SystemStateComponents

The purpose of SystemStateComponentData is to allow you to track resources internal to a system and have the opportunity to appropriately create and destroy those resources as needed without relying on individual callbacks.

SystemStateComponentData and SystemStateSharedComponentData are exactly like ComponentData and SharedComponentData, respectively, except in one important respect:

1. SystemStateComponentData is not deleted when an entity is destroyed.

DestroyEntity is shorthand for

1. Find all components which reference this particular entity ID.
2. Delete those components.
3. Recycle the entity id for reuse.

However, if SystemStateComponentData is present, it is not removed. This gives a system the opportunity to cleanup any resources or state associated with an entity ID. The entity ID will only be reused once all SystemStateComponentData has been removed.

Motivation

• Systems may need to keep an internal state based on ComponentData. For instance, resources may be allocated.
• Systems need to be able to manage that state as values and state changes are made by other systems. For example, when values in components change, or when relevant components are added or deleted.
• "No callbacks" is an important element of the ECS design rules.

Concept

The general use of SystemStateComponentData is expected to mirror a user component, providing the internal state.

For instance, given:

• FooComponent (ComponentData, user assigned)
• FooStateComponent (SystemComponentData, system assigned)

Detecting Component Add

When user adds FooComponent, FooStateComponent does not exist. The FooSystem update queries for FooComponent without FooStateComponent and can infer that they have been added. At that point, the FooSystem will add the FooStateComponent and any needed internal state.

Detecting Component Remove

When user removes FooComponent, FooStateComponent still exists. The FooSystem update queries for FooStateComponent without FooComponent and can infer that they have been removed. At that point, the FooSystem will remove the FooStateComponent and fix up any needed internal state.

Detecting Destroy Entity

DestroyEntity is actually a shorthand utility for:

• Find components which reference given entity ID.
• Delete components found.
• Recycle entity ID.

However, SystemStateComponentData are not removed on DestroyEntity and the entity ID is not recycled until the last component is deleted. This gives the system the opportunity to clean up the internal state in the exact same way as with component removal.

SystemStateComponent

A SystemStateComponentData is analogous to a ComponentData and used similarly.

struct FooStateComponent : ISystemStateComponentData
{
}

Visibility of a SystemStateComponentData is also controlled in the same way as a component (using private, public, internal) However, it's expected, as a general rule, that a SystemStateComponentData will be ReadOnly outside the system that creates it.

SystemStateSharedComponent

A SystemStateSharedComponentData is analogous to a SharedComponentData and used similarly.

struct FooStateSharedComponent : ISystemStateSharedComponentData
{
  public int Value;
}