Provide entities_iter() on EntityMapper to get an iterator of foreign entities
#13703
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# Objective - Fixes #13703 ## Solution - Added `mappings` to the `EntityMapper` trait, which returns an iterator over currently tracked `Entity` to `Entity` mappings. - Added `DynEntityMapper` as an [object safe](https://doc.rust-lang.org/reference/items/traits.html#object-safety) alternative to `EntityMapper`. - Added `assert_object_safe` as a helper for ensuring traits are object safe. ## Testing - Added new unit test `entity_mapper_iteration` which tests the `SceneEntityMapper` implementation of `EntityMapper::mappings`. - Added unit tests to ensure `DynEntityMapper`, `DynEq` and `DynHash` are object safe. - Passed CI on my Windows 10 development environment --- ## Changelog - Added `mappings` to `EntityMapper` trait. ## Migration Guide - If you are implementing `EntityMapper` yourself, you can use the below as a stub implementation: ```rust fn mappings(&self) -> impl Iterator<Item = (Entity, Entity)> { unimplemented!() } ``` - If you were using `EntityMapper` as a trait object (`dyn EntityMapper`), instead use `dyn DynEntityMapper` and its associated methods. ## Notes - The original issue proposed returning a `Vec` from `EntityMapper` instead of an `impl Iterator` to preserve its object safety. This is a simpler option, but also forces an allocation where it isn't strictly needed. I've opted for this split into `DynEntityMapper` and `EntityMapper` as it's been done several times across Bevy already, and provides maximum flexibility to users. - `assert_object_safe` is an empty function, since the assertion actually happens once you try to use a `dyn T` for some trait `T`. I have still added this function to clearly document what object safety is within Bevy, and to create a standard way to communicate that a given trait must be object safe. - Other traits should have tests added to ensure object safety, but I've left those off to avoid cluttering this PR further. --------- Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
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What problem does this solve or what need does it fill?
I want to get a list of all the entities that an
EntityMapperis tracking. If we have a concrete hash map this is easy, but in generic code we have to work with the trait.What solution would you like?
Add a new function to
EntityMapper(returningVecrather thanimpl Iteratorfor trait object reasons)What alternative(s) have you considered?
Add a separate trait with this function. This would reduce breakage by not requiring the ecosystem to immediately implement a new function.
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