

Here's an example Velocity component:

type Velocity struct {
	x, y float64
}

To make this implement the akara.Component interface, we need a New method:

func (*Velocity) New() akara.Component {
	return &Velocity{}
}

Now we can register the component:

// this only yields the component ID
velocityID := world.RegisterComponent(&Velocity{})

Component Factories

We could use the component ID to grab the abstract component factory:

factory := world.GetComponentFactory(velocityID)

Component factories are used to create, retrieve, and remove components from entities:

e := world.NewEntity()

// returns a akara.Component interface!
c := factory.Add(e) 

// we must cast the interface to our concrete component type
v := c.(*Velocity)

Notice how we always have to cast the returned interface back to our concrete component implementation? We can get around this annoyance by making a concrete component factory:

type VelocityFactory struct {
	*akara.ComponentFactory
}

func (m *VelocityFactory) Add(id akara.EID) *Velocity {
	return m.ComponentFactory.Add(id).(*Velocity)
}

func (m *VelocityFactory) Get(id akara.EID) (*Velocity, bool) {
	component, found := m.ComponentFactory.Get(id)
	if !found {
		return nil, found
	}

	return component.(*Velocity), found
}

this allows us to just use Add and Get without having to cast the returned value.

It's worth mentioning that each distinct component type that is registered will only have one component factory and one component ID.

Here, we try to register the same component twice, but nothing bad happens:

id1 := world.RegisterComponent(&Velocity{})
id2 := world.RegisterComponent(&Velocity{})

isSame := id1 == id2 // true

Entities

An Entity is just a unique uint64, nothing more.

Systems

Systems are fairly simple in that they need only implement this interface:

type System interface {
	Active() bool
	SetActive(bool)
	Update()
}

However, there are a couple of concrete system types provided which you can use to create your own systems.

The first is BaseSystem, and it has its own Active and SetActive methods.

type BaseSystem struct {
   *World
   active bool
}

func (s *BaseSystem) Active() bool {
   return s.active
}

func (s *BaseSystem) SetActive(b bool) {
   s.active = b
}

You can embed the BaseSystem in your own system like this:

type ExampleSystem struct {
	*BaseSystem
}

func (s *ExampleSystem) Update() {
	// do stuff
}

The second type of system is a SubscriberSystem, but before we talk about that we need to talk about subscriptions...

Component Subscription & Component Filters

Before we can talk about Subscriptions or ComponentFilters, we need to know what a BitSet is.

BitSets

BitSets are just a bunch of booleans, packed up in a slice of uint64's.

type BitSet struct {
	groups []uint64
}

These are used by the EntityManager to signify which components an entity currently has . Whenever a ComponentMap adds or removes a component for an entity, the EntityManager will update the entity's bitset.

Remember how the Component types all have a unique ID? That ID corresponds to the bit index that is toggled in the BitSet when a component is being added or removed!

ComponentFilters

ComponentFilters also use BitSets, but they use them for comparisons against an entity bitset .

type ComponentFilter struct {
	Required    *BitSet
	OneRequired *BitSet
	Forbidden   *BitSet
}

When an entity bitset is evaluated by a ComponentFilter, each of the Filter's Bitsets is used to determine if the entity should be allowed to pass through the filter.

When determining if an entity's component bitset will pass through the ComponentFilter:

Required -- The entity bitset must contain true bits for all true bits present in the Required bitset.
OneRequired -- The entity bitset must have at least one true bit in common with the OneRequired BitSet
Forbidden -- The entity bitset must not contain any true bits for any true bits present in the Forbidden bitset

Subscriptions

Subscriptions are simply a combination of a ComponentFilter and a slice of entity ID's:

type Subscription struct {
	Filter          *ComponentFilter
	entities        []EID
}

As Components are added and removed from entities, the entity manager will pass the updated entity bitset to the subscription. If the entity bitset passes through the subscription's filter, the entity ID is added to the slice of entities for that subscription.

This leads us to the second utility system that is provided... The SubscriberSystem!

type SubscriberSystem struct {
	*BaseSystem
	Subscriptions []*Subscription
}

Examples

Creating a world

// make a world config
cfg := akara.NewWorldConfig()

// add systems to the config
cfg.With(&MovementSystem{})

// create a world instance using the world config
world := akara.NewWorld(cfg)

Declaring a Component

Here is the bare minimum required to create a new component:

type Velocity struct {
	*Vector3
}

func (*Velocity) New() akara.Component {
	return &Velocity{
		Vector3: NewVector3(0, 0, 0),
	}
}

Initialization logic specific to this component (like creating instances of another *struct) belongs inside of the New method.

Concrete Component Factories

A concrete component factory is just a wrapper for the generic component factory, but it casts the returned values from Add and Get to the concrete component implementation. This is just to prevent you from having to cast the component interface to struct pointers.

type VelocityFactory struct {
	*akara.ComponentFactory
}

func (m *VelocityFactory) Add(id akara.EID) *Velocity {
	return m.ComponentFactory.Add(id).(*Velocity)
}

func (m *VelocityFactory) Get(id akara.EID) (*Velocity, bool) {
	component, found := m.ComponentFactory.Get(id)
	if !found {
		return nil, found
	}

	return component.(*Velocity), found
}

Creating a ComponentFilter

cfg := akara.NewFilter()

cfg.Require(
	components.Position,
	components.Velocity,
)

filter := cfg.Build()

Example System (with Subscriptions!)

For systems that use subscriptions, it is recommended that you embed an akara.BaseSubscriberSystem as it provides the generic methods for dealing with subscriptions. It also contains an akara.BaseSystem, which has other generic system methods.

It is also recommended that all component factories be placed inside of a common struct and given explicit names. This helps to keep the code clear when writing complicated systems.

As of writing, there is no general guide for how subscriptions are managed, but just embedding them in the system struct and giving them descriptive names is sufficient.

type MovementSystem struct {
	akara.SubscriberSystem
	components struct {
		positions   PositionFactory
		velocities  VelocityFactory
	}
	movingEntities []akara.EID
}

As of writing, all systems should set their World and call SetActive(false) if world is nil. After that, actual system initialization logic is added:

func (m *MovementSystem) Init(world *akara.World) {
	m.World = world

	if world == nil {
		m.SetActive(false)
		return
	}

	m.setupComponents()
	m.setupSubscriptions()
}

Here, we use BaseSystem.InjectComponent, which registers a component and assigns a component factory to the given destination.

func (m *MovementSystem) setupComponents() {
	m.InjectComponent(&Position{}, &m.components.Position.ComponentFactory)
	m.InjectComponent(&Velocity{}, &m.components.Velocity.ComponentFactory)
}

Here, we set up our only subscription. For this example, our MovementSystem is interested in Position and Velocity components.

func (m *MovementSystem) setupSubscriptions() {
	filterBuilder := m.NewComponentFilter()

	filterBuilder.Require(&Position{})
	filterBuilder.Require(&Velocity{})

	filter := filterBuilder.Build()

	m.movingEntities = m.World.AddSubscription(filter)
}

Our Update method is simple; we iterate over entities that are in our subscription. Remember, as components are added and removed from entities, all subscriptions are updated.

func (m *MovementSystem) Update() {
	for _, eid := range m.movingEntities.GetEntities() {
		m.moveEntity(eid)
	}
}

This is where our system actually does work. For the given incoming entity id, we retreive the position and velocity components and apply the velocity to the position. If either of those components does not exist for the entity, we return.

func (m *MovementSystem) moveEntity(id akara.EID) {
	position, found := m.components.positions.Get(id)
	if !found {
		return
	}

	velocity, found := m.components.velocities.Get(id)
	if !found {
		return
	}

	s := float64(m.World.TimeDelta) / float64(time.Second)
	position.Vector.Add(velocity.Vector.Clone().Scale(s))
}

Example: Static Checks for Interface Implementation

Wherever you define components and systems, it's good practice to add static checks. These prevent you from compiling if things don't implement the interfaces that they should.

// static check that MovementSystem implements the System interface
var _ akara.System = &MovementSystem{}

// static check that PositionComponent implements Component
var _ akara.Component = &PositionComponent{}

gravestench/akara