9e2bd8ac18
# Objective Provide a generic `impl SystemParam for Option<P>` that uses system parameter validation. This immediately gives useful impls for params like `EventReader` and `GizmosState` that are defined in terms of `Res`. It also allows third-party system parameters to be usable with `Option`, which was previously impossible due to orphan rules. Note that this is a behavior change for `Option<Single>`. It currently fails validation if there are multiple matching entities, but with this change it will pass validation and produce `None`. Also provide an impl for `Result<P, SystemParamValidationError>`. This allows systems to inspect the error if necessary, either for bubbling it up or for checking the `skipped` flag. Fixes #12634 Fixes #14949 Related to #18516 ## Solution Add generic `SystemParam` impls for `Option` and `Result`, and remove the impls for specific types. Update documentation and `fallible_params` example with the new semantics for `Option<Single>`.
29 lines
952 B
Markdown
29 lines
952 B
Markdown
---
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title: Generic `Option` Parameter
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pull_requests: [18766]
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---
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`Option<Single<D, F>>` will now resolve to `None` if there are multiple entities matching the query.
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Previously, it would only resolve to `None` if there were no entities, and would skip the system if there were multiple.
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We have introduced a blanket `impl SystemParam for Option` that resolves to `None` if the parameter is invalid.
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This allows third-party system parameters to work with `Option`, and makes the behavior more consistent.
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If you want a system to run when there are no matching entities but skip when there are multiple,
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you will need to use `Query<D, F>` and call `single()` yourself.
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```rust
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// 0.16
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fn my_system(single: Option<Single<&Player>>) {
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}
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// 0.17
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fn my_system(query: Query<&Player>) {
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let result = query.single();
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if matches!(r, Err(QuerySingleError(MultipleEntities(_)))) {
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return;
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}
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let single: Option<&Player> = r.ok();
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}
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```
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