Add an inverse to Changed and Added filters #7265

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1. nicopap

   added

   C-FeatureA new feature, making something new possibleA new feature, making something new possible

   A-ECSEntities, components, systems, and eventsEntities, components, systems, and events

   on Jan 18, 2023
2. djeedai commented on Jan 18, 2023

   

   djeedai

   on Jan 18, 2023

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   Should we consider a generic Not<>? I don't know how feasible this is though.

   👍React with 👍2Reacted by Robin KAY and James Liu
3. james7132 commented on Jan 18, 2023

   

   james7132

   on Jan 18, 2023

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   Should we consider a generic Not<>? I don't know how feasible this is though.

   Probably would just be a simple Not<T: ReadOnlyWorldQuery> that just inverts the output on filter_fetch. It probably should be noted that it wouldn't be an archetypal filter and would run against the split between With/Without.
4. james7132

   added

   S-Ready-For-ImplementationThis issue is ready for an implementation PR. Go for it!This issue is ready for an implementation PR. Go for it!

   on Jan 18, 2023
5. CatThingy commented on Jan 20, 2023

   

   CatThingy

   on Jan 20, 2023

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   I took a stab at Not<T> in #7271, but ran into some issues I could not resolve. matches_component_set has different "negation behaviours" for With(out) and Added/Changed: for With(out), it must be negated; for Added/Changed, it must not be. If individual implementations were implemented for the four "inner" filters, Or then becomes impossible to generically negate as it's possible to mix With(out) and Added/Changed within an Or. One possible solution moving forward is to have an additional trait that defines the negation behaviour.

   My implementation also had Not<Added<T>> imply With<T>, which is not a logical negation of Added<T>.
6. alice-i-cecile

   added

   D-ComplexQuite challenging from either a design or technical perspective. Ask for help!Quite challenging from either a design or technical perspective. Ask for help!

   and removed

   S-Ready-For-ImplementationThis issue is ready for an implementation PR. Go for it!This issue is ready for an implementation PR. Go for it!

   on Feb 27, 2023
7. ickk commented on Aug 10, 2023

   

   ickk

   on Aug 10, 2023

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   As a user-story, today I tried to do something like the following in order to filter out all the entities that had a component added in the same frame.

   Query<&T, Without<Added<T>>>

   I was somewhat surprised that this was not supported.

   ---

   Having set-like operations defined on Query could also be useful to solve similar problems instead with run-time compositions, i.e.

   fn system(
     a: Query<Entity>
     b: Query<Added<&T>
   ) {
     // set difference; C is the set of all entities that satisfy A but 
     // with the entities that satisfy B removed
     let c: Query<Entity> = a - b;
   }
8. maniwani commented on Aug 10, 2023

   

   maniwani

   on Aug 10, 2023

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   NotChanged<T> and NotAdded<T> return all entities that Changed<T> and Added<T> do not.

   Changed<T> and Added<T> have an implicit With<T>. Inverting their output won't change that. They'll return the entities that have a T that wasn't recently changed or added. Their inverses should not include entities matched by Without<T>.

   My implementation also had Not<Added<T>> imply With<T>, which is not a logical negation of Added<T>.

   This would be much clearer if Added<T> and is_added were named New<T> and is_new (which is what it means). IMO if you ask for "entities whose T is not new", it doesn't make any sense to include entities that don't even have T.

   It's not the complementary set of entities, but the implied With<T> is intuitive.