New Event/Observer API is unfriendly to Entity-like types

[Zeenobit]

What problem does this solve or what need does it fill?

Hello! I maintain the moonshine-kind crate, which adds the Instance<T> type. The new changes for entity events in Bevy 0.17 make hard assumptions that the underlying target is exactly an Entity and nothing else.

This makes implementation of newtype wrappers around Entity (like Instance<T>) very awkward.

For example:

• The EntityEvent trait demands a &mut Entity access which forces any Entity-like struct to provide mutable entity access. This could be avoided if this trait had a set_event_target method instead of event_target_mut.
• The EntityEvent derive macro expects exactly an Entity type. In my opinion, it should work with any ContainsEntity type.
• The EntityCommands::trigger method demands an impl FnOnce(Entity) constructor function. There should also be a function that lets you just trigger any EntityEvent as-is. I don't see the need to force the user to pass a constructor instead of the event itself.

What solution would you like?

• Refactor EntityEvent::event_target_mut to EntityEvent::set_event_target
• Support ContainsEntity on #[derive(EntityEvent)]
• Refactor EntityCommands::trigger to take an impl EntityEvent, and add trigger_with as the ctor version. trigger_with should also support ContainsEntity.

What alternative(s) have you considered?

N/A

[alice-i-cecile]

I'm fine with those changes: can you add some tests too to ensure that we don't regress? #1634 is near-and-dear to my heart, and the changes seem very sensible.

Split out the EntityCommands::trigger changes: I'd prefer to avoid breaking changes there, and the design is a bit more involved.