Trigger events with observer

[patrickariel]

Adds observer triggers on top of normal events so that users can now use observers to listen to tween events (this is similar to the approach used upstream).

This adds a new Clone constraint to tween_event_taking_system, because the event now needs to be moved to both the observer and the event writer.

[Multirious]

Thanks for the PR! Now I'm not sure if adding the Clone constraint is to taking-event appropriate. It's originally made for events that do not implemented Clone. Though I have no idea how common that usages is. We may need to do more researches regarding this but if you have any opinions please do tell!

[patrickariel]

I personally don't use it, so I'm not really sure if there are any legitimate use-cases for it. Another downside is that EntityEvent requires Clone, which means that non-clone events can't benefit from it. Maybe it could be removed?

[Multirious]

We probably could probably remove the entire thing entirely so TweenEvent wouldn't need to always have Option inside them.

[patrickariel]

Ok, removed the event taking system and the Option<Data> thing!

[rendaoer]

In fact, the trigger object here is not reasonable. Using animation() to add animation will create a sub-entity. The entity of the trigger here is the sub-entity, but it is difficult to obtain the id of this sub-entity when you listen.

[Multirious]

@rendaoer One way to solve this is via a custom event function/combinator.

pub fn my_event<Data>( event_data: Data, entity_id: &mut Option<Entity>, // Or use Entity::PLACE_HOLDER ) -> impl FnOnce(&mut AnimationCommands, &mut Duration) where Data: Send + Sync + 'static, { move |a, pos| { let entity = a.spawn(( TimeSpan::try_from(*pos..=*pos).unwrap(), TweenEventData::with_data(event_data), )); *entity_id = Some(entity.id()); } }

I have deliberately made AnimationCommands spawns entity individually instead of stuffing them all inside a custom Command (Which will result in the entity id only accessible after command flush).