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>`.
952 B
952 B
| title | pull_requests | |
|---|---|---|
| Generic `Option` Parameter |
|
Option<Single<D, F>> will now resolve to None if there are multiple entities matching the query.
Previously, it would only resolve to None if there were no entities, and would skip the system if there were multiple.
We have introduced a blanket impl SystemParam for Option that resolves to None if the parameter is invalid.
This allows third-party system parameters to work with Option, and makes the behavior more consistent.
If you want a system to run when there are no matching entities but skip when there are multiple,
you will need to use Query<D, F> and call single() yourself.
// 0.16
fn my_system(single: Option<Single<&Player>>) {
}
// 0.17
fn my_system(query: Query<&Player>) {
let result = query.single();
if matches!(r, Err(QuerySingleError(MultipleEntities(_)))) {
return;
}
let single: Option<&Player> = r.ok();
}