Commit Graph

9 Commits

Author SHA1 Message Date
Brezak
0f446d49a1 Add more methods to RelationshipSourceCollection (#18421)
# Objective

While working on #18058 I realized I could use
`RelationshipTargetCollection::new`, so I added it.

## Solution

- Add `RelationshipTargetCollection::new`
- Add `RelationshipTargetCollection::reserve`. Could generally be useful
when doing micro-optimizations.
- Add `RelationshipTargetCollection::shrink_to_fit`. Rust collections
generally don't shrink when removing elements. Might be a good idea to
call this once in a while.

## Testing

`cargo clippy`

---

## Showcase

`RelationshipSourceCollection` now implements `new`, `reserve` and
`shrink_to_fit` to give greater control over how much memory it
consumes.

## Migration Guide

Any type implementing `RelationshipSourceCollection` now needs to also
implement `new`, `reserve` and `shrink_to_fit`. `reserve` and
`shrink_to_fit` can be made no-ops if they conceptually mean nothing to
a collection.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Chris Russell <8494645+chescock@users.noreply.github.com>
2025-03-24 00:09:29 +01:00
Brezak
7cd729cf1a Add methods to bulk replace relationships on a entity (#18058)
# Objective

Add a way to efficiently replace a set of specifically related entities
with a new set.
Closes #18041 

## Solution

Add new `replace_(related/children)` to `EntityWorldMut` and friends.

## Testing

Added a new test to `hierarchy.rs` that specifically check if
`replace_children` actually correctly replaces the children on a entity
while keeping the original one.

---

## Showcase

`EntityWorldMut` and `EntityCommands` can now be used to efficiently
replace the entities a entity is related to.

```rust
/// `parent` has 2 children. `entity_a` and `entity_b`.
assert_eq!([entity_a, entity_b], world.entity(parent).get::<Children>());

/// Replace `parent`s children with `entity_a` and `entity_c`
world.entity_mut(parent).replace_related(&[entity_a, entity_c]);

/// `parent` now has 2 children. `entity_a` and `entity_c`.
///
/// `replace_children` has saved time by not removing and reading
/// the relationship between `entity_a` and `parent`
assert_eq!([entity_a, entity_c], world.entity(parent).get::<Children>());

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
2025-03-19 21:36:18 +01:00
Brezak
906d788420
Expand the RelationshipSourceCollection to return more information (#18241)
# Objective

While redoing #18058 I needed `RelationshipSourceCollection` (henceforth
referred to as the **Trait**) to implement `clear` so I added it.

## Solution

Add the `clear` method to the **Trait**.
Also make `add` and `remove` report if they succeeded.

## Testing

Eyeballs

---

## Showcase

The `RelationshipSourceCollection` trait now reports if adding or
removing an entity from it was successful.
It also not contains the `clear` method so you can easily clear the
collection in generic contexts.

## Changes

EDITED by Alice: We should get this into 0.16, so no migration guide
needed.

The `RelationshipSourceCollection` methods `add` and `remove` now need
to return a boolean indicating if they were successful (adding a entity
to a set that already contains it counts as failure). Additionally the
`clear` method has been added to support clearing the collection in
generic contexts.
2025-03-10 22:04:08 +00:00
Carter Anderson
a530c07bc5
Preserve spawned RelationshipTarget order and other improvements (#17858)
Fixes #17720

## Objective

Spawning RelationshipTargets from scenes currently fails to preserve
RelationshipTarget ordering (ex: `Children` has an arbitrary order).
This is because it uses the normal hook flow to set up the collection,
which means we are pushing onto the collection in _spawn order_ (which
is currently in archetype order, which will often produce mismatched
orderings).

We need to preserve the ordering in the original RelationshipTarget
collection. Ideally without expensive checking / fixups.

## Solution

One solution would be to spawn in hierarchy-order. However this gets
complicated as there can be multiple hierarchies, and it also means we
can't spawn in more cache-friendly orders (ex: the current per-archetype
spawning, or future even-smarter per-table spawning). Additionally,
same-world cloning has _slightly_ more nuanced needs (ex: recursively
clone linked relationships, while maintaining _original_ relationships
outside of the tree via normal hooks).

The preferred approach is to directly spawn the remapped
RelationshipTarget collection, as this trivially preserves the ordering.
Unfortunately we can't _just_ do that, as when we spawn the children
with their Relationships (ex: `ChildOf`), that will insert a duplicate.

We could "fixup" the collection retroactively by just removing the back
half of duplicates, but this requires another pass / more lookups /
allocating twice as much space. Additionally, it becomes complicated
because observers could insert additional children, making it harder
(aka more expensive) to determine which children are dupes and which are
not.

The path I chose is to support "opting out" of the relationship target
hook in the contexts that need that, as this allows us to just cheaply
clone the mapped collection. The relationship hook can look for this
configuration when it runs and skip its logic when that happens. A
"simple" / small-amount-of-code way to do this would be to add a "skip
relationship spawn" flag to World. Sadly, any hook / observer that runs
_as the result of an insert_ would also read this flag. We really need a
way to scope this setting to a _specific_ insert.

Therefore I opted to add a new `RelationshipInsertHookMode` enum and an
`entity.insert_with_relationship_insert_hook_mode` variant. Obviously
this is verbose and ugly. And nobody wants _more_ insert variants. But
sadly this was the best I could come up with from a performance and
capability perspective. If you have alternatives let me know!

There are three variants:

1. `RelationshipInsertHookMode::Run`: always run relationship insert
hooks (this is the default)
2. `RelationshipInsertHookMode::Skip`: do not run any relationship
insert hooks for this insert (this is used by spawner code)
3. `RelationshipInsertHookMode::RunIfNotLinked`: only run hooks for
_unlinked_ relationships (this is used in same-world recursive entity
cloning to preserve relationships outside of the deep-cloned tree)

Note that I have intentionally only added "insert with relationship hook
mode" variants to the cases we absolutely need (everything else uses the
default `Run` mode), just to keep the code size in check. I do not think
we should add more without real _very necessary_ use cases.

I also made some other minor tweaks:

1. I split out `SourceComponent` from `ComponentCloneCtx`. Reading the
source component no longer needlessly blocks mutable access to
`ComponentCloneCtx`.
2. Thanks to (1), I've removed the `RefCell` wrapper over the cloned
component queue.
3. (1) also allowed me to write to the EntityMapper while queuing up
clones, meaning we can reserve entities during the component clone and
write them to the mapper _before_ inserting the component, meaning
cloned collections can be mapped on insert.
4. I've removed the closure from `write_target_component_ptr` to
simplify the API / make it compatible with the split `SourceComponent`
approach.
5. I've renamed `EntityCloner::recursive` to
`EntityCloner::linked_cloning` to connect that feature more directly
with `RelationshipTarget::LINKED_SPAWN`
6. I've removed `EntityCloneBehavior::RelationshipTarget`. This was
always intended to be temporary, and this new behavior removes the need
for it.

---------

Co-authored-by: Viktor Gustavsson <villor94@gmail.com>
2025-03-05 22:18:57 +00:00
Dmytro Banin
7a1972ed3d
One-to-One Relationships (#18087)
# Objective

Minimal implementation of directed one-to-one relationships via
implementing `RelationshipSourceCollection` for `Entity`.

Now you can do

```rust
#[derive(Component)]
#[relationship(relationship_target = Below)]
pub struct Above(Entity);

#[derive(Component)]
#[relationship_target(relationship = Above)]
pub struct Below(Entity);
```

## Future Work

It would be nice if the relationships could be fully symmetrical in the
future - in the example above, since `Above` is the source of truth you
can't add `Below` to an entity and have `Above` added automatically.

## Testing

Wrote unit tests for new relationship sources and and verified
adding/removing relationships maintains connection as expected.
2025-03-04 07:57:35 +00:00
raldone01
1b7db895b7
Harden proc macro path resolution and add integration tests. (#17330)
This pr uses the `extern crate self as` trick to make proc macros behave
the same way inside and outside bevy.

# Objective

- Removes noise introduced by `crate as` in the whole bevy repo.
- Fixes #17004.
- Hardens proc macro path resolution.

## TODO

- [x] `BevyManifest` needs cleanup.
- [x] Cleanup remaining `crate as`.
- [x] Add proper integration tests to the ci.

## Notes

- `cargo-manifest-proc-macros` is written by me and based/inspired by
the old `BevyManifest` implementation and
[`bkchr/proc-macro-crate`](https://github.com/bkchr/proc-macro-crate).
- What do you think about the new integration test machinery I added to
the `ci`?
  More and better integration tests can be added at a later stage.
The goal of these integration tests is to simulate an actual separate
crate that uses bevy. Ideally they would lightly touch all bevy crates.

## Testing

- Needs RA test
- Needs testing from other users
- Others need to run at least `cargo run -p ci integration-test` and
verify that they work.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
2025-02-09 19:45:45 +00:00
Carter Anderson
3c8fae2390
Improved Entity Mapping and Cloning (#17687)
Fixes #17535

Bevy's approach to handling "entity mapping" during spawning and cloning
needs some work. The addition of
[Relations](https://github.com/bevyengine/bevy/pull/17398) both
[introduced a new "duplicate entities" bug when spawning scenes in the
scene system](#17535) and made the weaknesses of the current mapping
system exceedingly clear:

1. Entity mapping requires _a ton_ of boilerplate (implement or derive
VisitEntities and VisitEntitesMut, then register / reflect MapEntities).
Knowing the incantation is challenging and if you forget to do it in
part or in whole, spawning subtly breaks.
2. Entity mapping a spawned component in scenes incurs unnecessary
overhead: look up ReflectMapEntities, create a _brand new temporary
instance_ of the component using FromReflect, map the entities in that
instance, and then apply that on top of the actual component using
reflection. We can do much better.

Additionally, while our new [Entity cloning
system](https://github.com/bevyengine/bevy/pull/16132) is already pretty
great, it has some areas we can make better:

* It doesn't expose semantic info about the clone (ex: ignore or "clone
empty"), meaning we can't key off of that in places where it would be
useful, such as scene spawning. Rather than duplicating this info across
contexts, I think it makes more sense to add that info to the clone
system, especially given that we'd like to use cloning code in some of
our spawning scenarios.
* EntityCloner is currently built in a way that prioritizes a single
entity clone
* EntityCloner's recursive cloning is built to be done "inside out" in a
parallel context (queue commands that each have a clone of
EntityCloner). By making EntityCloner the orchestrator of the clone we
can remove internal arcs, improve the clarity of the code, make
EntityCloner mutable again, and simplify the builder code.
* EntityCloner does not currently take into account entity mapping. This
is necessary to do true "bullet proof" cloning, would allow us to unify
the per-component scene spawning and cloning UX, and ultimately would
allow us to use EntityCloner in place of raw reflection for scenes like
`Scene(World)` (which would give us a nice performance boost: fewer
archetype moves, less reflection overhead).

## Solution

### Improved Entity Mapping

First, components now have first-class "entity visiting and mapping"
behavior:

```rust
#[derive(Component, Reflect)]
#[reflect(Component)]
struct Inventory {
    size: usize,
    #[entities]
    items: Vec<Entity>,
}
```

Any field with the `#[entities]` annotation will be viewable and
mappable when cloning and spawning scenes.

Compare that to what was required before!

```rust
#[derive(Component, Reflect, VisitEntities, VisitEntitiesMut)]
#[reflect(Component, MapEntities)]
struct Inventory {
    #[visit_entities(ignore)]
    size: usize,
    items: Vec<Entity>,
}
```

Additionally, for relationships `#[entities]` is implied, meaning this
"just works" in scenes and cloning:

```rust
#[derive(Component, Reflect)]
#[relationship(relationship_target = Children)]
#[reflect(Component)]
struct ChildOf(pub Entity);
```

Note that Component _does not_ implement `VisitEntities` directly.
Instead, it has `Component::visit_entities` and
`Component::visit_entities_mut` methods. This is for a few reasons:

1. We cannot implement `VisitEntities for C: Component` because that
would conflict with our impl of VisitEntities for anything that
implements `IntoIterator<Item=Entity>`. Preserving that impl is more
important from a UX perspective.
2. We should not implement `Component: VisitEntities` VisitEntities in
the Component derive, as that would increase the burden of manual
Component trait implementors.
3. Making VisitEntitiesMut directly callable for components would make
it easy to invalidate invariants defined by a component author. By
putting it in the `Component` impl, we can make it harder to call
naturally / unavailable to autocomplete using `fn
visit_entities_mut(this: &mut Self, ...)`.

`ReflectComponent::apply_or_insert` is now
`ReflectComponent::apply_or_insert_mapped`. By moving mapping inside
this impl, we remove the need to go through the reflection system to do
entity mapping, meaning we no longer need to create a clone of the
target component, map the entities in that component, and patch those
values on top. This will make spawning mapped entities _much_ faster
(The default `Component::visit_entities_mut` impl is an inlined empty
function, so it will incur no overhead for unmapped entities).

### The Bug Fix

To solve #17535, spawning code now skips entities with the new
`ComponentCloneBehavior::Ignore` and
`ComponentCloneBehavior::RelationshipTarget` variants (note
RelationshipTarget is a temporary "workaround" variant that allows
scenes to skip these components. This is a temporary workaround that can
be removed as these cases should _really_ be using EntityCloner logic,
which should be done in a followup PR. When that is done,
`ComponentCloneBehavior::RelationshipTarget` can be merged into the
normal `ComponentCloneBehavior::Custom`).

### Improved Cloning

* `Option<ComponentCloneHandler>` has been replaced by
`ComponentCloneBehavior`, which encodes additional intent and context
(ex: `Default`, `Ignore`, `Custom`, `RelationshipTarget` (this last one
is temporary)).
* Global per-world entity cloning configuration has been removed. This
felt overly complicated, increased our API surface, and felt too
generic. Each clone context can have different requirements (ex: what a
user wants in a specific system, what a scene spawner wants, etc). I'd
prefer to see how far context-specific EntityCloners get us first.
* EntityCloner's internals have been reworked to remove Arcs and make it
mutable.
* EntityCloner is now directly stored on EntityClonerBuilder,
simplifying the code somewhat
* EntityCloner's "bundle scratch" pattern has been moved into the new
BundleScratch type, improving its usability and making it usable in
other contexts (such as future cross-world cloning code). Currently this
is still private, but with some higher level safe APIs it could be used
externally for making dynamic bundles
* EntityCloner's recursive cloning behavior has been "externalized". It
is now responsible for orchestrating recursive clones, meaning it no
longer needs to be sharable/clone-able across threads / read-only.
* EntityCloner now does entity mapping during clones, like scenes do.
This gives behavior parity and also makes it more generically useful.
* `RelatonshipTarget::RECURSIVE_SPAWN` is now
`RelationshipTarget::LINKED_SPAWN`, and this field is used when cloning
relationship targets to determine if cloning should happen recursively.
The new `LINKED_SPAWN` term was picked to make it more generically
applicable across spawning and cloning scenarios.

## Next Steps

* I think we should adapt EntityCloner to support cross world cloning. I
think this PR helps set the stage for that by making the internals
slightly more generalized. We could have a CrossWorldEntityCloner that
reuses a lot of this infrastructure.
* Once we support cross world cloning, we should use EntityCloner to
spawn `Scene(World)` scenes. This would yield significant performance
benefits (no archetype moves, less reflection overhead).

---------

Co-authored-by: eugineerd <70062110+eugineerd@users.noreply.github.com>
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
2025-02-06 22:13:41 +00:00
Alice Cecile
5a9bc28502
Support non-Vec data structures in relations (#17447)
# Objective

The existing `RelationshipSourceCollection` uses `Vec` as the only
possible backing for our relationships. While a reasonable choice,
benchmarking use cases might reveal that a different data type is better
or faster.

For example:

- Not all relationships require a stable ordering between the
relationship sources (i.e. children). In cases where we a) have many
such relations and b) don't care about the ordering between them, a hash
set is likely a better datastructure than a `Vec`.
- The number of children-like entities may be small on average, and a
`smallvec` may be faster

## Solution

- Implement `RelationshipSourceCollection` for `EntityHashSet`, our
custom entity-optimized `HashSet`.
-~~Implement `DoubleEndedIterator` for `EntityHashSet` to make things
compile.~~
   -  This implementation was cursed and very surprising.
- Instead, by moving the iterator type on `RelationshipSourceCollection`
from an erased RPTIT to an explicit associated type we can add a trait
bound on the offending methods!
- Implement `RelationshipSourceCollection` for `SmallVec`

## Testing

I've added a pair of new tests to make sure this pattern compiles
successfully in practice!

## Migration Guide

`EntityHashSet` and `EntityHashMap` are no longer re-exported in
`bevy_ecs::entity` directly. If you were not using `bevy_ecs` / `bevy`'s
`prelude`, you can access them through their now-public modules,
`hash_set` and `hash_map` instead.

## Notes to reviewers

The `EntityHashSet::Iter` type needs to be public for this impl to be
allowed. I initially renamed it to something that wasn't ambiguous and
re-exported it, but as @Victoronz pointed out, that was somewhat
unidiomatic.

In
1a8564898f,
I instead made the `entity_hash_set` public (and its `entity_hash_set`)
sister public, and removed the re-export. I prefer this design (give me
module docs please), but it leads to a lot of churn in this PR.

Let me know which you'd prefer, and if you'd like me to split that
change out into its own micro PR.
2025-01-20 21:26:08 +00:00
Carter Anderson
21f1e3045c
Relationships (non-fragmenting, one-to-many) (#17398)
This adds support for one-to-many non-fragmenting relationships (with
planned paths for fragmenting and non-fragmenting many-to-many
relationships). "Non-fragmenting" means that entities with the same
relationship type, but different relationship targets, are not forced
into separate tables (which would cause "table fragmentation").

Functionally, this fills a similar niche as the current Parent/Children
system. The biggest differences are:

1. Relationships have simpler internals and significantly improved
performance and UX. Commands and specialized APIs are no longer
necessary to keep everything in sync. Just spawn entities with the
relationship components you want and everything "just works".
2. Relationships are generalized. Bevy can provide additional built in
relationships, and users can define their own.

**REQUEST TO REVIEWERS**: _please don't leave top level comments and
instead comment on specific lines of code. That way we can take
advantage of threaded discussions. Also dont leave comments simply
pointing out CI failures as I can read those just fine._

## Built on top of what we have

Relationships are implemented on top of the Bevy ECS features we already
have: components, immutability, and hooks. This makes them immediately
compatible with all of our existing (and future) APIs for querying,
spawning, removing, scenes, reflection, etc. The fewer specialized APIs
we need to build, maintain, and teach, the better.

## Why focus on one-to-many non-fragmenting first?

1. This allows us to improve Parent/Children relationships immediately,
in a way that is reasonably uncontroversial. Switching our hierarchy to
fragmenting relationships would have significant performance
implications. ~~Flecs is heavily considering a switch to non-fragmenting
relations after careful considerations of the performance tradeoffs.~~
_(Correction from @SanderMertens: Flecs is implementing non-fragmenting
storage specialized for asset hierarchies, where asset hierarchies are
many instances of small trees that have a well defined structure)_
2. Adding generalized one-to-many relationships is currently a priority
for the [Next Generation Scene / UI
effort](https://github.com/bevyengine/bevy/discussions/14437).
Specifically, we're interested in building reactions and observers on
top.

## The changes

This PR does the following:

1. Adds a generic one-to-many Relationship system
3. Ports the existing Parent/Children system to Relationships, which now
lives in `bevy_ecs::hierarchy`. The old `bevy_hierarchy` crate has been
removed.
4. Adds on_despawn component hooks
5. Relationships can opt-in to "despawn descendants" behavior, meaning
that the entire relationship hierarchy is despawned when
`entity.despawn()` is called. The built in Parent/Children hierarchies
enable this behavior, and `entity.despawn_recursive()` has been removed.
6. `world.spawn` now applies commands after spawning. This ensures that
relationship bookkeeping happens immediately and removes the need to
manually flush. This is in line with the equivalent behaviors recently
added to the other APIs (ex: insert).
7. Removes the ValidParentCheckPlugin (system-driven / poll based) in
favor of a `validate_parent_has_component` hook.

## Using Relationships

The `Relationship` trait looks like this:

```rust
pub trait Relationship: Component + Sized {
    type RelationshipSources: RelationshipSources<Relationship = Self>;
    fn get(&self) -> Entity;
    fn from(entity: Entity) -> Self;
}
```

A relationship is a component that:

1. Is a simple wrapper over a "target" Entity.
2. Has a corresponding `RelationshipSources` component, which is a
simple wrapper over a collection of entities. Every "target entity"
targeted by a "source entity" with a `Relationship` has a
`RelationshipSources` component, which contains every "source entity"
that targets it.

For example, the `Parent` component (as it currently exists in Bevy) is
the `Relationship` component and the entity containing the Parent is the
"source entity". The entity _inside_ the `Parent(Entity)` component is
the "target entity". And that target entity has a `Children` component
(which implements `RelationshipSources`).

In practice, the Parent/Children relationship looks like this:

```rust
#[derive(Relationship)]
#[relationship(relationship_sources = Children)]
pub struct Parent(pub Entity);

#[derive(RelationshipSources)]
#[relationship_sources(relationship = Parent)]
pub struct Children(Vec<Entity>);
```

The Relationship and RelationshipSources derives automatically implement
Component with the relevant configuration (namely, the hooks necessary
to keep everything in sync).

The most direct way to add relationships is to spawn entities with
relationship components:

```rust
let a = world.spawn_empty().id();
let b = world.spawn(Parent(a)).id();

assert_eq!(world.entity(a).get::<Children>().unwrap(), &[b]);
```

There are also convenience APIs for spawning more than one entity with
the same relationship:

```rust
world.spawn_empty().with_related::<Children>(|s| {
    s.spawn_empty();
    s.spawn_empty();
})
```

The existing `with_children` API is now a simpler wrapper over
`with_related`. This makes this change largely non-breaking for existing
spawn patterns.

```rust
world.spawn_empty().with_children(|s| {
    s.spawn_empty();
    s.spawn_empty();
})
```

There are also other relationship APIs, such as `add_related` and
`despawn_related`.

## Automatic recursive despawn via the new on_despawn hook

`RelationshipSources` can opt-in to "despawn descendants" behavior,
which will despawn all related entities in the relationship hierarchy:

```rust
#[derive(RelationshipSources)]
#[relationship_sources(relationship = Parent, despawn_descendants)]
pub struct Children(Vec<Entity>);
```

This means that `entity.despawn_recursive()` is no longer required.
Instead, just use `entity.despawn()` and the relevant related entities
will also be despawned.

To despawn an entity _without_ despawning its parent/child descendants,
you should remove the `Children` component first, which will also remove
the related `Parent` components:

```rust
entity
    .remove::<Children>()
    .despawn()
```

This builds on the on_despawn hook introduced in this PR, which is fired
when an entity is despawned (before other hooks).

## Relationships are the source of truth

`Relationship` is the _single_ source of truth component.
`RelationshipSources` is merely a reflection of what all the
`Relationship` components say. By embracing this, we are able to
significantly improve the performance of the system as a whole. We can
rely on component lifecycles to protect us against duplicates, rather
than needing to scan at runtime to ensure entities don't already exist
(which results in quadratic runtime). A single source of truth gives us
constant-time inserts. This does mean that we cannot directly spawn
populated `Children` components (or directly add or remove entities from
those components). I personally think this is a worthwhile tradeoff,
both because it makes the performance much better _and_ because it means
theres exactly one way to do things (which is a philosophy we try to
employ for Bevy APIs).

As an aside: treating both sides of the relationship as "equivalent
source of truth relations" does enable building simple and flexible
many-to-many relationships. But this introduces an _inherent_ need to
scan (or hash) to protect against duplicates.
[`evergreen_relations`](https://github.com/EvergreenNest/evergreen_relations)
has a very nice implementation of the "symmetrical many-to-many"
approach. Unfortunately I think the performance issues inherent to that
approach make it a poor choice for Bevy's default relationship system.

## Followup Work

* Discuss renaming `Parent` to `ChildOf`. I refrained from doing that in
this PR to keep the diff reasonable, but I'm personally biased toward
this change (and using that naming pattern generally for relationships).
* [Improved spawning
ergonomics](https://github.com/bevyengine/bevy/discussions/16920)
* Consider adding relationship observers/triggers for "relationship
targets" whenever a source is added or removed. This would replace the
current "hierarchy events" system, which is unused upstream but may have
existing users downstream. I think triggers are the better fit for this
than a buffered event queue, and would prefer not to add that back.
* Fragmenting relations: My current idea hinges on the introduction of
"value components" (aka: components whose type _and_ value determines
their ComponentId, via something like Hashing / PartialEq). By labeling
a Relationship component such as `ChildOf(Entity)` as a "value
component", `ChildOf(e1)` and `ChildOf(e2)` would be considered
"different components". This makes the transition between fragmenting
and non-fragmenting a single flag, and everything else continues to work
as expected.
* Many-to-many support
* Non-fragmenting: We can expand Relationship to be a list of entities
instead of a single entity. I have largely already written the code for
this.
* Fragmenting: With the "value component" impl mentioned above, we get
many-to-many support "for free", as it would allow inserting multiple
copies of a Relationship component with different target entities.

Fixes #3742 (If this PR is merged, I think we should open more targeted
followup issues for the work above, with a fresh tracking issue free of
the large amount of less-directed historical context)
Fixes #17301
Fixes #12235 
Fixes #15299
Fixes #15308 

## Migration Guide

* Replace `ChildBuilder` with `ChildSpawnerCommands`.
* Replace calls to `.set_parent(parent_id)` with
`.insert(Parent(parent_id))`.
* Replace calls to `.replace_children()` with `.remove::<Children>()`
followed by `.add_children()`. Note that you'll need to manually despawn
any children that are not carried over.
* Replace calls to `.despawn_recursive()` with `.despawn()`.
* Replace calls to `.despawn_descendants()` with
`.despawn_related::<Children>()`.
* If you have any calls to `.despawn()` which depend on the children
being preserved, you'll need to remove the `Children` component first.

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Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
2025-01-18 22:20:30 +00:00