Add support for custom picking data

[m-edlund]

Objective

• Adds the ability for picking backends to add custom hit data
• Fixes Extend the information included in HitData #16186

Solution

• Adds an extra field to the HitData struct which can take any Data that implements HitDataExtra. This is stored in an Arc which can be downcast with a helper function in the system that consumes the hit.
• In the original ticket I suggested using a generic, however this caused extensive code changes and complications down the line with the HoverMap and OverMap so I decided against it.

Testing

• I added an example custom_hit_data to test the new feature
• I tested all other picking examples to ensure they still work as expected
• I tested the examples on Ubuntu
• I additionally tested the custom_hit_data example in wasm

---

Showcase

Click to view showcase

Creating custom hits:

let picks: Vec<(Entity, HitData)> = ray_cast
            .cast_ray(ray, &settings)
            .iter()
            .map(|(entity, hit)| {
                let extra = TriangleHitInfo {
                    triangle_vertices: hit.triangle,
                };

                let hit_data = HitData::new_with_extra(
                    ray_id.camera,
                    hit.distance,
                    Some(hit.point),
                    Some(hit.normal),
                    extra,
                );

                (*entity, hit_data)
            })
            .collect();

Reading custom hits:

   for hits in pointer_hits.read() {
        for (_, hit) in &hits.picks {
            let Some(info) = hit.extra_as::<TriangleHitInfo>() else {
                continue;
            };
            let Some(vertices) = info.triangle_vertices else {
                continue;
            };

            // do something cool with your custom hit data
        }
    }

An example of what you can do with this

[kfc35]

This is cool. I approve. I checked out the branch and the example works.

The overarching comment I have is that the example can be improved to be more beginner friendly for comprehension, since examples serve as documentation of sorts and that code is accessible for people to read. I made a bunch of smaller comments to help with that.

[m-edlund]

Is #23285 considered a blocker? The new example uses no Interaction, so I assumed it should be independent of the changes in that PR.

[kfc35]

@m-edlund I don’t think it’s considered a blocker. As someone who reviewed that PR, I think the author may have been mistaken.

Also, I saw your message on Discord regarding this PR. Sorry others haven’t been able to review! I think after the 0.19 milestone is cut, people will have more bandwidth to consider reviewing this. I’m not sure if this will get merged because we are only focusing on merging bug fixes currently, but I’ll try bumping your message one time to see if we can get anyone else to look at the PR. Since I don’t think it would add that much complication, maybe it can be considered.

[HackerFoo]

Why doesn't this check extra? This should be documented and justified in that documentation.

It looks like PartialEq is derived for all events, I'm not sure if they are deduplicated somewhere.

[m-edlund]

Equality would require downcasting both sides every time there is a comparison (https://users.rust-lang.org/t/how-to-compare-two-trait-objects-for-equality/88063), which feels overkill. The explanation was added in e7c410d

[alice-i-cecile]

Should this have a name method or similar to help aid debugging / figure out what to downcast to?

[m-edlund]

Since HitDataExtra requires Debug, devs can just print the hit data to help with debugging at runtime. So unless I'm missing something, a name function feels like it just duplicates what Debug is already doing.

[alice-i-cecile]

On board with the motivation. The prior art was really compelling, thanks! Good docs, broadly solid implementation. Fixes needed:

• the upcasting strategy needs cleanup
• I would like a unit test
• the choice of Arc is good but needs clear documentation

Other feedback is non-blocking, but I am curious about your thoughts :)