State conformance to ObservableState store runtime diagnostics

[maximkrouk]

Just got SIGABRT crash while playing with observation-beta

Can be reproduced in Example project here.

Steps to reprocuce:

• Launch the app
• Open first user profile
• Go back
• Open first tweet details

Console error output:

Could not cast value of type 'Swift.Optional<()>' (0x1c1d89568) to 'ProfileFeature.ProfileFeature.State' (0x1032fe4e0).

Some notes from the callstack:

Store.send(_:originatingFrom:)
withoutPerceptionChecking(_:) // Kinda hints too look for missing `ObservableState`s, but it's not obvious at all
...
Store.init(rootStore:toState:fromAction).subscribeToDidSet()
PartialToState.keyPath.callAsFunction(_:) // on failure Xcode navigates to here (`return state❌[keyPath: keyPath] as! State`)

I already solved it by accidentally finding that I didn't mark ProfileTabFeature.State as ObservableState, but none of errors were helpful for finding whats wrong...

I think the MVP for the fix could be just a simple comment in PartialToState.keyPath.callAsFunction(_:) that "if you got a SIGABRT crash here, probably you forgot to add @ObservableState to one of your features"

Ideally it would be cool to use some failure with a message containing specific type that needs to conform to ObservableState, but I don't see how to do that without a bunch on reflection (which will probably drain performance, but probably it's ok for debug builds)

[mbrandonw]

Hi @maximkrouk, thanks for reporting this. Unfortunately this is due to Swift bug in key paths. We filed this yesterday and I just pushed a fix (60b5fe4) for the part that you were running into.

I was also able to verify that marking FeedTabFeature.State with @ObservableState did somehow fix the issue, but that is completely mystifying to me. FWIW, this crash should not happen at all, regardless of using @ObservableState or not. And so we don't think we need to create any documentation at the crash site. It's just a Swift bug that will hopefully be fixed sometime soon.

[maximkrouk]

but that is completely mystifying to me

I find this part of the reply really hilarious 😂

We filed swiftlang/swift#70611 yesterday and I just pushed a fix (60b5fe4) for the part that you were running into.

👍

[mbrandonw]

Also something stood out in your demo is that you have some SwitchStores, ForEachStores, etc that can all be done away with using the new observation tools.

[maximkrouk]

I began to write the demo before updating to observation-beta, today I finished shaping the project and gonna start scaling it with new features and refactoring existing ones tomorrow 👌

But it's nice that having those Stores didn't affect migration 👍