Project Url: Hexworks/amethyst
Introduction: Simple multiplatform SEA (Systems, Entities, Attributes) library written in multiplatform Kotlin.
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So what is Amethyst, and why is it useful for us? Amethyst is a small library which takes care of managing your game entities in a nutshell. It is similar to an Entity Component System but it is a bit different in some important aspects.

"So what is a game entity anyway?", you might ask. A game entity can be anything which has its own behavior or state in your game, from a goblin, to a wall, or even immaterial effects, like shadows. What's important about a game entity is that it encapsulates all internal state of our goblin for example and it also contains all actions it can perform. This means that by using this concept we can maintain the Cohesion) of our game objects while still retaining the flexibility.

So how does it work? Let's look at the SEA:

Systems, Entities, Attributes

When working with an ECS, you'll probably see things like Entities, Components, and Systems. In Amethyst we have Systems, Entities and Attributes. I'm not going into detail about the difference between ECS and SEA, but explain how SEA works:

Let's say that we want to add a goblin to our game. If we think about how things can behave and change over time we can conclude, that there are essentially two ways: internal, and external. For example a goblin can decide that it wants to explore the dungeon, or just take a look around (internal change) or the player can decide to bash the goblin to a pulpy mass with their club (external change). For this Amethyst has Systems in place:

 * A [System] is responsible for updating the internal state
 * ([Attribute]s) of an [Entity].
interface System<C : Context>

There are several types of Systems, and for our discussion here we need to know about Behavior:

 * A [Behavior] is a [System] which performs actions autonomously
 * with entities whenever they are [update]d.
interface Behavior<C : Context> : System<C>

which lets our goblin interact with the world and Facet:

 * A [Facet] is a [System] which performs actions based on the
 * [Command]s they receive.
interface Facet<C : Context> : System<C>

which lets whe world interact with our goblin.

There is also Actor which combines Facet and Behavior. We'll talk about it later.

When a change happens over time to an entity (our goblin in this example) its state might change. To represent this, Amethyst gives us Attributes:

 * An [Attribute] represents metadata about an entity which can
 * change over time.
interface Attribute

An Attribute can be anything which you add to your entity, from health points to stomach contents. What's important is that Attributes should never have behavior, they are supposed to be dumb data structures.

On the other hand, Systems should never have internal state. These two important rules allow us to compose Systems and Attributes into Entity objects which are flexible, cohesive and safe to use.

So how do these entities work together? We have Engine for that which handles them so we don't have to do it by hand:

interface Engine<T : Context> {

     * Adds the given [Entity] to this [Engine].
    fun addEntity(entity: Entity<EntityType, T>)

     * Removes the given [Entity] from this [Engine].
    fun removeEntity(entity: Entity<EntityType, T>)

     * Updates the [Entity] objects in this [Engine] with the given [context].
    fun update(context: T)
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