Component Lifecycle Terms

[xentripetal]

Hey I really like the direction you're taking this library! I've been bouncing between different ECS libs and this seems like a great option.

It would be very helpful if queries had support for defining filters based on the lifecycle of components.

Mainly:

1. Changed<T> - Evaluates to true when component T was accessed mutably or set for the entity since the last execution of the query

2. Added<T> - Evaluates to true when component T was added to an entity since the last execution of the query

I'm not fully sure how bevy does this but it seems like they store a vec of watermarks alongside each column that gets updated each time an entry in the column is accessed mutably or added. Then each query tracks a watermark of when it was last ran it uses for comparison.
https://github.com/bevyengine/bevy/blob/main/crates/bevy_ecs/src/change_detection.rs
https://bevy-cheatbook.github.io/programming/change-detection.html

3. Removed<T> - Evaluates to true when component T was removed from an entity since the last execution of the query

In bevy, removals publish out an event that systems can read if they run the next frame https://github.com/bevyengine/bevy/blob/main/crates/bevy_ecs/src/removal_detection.rs

[xentripetal]

Also wanted to give you a heads up that I posted a MR benchmarking this lib Doraku/Ecs.CSharp.Benchmark#29

Showed really great results

[andreakarasho]

On my list!
but not sure what would be the right strategy to apply honestly, because components access is always by ref. So everytime you access ref entity.Get<T>() (or when quering) it will set the component as Changed<T> [?]

[xentripetal]

Hmm, yeah adding variations for ref vs non ref would make the query gen way too large.

I wonder if you could introduce wrapper component types like

public ref struct ChangeTracked<T> where T : struct
{
    public ChangeTracked(Ref<T> value)
    {
        _value = value;
    }

    private Ref<T> _value;
    public bool Changed = false;

    public T Value
    {
        get => _value.Value;
        set
        {
            _value.Value = value;
            Changed = true;
        }
    }
}

that would be automatically handled by the query system to resolve down to T ComponentInfo and update the cell watermark when changed

[andreakarasho]

I'm not sure about that.
I put on the table this too: https://github.com/SanderMertens/flecs/blob/master/docs/Queries.md#change-detection

[xentripetal]

I've started working on a POC where that's stored alongside the component array for chunks
xentripetal@8dca1e2#diff-58f635af42e8d7ed9ebdcb7026ea8163e9fda76b51eefc446bb8ca91fe50a015R11

Add tracking is easy, but I'm not sure how to do the Change notifications either. I think your only options are

1. Introduce a wrapper type for components
2. Have a manual NotifyChanged method you can do on the EntityView
3. Support non-ref struct queries

[xentripetal]

Though you likely want some way for a query to denote what components it reads vs writes, that way you can start tracking the dependencies of systems for parallelization. So IMO if theres a path to 3, thats the best option.

I believe flecs does it with a NotifyChange and supports parallelization via manually annotating queries

[andreakarasho]

More a proof of concept than anything, not sure if it will work with the full functionalities set like bevy:
main...feat/change-detection

[xentripetal]

Nice! How does that Removed flag work? If it gets flagged on the old archetype then the old arch row is marked as invalid and won't track anything

[andreakarasho]

Pushed more commits.
Looks like it starting work. Unfortunately I have no clue how to implement the Each<> methods.

Good catch about the removed component. I didn't realize until now that you just posted the solution with your first post! Basically bevy uses the events under the hood