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Add Has<C> WorldQuery type to replace Option<&C> when checking existence #7811

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alice-i-cecile opened this issue Feb 24, 2023 · 4 comments · Fixed by #8844
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Add Has<C> WorldQuery type to replace Option<&C> when checking existence #7811

alice-i-cecile opened this issue Feb 24, 2023 · 4 comments · Fixed by #8844
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A-ECS Entities, components, systems, and events C-Feature A new feature, making something new possible

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@alice-i-cecile
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What problem does this solve or what need does it fill?

I commonly find myself reaching for Option<&C> when I just need to check whether or not a component exists, and then branching on Some versus None.

This works well enough, but:

  1. The access is more restrictive than it needs to be: we request read access, but only actually need existence checking. This limits parallelism, as these systems could run in parallel with mutating systems.
  2. It's less clear to read: the pattern is a bit strange and implicit.
  3. It may be measurably slower.

What solution would you like?

Add a Has<C: Component> type. Implement WorldQuery for this, with no access required.

The item type of each query element should be bool.

What alternative(s) have you considered?

We could do nothing, the eixisting pattern isn't terrible.

We could name this Is<C>: I'm not sure which one is clearer.
@alice-i-cecile alice-i-cecile added C-Feature A new feature, making something new possible A-ECS Entities, components, systems, and events labels Feb 24, 2023
@SkiFire13
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It's pretty horrible but Or<(With<T>,)> (when used as query rather than filter) does yield a bool indicating whether the T is present or not. This is kinda weird though since With<T> by itself does not do this.

@alice-i-cecile
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Ah right, that's caused by #7680.

@BoxyUwU
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BoxyUwU commented Feb 24, 2023 •
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Option<With<T>> works which is nicer than Or<(With<T>,) imo

@BoxyUwU
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BoxyUwU commented Feb 24, 2023

sidenote: Or does not return bool it returns ()

@wainwrightmark wainwrightmark mentioned this issue Jun 14, 2023
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james7132 pushed a commit to james7132/bevy that referenced this issue Jun 19, 2023
@wainwrightmark @alice-i-cecile
@joseph-gio @james7132
Added Has<T> WorldQuery type (bevyengine#8844)
f673ee4 
# Objective

- Fixes bevyengine#7811 

## Solution

- I added `Has<T>` (and `HasFetch<T>` ) and implemented `WorldQuery`,
`ReadonlyWorldQuery`, and `ArchetypeFilter` it
- I also added documentation with an example and a unit test


I believe I've done everything right but this is my first contribution
and I'm not an ECS expert so someone who is should probably check my
implementation. I based it on what `Or<With<T>,>`, would do. The only
difference is that `Has` does not update component access - adding `Has`
to a query should never affect whether or not it is disjoint with
another query *I think*.

---

## Changelog

## Added
- Added `Has<T>` WorldQuery to find out whether or not an entity has a
particular component.

---------

Co-authored-by: Alice Cecile <[email protected]>
Co-authored-by: JoJoJet <[email protected]>
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Added Has<T> WorldQuery type wainwrightmark/bevy
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@alice-i-cecile @SkiFire13 @BoxyUwU