Better ergonomics for inline @Shared updates

[jegnux]

If we want to modify the value of a shared state "inline" (from a function) for test or previews, we have to go through this syntax:

func testFeature() {
  @Shared(.appStorage("count")) var count = 42
  count = 42  // NB: Set again to override any value set by the app target.


  // Shared state will be 42 for all features using it.
  let store = TestStore(…)
}

And if we want to restore the default value of a PersistenceKeyDefault there's no real way to do it as

swift-composable-architecture/Sources/ComposableArchitecture/SharedState/PersistenceKey/PersistenceKeyDefault.swift

Line 27 in 8a40e93

let defaultValue: () -> Base.Value

is internal.

I would like to introduce 3 new APIs to provide better ergonomics to Shared:

extension Shared {
    public static func reset<T: PersistenceKey>(_ key: PersistenceKeyDefault<T>) where T.Value == Value {
        Self.set(key.defaultValue(), for: key)
    }

    public static func set<T: PersistenceKey>(_ value: Value, for key: PersistenceKeyDefault<T>) where T.Value == Value {
        Self.update(key) {
            $0 = value
        }
    }
    
    public static func update<Key: PersistenceKey>(_ key: Key, handler: (inout Key.Value) -> Void) where Key.Value == Value {
        guard let shared = try? Shared(key) else {
            return
        }
        handler(&shared.wrappedValue)
    }
}

So we can write:

func testFeature() {
  Shared.update(.appStorage("count")) {
    $0 = 42
  }

  // Shared state will be 42 for all features using it.
  let store = TestStore(…)
}

Or:

func testFeature() {
  Shared.set(42, for: .appStorage("count"))

  // Shared state will be 42 for all features using it.
  let store = TestStore(…)
}

Or:

extension PersistenceKey where Self == PersistenceKeyDefault<AppStorageKey<Int>> {
    public static var sharedCount: Self {
        .init(.appStorage("some_shared_count"), 42)
    }
}

func testFeature() {
  Shared.reset(.sharedCount)

  // Shared state will be 42 for all features using it.
  let store = TestStore(…)
}

If the team/community doesn't think it improves the Shared api ergonomics enough to add this to the library, can we at least make PersistenceKeyDefault.defaultValue public, so I (or anyone) can write these extensions in my project?

[jegnux]

Of course, if we introduce this in the library, we can consider other naming or error management (update func currently try? but we can rethrows the error)

[jegnux]

An alternative could be to add a reset() requirement to PersistenceKey so all keys can have a way to reset themselves (delete file, remove user defaults, etc)

[stephencelis]

@jegnux Generally in tests and previews you should not have to reset shared state so long as you follow this documented gotcha: https://pointfreeco.github.io/swift-composable-architecture/main/documentation/composablearchitecture/sharingstate#Previews

And so your first example becomes simply:

func testFeature() {
  @Shared(.appStorage("count")) var count = 42

  // Shared state will be 42 for all features using it.
  let store = TestStore(…)
}

And as such there should also be no need to call reset.

If you prefer the ergonomics of your helpers, you can totally define them in your own projects, and if so we could consider making defaultValue public, but I think we'd like to keep the library surface area a bit leaner. What do you think?

[jegnux]

@Shared(.appStorage("count")) var count = 42

I personnaly don't like it much because it uses the "default" syntax to set the value and rely on an implementation detail that I don't want to care about in tests, previews or elsewhere.

Also, my use case is beyond tests and previews, as I build a Debug Sheet in my application where I want to be able to inspect and reset my Shared states (see below, although for now it only just reset everything)

Actually, it would be really great for my use case to add a reset() requirement on all PersistenceKey because right now I'm only able to reset a PersistenceKeyDefault to its default, but the underlying key is not reset, it only replaces its current value with the default value which is saved/persisted on disk/user defaults/etc. With a reset() requirement I would have a real reset behavior on any key. But T can accommodate having only a public defaultValue for now.

[stephencelis]

@jegnux Thanks for the context!

rely on an implementation detail that I don't want to care about in tests, previews or elsewhere

I don't think we consider this an "implementation detail" since it's not some internal thing, but just the way the feature is expected to and must behave.

Your debug sheet is a cool example, but I think the lack of public defaultValue is the only thing preventing you from implementing it, correct? As mentioned earlier, we'd definitely be open to making this public, or at least @_spi-available in some capacity for now.

[jegnux]

As mentioned earlier, we'd definitely be open to making this public

Cool! What's the process? Can I simply open a PR?

[stephencelis]

Sure, let's start with that!