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# Objective Expand the existing `Query` API to support more dynamic use cases i.e. scripting. ## Prior Art - #6390 - #8308 - #10037 ## Solution - Create a `QueryBuilder` with runtime methods to define the set of component accesses for a built query. - Create new `WorldQueryData` implementations `FilteredEntityMut` and `FilteredEntityRef` as variants of `EntityMut` and `EntityRef` that provide run time checked access to the components included in a given query. - Add new methods to `Query` to create "query lens" with a subset of the access of the initial query. ### Query Builder The `QueryBuilder` API allows you to define a query at runtime. At it's most basic use it will simply create a query with the corresponding type signature: ```rust let query = QueryBuilder::<Entity, With<A>>::new(&mut world).build(); // is equivalent to let query = QueryState::<Entity, With<A>>::new(&mut world); ``` Before calling `.build()` you also have the opportunity to add additional accesses and filters. Here is a simple example where we add additional filter terms: ```rust let entity_a = world.spawn((A(0), B(0))).id(); let entity_b = world.spawn((A(0), C(0))).id(); let mut query_a = QueryBuilder::<Entity>::new(&mut world) .with::<A>() .without::<C>() .build(); assert_eq!(entity_a, query_a.single(&world)); ``` This alone is useful in that allows you to decide which archetypes your query will match at runtime. However it is also very limited, consider a case like the following: ```rust let query_a = QueryBuilder::<&A>::new(&mut world) // Add an additional access .data::<&B>() .build(); ``` This will grant the query an additional read access to component B however we have no way of accessing the data while iterating as the type signature still only includes &A. For an even more concrete example of this consider dynamic components: ```rust let query_a = QueryBuilder::<Entity>::new(&mut world) // Adding a filter is easy since it doesn't need be read later .with_id(component_id_a) // How do I access the data of this component? .ref_id(component_id_b) .build(); ``` With this in mind the `QueryBuilder` API seems somewhat incomplete by itself, we need some way method of accessing the components dynamically. So here's one: ### Query Transmutation If the problem is not having the component in the type signature why not just add it? This PR also adds transmute methods to `QueryBuilder` and `QueryState`. Here's a simple example: ```rust world.spawn(A(0)); world.spawn((A(1), B(0))); let mut query = QueryBuilder::<()>::new(&mut world) .with::<B>() .transmute::<&A>() .build(); query.iter(&world).for_each(|a| assert_eq!(a.0, 1)); ``` The `QueryState` and `QueryBuilder` transmute methods look quite similar but are different in one respect. Transmuting a builder will always succeed as it will just add the additional accesses needed for the new terms if they weren't already included. Transmuting a `QueryState` will panic in the case that the new type signature would give it access it didn't already have, for example: ```rust let query = QueryState::<&A, Option<&B>>::new(&mut world); /// This is fine, the access for Option<&A> is less restrictive than &A query.transmute::<Option<&A>>(&world); /// Oh no, this would allow access to &B on entities that might not have it, so it panics query.transmute::<&B>(&world); /// This is right out query.transmute::<&C>(&world); ``` This is quite an appealing API to also have available on `Query` however it does pose one additional wrinkle: In order to to change the iterator we need to create a new `QueryState` to back it. `Query` doesn't own it's own state though, it just borrows it, so we need a place to borrow it from. This is why `QueryLens` exists, it is a place to store the new state so it can be borrowed when you call `.query()` leaving you with an API like this: ```rust fn function_that_takes_a_query(query: &Query<&A>) { // ... } fn system(query: Query<(&A, &B)>) { let lens = query.transmute_lens::<&A>(); let q = lens.query(); function_that_takes_a_query(&q); } ``` Now you may be thinking: Hey, wait a second, you introduced the problem with dynamic components and then described a solution that only works for static components! Ok, you got me, I guess we need a bit more: ### Filtered Entity References Currently the only way you can access dynamic components on entities through a query is with either `EntityMut` or `EntityRef`, however these can access all components and so conflict with all other accesses. This PR introduces `FilteredEntityMut` and `FilteredEntityRef` as alternatives that have additional runtime checking to prevent accessing components that you shouldn't. This way you can build a query with a `QueryBuilder` and actually access the components you asked for: ```rust let mut query = QueryBuilder::<FilteredEntityRef>::new(&mut world) .ref_id(component_id_a) .with(component_id_b) .build(); let entity_ref = query.single(&world); // Returns Some(Ptr) as we have that component and are allowed to read it let a = entity_ref.get_by_id(component_id_a); // Will return None even though the entity does have the component, as we are not allowed to read it let b = entity_ref.get_by_id(component_id_b); ``` For the most part these new structs have the exact same methods as their non-filtered equivalents. Putting all of this together we can do some truly dynamic ECS queries, check out the `dynamic` example to see it in action: ``` Commands: comp, c Create new components spawn, s Spawn entities query, q Query for entities Enter a command with no parameters for usage. > c A, B, C, Data 4 Component A created with id: 0 Component B created with id: 1 Component C created with id: 2 Component Data created with id: 3 > s A, B, Data 1 Entity spawned with id: 0v0 > s A, C, Data 0 Entity spawned with id: 1v0 > q &Data 0v0: Data: [1, 0, 0, 0] 1v0: Data: [0, 0, 0, 0] > q B, &mut Data 0v0: Data: [2, 1, 1, 1] > q B || C, &Data 0v0: Data: [2, 1, 1, 1] 1v0: Data: [0, 0, 0, 0] ``` ## Changelog - Add new `transmute_lens` methods to `Query`. - Add new types `QueryBuilder`, `FilteredEntityMut`, `FilteredEntityRef` and `QueryLens` - `update_archetype_component_access` has been removed, archetype component accesses are now determined by the accesses set in `update_component_access` - Added method `set_access` to `WorldQuery`, this is called before `update_component_access` for queries that have a restricted set of accesses, such as those built by `QueryBuilder` or `QueryLens`. This is primarily used by the `FilteredEntity*` variants and has an empty trait implementation. - Added method `get_state` to `WorldQuery` as a fallible version of `init_state` when you don't have `&mut World` access. ## Future Work Improve performance of `FilteredEntityMut` and `FilteredEntityRef`, currently they have to determine the accesses a query has in a given archetype during iteration which is far from ideal, especially since we already did the work when matching the archetype in the first place. To avoid making more internal API changes I have left it out of this PR. --------- Co-authored-by: Mike Hsu <mike.hsu@gmail.com> |
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Compile fail tests for bevy_ecs
This crate is separate from bevy_ecs and not part of the Bevy workspace in order to not fail crater tests for Bevy. The tests assert on the exact compiler errors and can easily fail for new Rust versions due to updated compiler errors (e.g. changes in spans).
The CI workflow executes these tests on the stable rust toolchain (see tools/ci).