When & WhenMut: skip system if Res is missing.

[mrchantey]

Objective

ParamWarnPolicy has been removed in 0.16, deprecating never_param_warn(). Like Single and Populated, we need a Res type that will skip the system if the resource is not present.

Solution

Implement two new system params: When & WhenMut, identical to Res & ResMut but with a validate_param that will skip instead of panic.

These types can also be extended to 'wrap' other types beyond Res & ResMut.

Testing

Tested both should skip and should not skip cases, verify by running cargo test -p bevy_ecs system::when

---

Showcase

Res & ResMut now have two alternatives When and WhenMut for when a system should skip instead of panic.

// existing options
fn panics_if_not_present(res: Res<Foo>){}
fn runs_even_if_not_present(res: Option<Res<Foo>>){}

// new - skip the system if the resource is missing
fn skips_if_not_present(res: When<Foo>){}

[github-actions]

It looks like your PR has been selected for a highlight in the next release blog post, but you didn't provide a release note.

Please review the instructions for writing release notes, then expand or revise the content in the release notes directory to showcase your changes.

[bushrat011899]

I like the idea, but When feels very general. To me, it feels like something you could use with any SystemParam and doesn't communicate its link to Resource well. Bike shedding some other names:

• ResIf<R>
• When<Res<R>> (this would be nice but might require more work than this PR covers to be generic)

[mrchantey]

Good point, I think the name was chosen so that the same convention can be implemented for more types beyond Res & ResMut in the future, more context in discord. Definitely possible to wrap Res, the first version did use fn my_system(res: When<Res<Foo>>) but maybe this brings it more in line with the other utility types like Single<T>?

[MrGVSV]

Definitely possible to wrap Res, the first version did use fn my_system(res: When<Res<Foo>>) but maybe this brings it more in line with the other utility types like Single<T>?

The cools thing about making When a wrapper is that we can possibly make Single and friends panic again. I like that it can skip systems but sometimes there are times where you want to fail fast if the expected single-ness is ever invalidated.

So I think When<Single<T>> actually makes a lot of sense.

Alternatively, we could do the inverse if skipping systems is a more commonly-desired scenario. Maybe we make Res skip by default but then allow users to require its existence with Require<Res<T>> or something.

[NthTensor]

@alice-i-cecile May need to make a call on M-Needs-Release-Note. This is typically a maintainer-level publishing choice.

I'm really not sure what the right usage of When would be -- do we want it to work for all system params, do we want it to be just for resources -- so I'm also tagging this with Needs-Design.

The cools thing about making When a wrapper is that we can possibly make Single and friends panic again.

Sort of don't want to break Single again personally, that's a lot of churn over several releases.

[alice-i-cecile]

Sort of don't want to break Single again personally, that's a lot of churn over several releases.

Strongly agree here.

@alice-i-cecile May need to make a call on M-Needs-Release-Note. This is typically a maintainer-level publishing choice.

Yes, worth having.

I'm really not sure what the right usage of When would be -- do we want it to work for all system params, do we want it to be just for resources -- so I'm also tagging this with Needs-Design.

All system params please!

[chescock]

Note that there is also an existing PR for a generic When: #18765

[MrGVSV]

Sort of don't want to break Single again personally, that's a lot of churn over several releases.

Fair but I think it's going to benefit readability if skipping (or not skipping) is explicit. And I do think there's benefit to having a fail fast variant for these sorts of parameters. If I expect a system to run, it might not be helpful for it to silently skip (of course, you can panic manually with Query::single so that's at least an option).

This could also be confusing for new users who see that When causes a system to be skipped, but find their Single systems randomly being skipped.

So imo adding a generic When puts Single as is in an unfinished/inconsistent state. And it's probably better to deal with another bit of churn now than decide to do it further down the road.

If it's even desired behavior haha I could be alone in this thought process.

[mrchantey]

it's probably better to deal with another bit of churn now than decide to do it further down the road.

I agree, this feels like one thats worth thinking deeply about and getting right. In terms of design it sounds like there are two paths forward:

Option A: Single and friends Panic

• Res and friends still panic
• Single and friends should panic again

fn system(
  res: When<Res<Foo>>,
  single: When<Single<Bar>>,
)

Option B: Only Resources Panic

• Res and friends still panic
• Single and friends still skip

In this case we'd need to make some implementation decisions:

// b.1 - No wrapper for Res & ResMut
fn system(
  res: When<Foo>,
  res_mut: WhenMut<Foo>,
  reader: WhenMut<EventReader<Bar>>,
)
// b.2 - Blanket Wrapper
fn system(
  res: When<Res<Foo>>,
  res_mut: When<ResMut<Foo>>,
  reader: When<EventReader<Bar>>,
)
// b.3 Required Single panics
fn system(
  query: Required<Single<Foo>>,
)

In my opinion Option B better reflects the nature of Components vs Resources. Looking at my codebase I'd like most Populated<Window> systems to skip, but Res<Time> systems to panic by default.

As for the implementation I think we should close this pr in favor of the blanket wrapper in #18765, and When<Single<T>> is valid but redundant. This closes the door on When<MyResource> but thats controversial and a wrapper is far simpler to implement and maintain.

[bushrat011899]

Personally, I'm in favour of When being a generic wrapper for any system parameter. For one thing, it means we don't need to do WhenMut, WhenNonSend, WhenNonSendMut, etc. etc., making the API surface area smaller but more powerful.

I also like that this opens the door for a form of per-parameter run conditions as a general API, expanding on Option and ParamSet.

[chescock]

If it's even desired behavior haha I could be alone in this thought process.

You're not alone! I also think we'll eventually want Single to fail and When<Single> to skip. The breaking change will feel really bad, though, especially for users who tried Single in 15.0 and had it break in the 15.1 patch release.

I'd vote for first getting some version of this or #18765 merged first, and then making a follow-up PR to debate changing the behavior of Single.

[mrchantey]

closing in favor of #18765 which is a more maintainable implementation.