Add deprecation guide for Ember.Evented and @ember/object/events

[wagenet]

RFC: emberjs/rfcs#1111

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[NullVoxPopuli]

also, maybe

on(event, callbark) {
  return this.#emitter.on(event, callbark);
}

[NullVoxPopuli]

but what you have is easier to be type-safe

[bendemboski]

Oof, for any application at scale this is going to be a pretty huge change. Switching from synchronous events to asynchronous events is almost certain to not just be a drop-in replacement like this, and this deprecation guide doesn't mention that at all.

It looks like this deprecation guide is simultaneously doing two things:

1. Describing the deprecation of Evented, and providing a possible path away from it
2. Recommending that users make the architectural change of switching from synchronous event emitting to asynchronous event emitting.

I think these really need to be called out separately. Users should be aware that if they just follow this deprecation guide mechanically they'll be changing timing characteristics of their application in ways that can (and in many cases almost certainly will) break things. So I think we need to be more explicit that we're both recommending a new library to use and the change from synchronous to asynchronous events, and ideally offer up a recommendation of a synchronous eventing library for users that aren't ready to make that change but still need to deal with this deprecation.

[wagenet]

Thanks for this callout. I would have thought existing events are async via the runloop. However, if we're sure this isn't the case, then we can definitely call it out in this guide.

[bendemboski]

Observers are async by default, @ember/object/events and Evented are synchronous

[wagenet]

I've updated the guides to make this clear.

[bendemboski]

Is there a deprecation plan for @ember/object/events? The RFC referenced here (and the only open one I can find) is for Evented, and if all we are doing is deprecating Evented then why would the recommended path not be to move to @ember/object/events? Perhaps this is trying to be forward-looking to a time when we also deprecate @ember/object/events?

But deprecating @ember/object/events will also include changes to the framework since it is used for events such as the Router sending routeWillChange events...is the plan to remove those events entirely and replace them with something else? To make them asynchronous?

This guide seems to be addressing cases where Evented and @ember/object/events are used to send events from application code to application code, but I'm having trouble even thinking about that without not knowing the story for sending events from the framework to application code, so I'd think the deprecation guide would need to address that as well...

[wagenet]

@bendemboski I've updated the RFC to match this deprecation guide.