jecs/docs/learn/concepts/component-traits.md

# Component Traits

Component traits in Jecs allow you to modify component behavior through special IDs and pairs. They provide a way to configure how components interact with entities and the world.

## Built-in Traits

### Component
```lua
jecs.Component
```
Identifies an ID as a component. Every component created with `world:component()` automatically has this trait.

### Tag
```lua
jecs.Tag
```
Marks a component as a tag that never contains data. This improves performance for structural changes.

## Cleanup Traits

Cleanup traits define what happens when entities used as components or relationship targets are deleted.

### OnDelete
Specifies what happens when a component or relationship is deleted:

::: code-group
```lua [luau]
local Archer = world:component()
world:add(Archer, pair(jecs.OnDelete, jecs.Remove))

local entity = world:entity()
world:add(entity, Archer)

-- This will remove Archer from entity
world:delete(Archer)
```
```typescript [typescript]
const Archer = world.component();
world.add(Archer, pair(jecs.OnDelete, jecs.Remove));

const entity = world.entity();
world.add(entity, Archer);

// This will remove Archer from entity
world.delete(Archer);
```
:::

### OnDeleteTarget
Specifies what happens when a relationship target is deleted:

```lua
local OwnedBy = world:component()
world:add(OwnedBy, pair(jecs.OnDeleteTarget, jecs.Remove))

local item = world:entity()
local player = world:entity()
world:add(item, pair(OwnedBy, player))

-- This will remove (OwnedBy, player) from item
world:delete(player)
```

## Cleanup Actions

Two cleanup actions are available:

1. **Remove**: Removes instances of the specified component/relationship (default)
2. **Delete**: Deletes all entities with the specified component/relationship

## Example Usage

### Tag Components
```lua
local IsEnemy = world:component()
world:add(IsEnemy, jecs.Tag)

-- More efficient than storing a boolean
world:add(entity, IsEnemy)
```

### Cleanup Configuration
```lua
-- Delete children when parent is deleted
local ChildOf = world:component()
world:add(ChildOf, pair(jecs.OnDeleteTarget, jecs.Delete))

-- Remove ownership when owner is deleted
local OwnedBy = world:component()
world:add(OwnedBy, pair(jecs.OnDeleteTarget, jecs.Remove))
```

## Best Practices

1. **Use Tags Appropriately**
   - Use tags for boolean-like components
   - Configure tags early in component setup
   - Document tag usage

2. **Cleanup Configuration**
   - Consider cleanup behavior during design
   - Document cleanup policies
   - Test cleanup behavior

3. **Performance Considerations**
   - Use tags for better performance
   - Configure cleanup for efficient entity management
   - Consider relationship cleanup impact

# Component

Every (component) ID comes with a `Component` which helps with the distinction between normal entities and component IDs.

# Tag

A (component) ID can be marked with `Tag´ in which the component will never contain any data. This allows for zero-cost components which improves performance for structural changes.

# Hooks

Hooks are part of the "interface" of a component. You could consider hooks as the counterpart to OOP methods in ECS. They define the behavior of a component, but can only be invoked through mutations on the component data. You can only configure a single `OnAdd`, `OnRemove` and `OnSet` hook per component, just like you can only have a single constructor and destructor.

## Examples

::: code-group

```luau [luau]
local Transform= world:component()
world:set(Transform, OnAdd, function(entity)
    -- A transform component has been added to an entity
end)
world:set(Transform, OnRemove, function(entity)
    -- A transform component has been removed from the entity
end)
world:set(Transform, OnSet, function(entity, value)
    -- A transform component has been assigned/changed to value on the entity
end)
```

```typescript [typescript]
const Transform = world.component();
world.set(Transform, OnAdd, (entity) => {
	// A transform component has been added to an entity
});
world.set(Transform, OnRemove, (entity) => {
	// A transform component has been removed from the entity
});
world.set(Transform, OnSet, (entity, value) => {
	// A transform component has been assigned/changed to value on the entity
});
```

:::

# Cleanup Traits

When entities that are used as tags, components, relationships or relationship targets are deleted, cleanup traits ensure that the store does not contain any dangling references. Any cleanup policy provides this guarantee, so while they are configurable, games cannot configure traits that allows for dangling references.

We also want to specify this per relationship. If an entity has `(Likes, parent)` we may not want to delete that entity, meaning the cleanup we want to perform for `Likes` and `ChildOf` may not be the same.

This is what cleanup traits are for: to specify which action needs to be executed under which condition. They are applied to entities that have a reference to the entity being deleted: if I delete the `Archer` tag I remove the tag from all entities that have it.

To configure a cleanup policy for an entity, a `(Condition, Action)` pair can be added to it. If no policy is specified, the default cleanup action (`Remove`) is performed.

There are two cleanup actions:

-   `Remove`: removes instances of the specified (component) id from all entities (default)
-   `Delete`: deletes all entities with specified id

There are two cleanup conditions:

-   `OnDelete`: the component, tag or relationship is deleted
-   `OnDeleteTarget`: a target used with the relationship is deleted

## Examples

The following examples show how to use cleanup traits

### (OnDelete, Remove)

::: code-group

```luau [luau]
local Archer = world:component()
world:add(Archer, pair(jecs.OnDelete, jecs.Remove))

local e = world:entity()
world:add(e, Archer)

-- This will remove Archer from e
world:delete(Archer)
```

```typescript [typescript]
const Archer = world.component();
world.add(Archer, pair(jecs.OnDelete, jecs.Remove));

const e = world.entity();
world.add(e, Archer);

// This will remove Archer from e
world.delete(Archer);
```

:::

### (OnDelete, Delete)

::: code-group

```luau [luau]
local Archer = world:component()
world:add(Archer, pair(jecs.OnDelete, jecs.Delete))

local e = world:entity()
world:add(e, Archer)

-- This will delete entity e because the Archer component has a (OnDelete, Delete) cleanup trait
world:delete(Archer)
```

```typescript [typescript]
const Archer = world.component();
world.add(Archer, pair(jecs.OnDelete, jecs.Delete));

const e = world.entity();
world.add(e, Archer);

// This will delete entity e because the Archer component has a (OnDelete, Delete) cleanup trait
world.delete(Archer);
```

:::

### (OnDeleteTarget, Remove)

::: code-group

```luau [luau]
local OwnedBy = world:component()
world:add(OwnedBy, pair(jecs.OnDeleteTarget, jecs.Remove))
local loot = world:entity()
local player = world:entity()
world:add(loot, pair(OwnedBy, player))

-- This will remove (OwnedBy, player) from loot
world:delete(player)
```

```typescript [typescript]
const OwnedBy = world.component();
world.add(OwnedBy, pair(jecs.OnDeleteTarget, jecs.Remove));
const loot = world.entity();
const player = world.entity();
world.add(loot, pair(OwnedBy, player));

// This will remove (OwnedBy, player) from loot
world.delete(player);
```

### (OnDeleteTarget, Delete)

::: code-group

```luau [luau]
local ChildOf = world:component()
world:add(ChildOf, pair(jecs.OnDeleteTarget, jecs.Delete))

local parent = world:entity()
local child = world:entity()
world:add(child, pair(ChildOf, parent))

-- This will delete both parent and child
world:delete(parent)
```

```typescript [typescript]
const ChildOf = world.component();
world.add(ChildOf, pair(jecs.OnDeleteTarget, jecs.Delete));

const parent = world.entity();
const child = world.entity();
world.add(child, pair(ChildOf, parent));

// This will delete both parent and child
world.delete(parent);
```

:::

This page takes wording and terminology directly from Flecs [documentation](https://www.flecs.dev/flecs/md_docs_2ComponentTraits.html)