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[Merged by Bors] - impl SystemParam for Option<Res<T>> / Option<ResMut<T>> #1494

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[Merged by Bors] - impl SystemParam for Option<Res<T>> / Option<ResMut<T>> #1494

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inodentry
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This allows users to write systems that do not panic if a resource does not exist at runtime (such as if it has not been inserted yet).

This is a copy-paste of the impls for Res and ResMut, with an extra check to see if the resource exists.

There might be a cleaner way to do it than this check. I don't know.

@alice-i-cecile alice-i-cecile added A-ECS Entities, components, systems, and events C-Usability A targeted quality-of-life change that makes Bevy easier to use labels Feb 21, 2021
TheRawMeatball
TheRawMeatball reviewed Feb 21, 2021
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Overall, I really like this

crates/bevy_ecs/src/system/system_param.rs Outdated
Comment on lines 246 to 532
if resources.contains::<T>() {
Some(Some(Res::new(
resources.get_unsafe_ref::<T>(ResourceIndex::Global),
)))
} else {
Some(None)
}
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I'm not sure if this is cleaner or not, but it's worth considering

Suggested change
if resources.contains::<T>() {
Some(Some(Res::new(
resources.get_unsafe_ref::<T>(ResourceIndex::Global),
)))
} else {
Some(None)
}
Some(
resources
.contains::<T>()
.then(|| Res::new(resources.get_unsafe_ref::<T>(ResourceIndex::Global))),
)

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@inodentry inodentry Feb 21, 2021 •
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I prefer the imperative style over the functional style.

So i think the original is clearer and more readable.

(and probably also easier for the compiler)

@alice-i-cecile alice-i-cecile added the S-Ready-For-Final-Review This PR has been approved by the community. It's ready for a maintainer to consider merging it label Feb 21, 2021
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I think this needs an example and a mention in the Res / ResMut docstrings. I really like this functionality though!

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Added doc comments.

I can't come up with a meaningful way to add it to the ecs_guide example right now.

DJMcNab
DJMcNab reviewed Feb 23, 2021
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This is useful functionality to expse.

Also this will almost certainly need to be changed for the ecs pr when that arrives, since the SystemParam trait will be changing.

crates/bevy_ecs/src/system/system_param.rs Outdated
_world: &'a World,
resources: &'a Resources,
) -> Option<Self::Item> {
if resources.contains::<T>() {
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This double lookup is not good - ideally we'd just have a get_unsafe_ref which returns an Option<NonNull<T>>.

(I think the most likely response from cart will be to point this out)

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cart commented Mar 3, 2021

I'm in favor of these changes. Lets just update this when #1525 is merged.

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Updated for ECS v2.

This time, thanks to the functionality being split across 2 separate traits, I could implement it more cleanly by wrapping an underlying ResState and delegating to it, rather than copypasting the trait impl code.

There is also no extra check, because the resource fetch API returns an Option now, so we can just map that.

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cart commented Mar 8, 2021

bors r+

bors bot pushed a commit that referenced this pull request Mar 8, 2021
@inodentry
impl SystemParam for Option<Res<T>> / Option<ResMut<T>> (#1494)
13aef05 
This allows users to write systems that do not panic if a resource does not exist at runtime (such as if it has not been inserted yet).

This is a copy-paste of the impls for `Res` and `ResMut`, with an extra check to see if the resource exists.

There might be a cleaner way to do it than this check. I don't know.
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bors bot commented Mar 8, 2021

Pull request successfully merged into main.

Build succeeded:

@bors bors bot changed the title impl SystemParam for Option<Res<T>> / Option<ResMut<T>> [Merged by Bors] - impl SystemParam for Option<Res<T>> / Option<ResMut<T>> Mar 8, 2021
@bors bors bot closed this Mar 8, 2021
@DJMcNab DJMcNab mentioned this pull request Mar 9, 2021
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