event.destination should not be an AppHistoryEntry. How should it behave?

[domenic]

As I started trying to spec event.destination, I realized making it an AppHistoryEntry would not work very well. In particular, only sometimes would we be able to get the right object identity, i.e. the one that matches up with appHistory.entries() or appHistory.current after the navigation. And, adding event listeners doesn't make much sense. And, the semantics of index were strange.

So we should definitely divorce event.destination from the AppHistoryEntry instances that live in appHistory.entries(), and instead make it a more abstraction representation of the destination. I think reusing much of the AppHistoryEntry properties makes sense, but I have some questions for each of them:

• url: OK, well, this is still obvious
• getState(): this seems fine and useful to include
• key: For push navigations, should key be the future key, or should it maybe be null? This is clearly a must for traverse navigations, and is easy to compute for replace navigations.
• id: should this even be included? Is it useful to make the browser figure this out early?
• index: a few options...
  • Don't include this
  • -1 for push navigations, accurate index for other navigations
  • appHistory.current.index + 1 for push navigations, accurate index for other navigations
• sameDocument: If you call event.respondWith(), should this change, or should it always reflect the "original" navigation? (AppHistoryEntry.sameDocument #70)

[tbondwilkinson]

Thoughts:

1. key: If possible, I think it would be useful. But if not, it's not a big deal.
2. id: If possible, I think it would be useful. But if not, it's not a big deal.
3. index: I lean towards not including. I think in general we want to discourage positional-type behavior. If this is a directional navigation, I think you could figure this out using key/id if you really wanted it by looking in appHistory.entries()
4. Tricky! I think it is probably simpler to think of the event Object as a static, immutable thing, rather than as a mutable reflection of state. In that sense, I lean towards not updating it.

[domenic]

I'd love to hear more cases for using key and id (for pushes) in a navigate handler. I dislike the special case of making them null for pushes, but having real use cases would help me justify fixing that. Otherwise, the path of least resistance for spec/implementation is to probably leave them null.

[tbondwilkinson]

I've seen a few use cases of storing the key so that you can detect when the state changes away and take some action. But perhaps they could set up those listeners directly on the entry?

If we're reasonable sure no one wants to correlate an in-progress entry with a serialized-entry, then I think they might not be necessary.

[dvoytenko]

I think it should be whatever appHistory.navigate() accepts. I'd think of it as a not-yet-materialized history entry, so I don't think it'd have an id yet or a key (unless the key has been provided in the navigate() call).

[domenic]

I started working on a PR to add key to "traverse" navigations. However then I thought that for traverse navigations, key/id/index are all on equal footing, and probably we should include all of them if we're going to include any of them. Hmm.

Maybe exclude index because it's really about the entry that lives in appHistory.entries(), whereas we're trying to say that e.destination is not the entry?

Or maybe e.destination should be either an AppHistoryEntry (for traverse) or an AppHistoryDestination (for push/replace/reload)? Reload being on that side feels a bit weird but it seems necessary since appHistory.reload({ state: newState }) means e.destination.getState() needs to be different from appHistory.current.getState()...