Upstream and use bevy_mod_picking

[alice-i-cecile]

Why we need picking (and raycasting)

To be glib, bevy_mod_picking by @aevyrie allows users to point at things and click on them.
Picking is the pattern of turning pointer events (like mouse clicks or touches) into the hover/selection/deselection of objects (meshes, sprites, UI...) via hit testing, of which raycasting is one common form.

Bevy needs an equivalent set of tools for:

1. General gameplay usage: raycasting is incredibly useful for everything from physics to line-of-sight to selecting units
2. To replace our bespoke and inconsistent UI Interaction component and focus handling
3. As an essential part of any balanced breakfast scene editor or interactive debugger

While the first application can and currently is solved by relying on an external crate, this is still limiting because:

1. It puts pressure on the maintainer to continue to promptly update a vital dependency.
2. It allows us to accidentally break a vital dependency on main, as it's not part of our CI.
3. It worsens the new user experience, as they have to find and figure out external dependencies.
4. It prevents us from building more realistic examples.

For the remaining use cases, we simply can't rely on an external crate that relies on Bevy due to circular dependencies which present severe technical and organizational challenges.

Why we should upstream rather than re-implement

Of course, the next question is "why should we upstream a crate, rather than simply building it ourselves!".
After all, it wasn't invented here, for a very narrow definition of "here".

However, as I previously discussed in bevyengine/rfcs#63, reinventing rather than upstreaming popular, functioning crates is both risky and wasteful.
While this functionality may seem simple at first glance, hard-won experience shows that there are a large number of ergonomic, design and performance concerns that must be carefully navigated.

bevy_mod_picking is a stable, beloved part of Bevy's ecosystem. While it may not be perfect in its current form, it decidedly works: with good performance, a lovely API and a generally low-bug experience. Building our own equivalent will take longer, involve dramatically more controversy, and leave us at a worse starting point.

Organizational strategy for upstreaming

I've been working closely with @aevyrie and other members of the community to plan this migration.
As part of that, we have a pair of junior volunteers from Purdue: Avi and Xavier who are helping us with this work (this issue is a sibling to #12349). Please be patient and kind with them as they work, but remember that they're here to learn and improve.

During this discussion, we've outlined a few key strategies to minimize the pain of this complex migration:

1. Identify blockers and perform vital improvements in the upstream crate(s) before starting the migration PR. Once the entire crate(s) are updated, we can begin upstreaming.
2. Split up the code into sizable and useful components, and upstream those in chunks. These PRs should be merged with minimal changes, to avoid stalling in review and accidentally introducing incompatibilities between our components.
3. Perform code quality and documentation work on individual components in small PRs once they're merged. Do not make breaking changes here until the entire system is migrated, tested and integrated.
4. Integrate the various components into the engine as they added, one area at a time, removing existing redundant solutions if we can do so without making breaking changes to the fragile new code.
5. Once everything is integrated, we can take a look at how it behaves in context, and can then discuss more serious architectural refactors and small breaking changes.

The essential problems to avoid are:

1. Complex PRs getting stuck in review and accumulating merge conflicts.
2. Making a release of Bevy where the old solution is removed and the new solution is not yet ready.
3. Making breaking changes part way through the migration, breaking the integration of other components.

If you have breaking concerns or ideas, please, please open issues to record them (and link this issue). We want to hear them, and to improve all areas of the engine: we just don't want the migration to fall into disarray :)

Technical pathway to upstreaming

• upstream bevy_eventlistener, a reactive programming solution used to dispatch and bubble events up hierarchies that is used by bevy_mod_picking.
  • pick a name for the new crate: bevy_event_dispatch by @viridia has won the bikeshed contest
  • Add the ability to have multiple callbacks in a single listener aevyrie/bevy_eventlistener#21
  • actually upstream this crate
• adapt bevy_mod_picking to the new version of bevy_event_listener
• upstream bevy_mod_picking's components into a new bevy_picking crate
  • intermediate focus layer
  • pointer and pointer event abstractions
  • add a convenient selection plugin
• integrate bevy_mod_picking
  • add a frontend for Bevy inputs
  • add a backend for UI picking
  • add a crude CPU backend for [sprite picking]*https://github.com/aevyrie/bevy_mod_picking/tree/main/backends/bevy_picking_sprite)
  • add a crude CPU backend for mesh picking based on https://github.com/aevyrie/bevy_mod_raycast
  • add backend(s?) for primitives picking e.g AABB based on https://github.com/aevyrie/bevy_mod_raycast
  • eliminate bevy_ui's focus system and Interaction
• add aggressive tests and docs
  • bevy_event_dispatch
  • bevy_picking
  • raycasting functionality
  • highlight dev tool and example

Ideas for breaking improvements

• swap bevy_event_dispatch over to observers (Generalised ECS reactivity with Observers #10839) to enable endlessly loopable reactivity as discussed here

Related work

• Initial HackMD notes
• High Level Physics Api #83
• Raycasting #615
• Smart method to get Entity when hovering it with mouse for 2d  #3805
• First-class technique for splitting input streams or absorbing input events #3570
• refactor and test ui_focus_system #4264
• Add support for detecting release of a button #5769
• UI- Interaction Released state for nodes #8157
• Add event for clicks #10141

[aevyrie]

Thank you Alice, reading through this now. 🙂

Small correction of a common misconception. Picking does not require raycasting, that is just one of the hit testing methods.

[brandon-reinhart]

I use both crates extensively and have written extensions in my own game. I will keep an eye on this and will be available for code reviews. Excited to see this start!