`@Shared` with `appStorage` doesn't update in extensions

[bcapps]

Description

Recently, we were attempting to use @Shared(.appStorage(key)) with several TCA reducer properties in a widget extension, having already used it successfully in the parent app. We updated dependencies.defaultAppStorage to our own UserDefaults with the correct suite in both the main app and extension, and while this allowed the correct value to be read into each property in the widget initially, the extension never receives updates when the main app writes to the defaults.

While tracing down this issue, we found that it was due to the implementation of AppStorageKey: it's using UserDefaults.didChangeNotification to observe changes and update the value, but this notification is documented to not work across processes. Therefore, the values in the extension never update as expected.

The documentation points to KVO as the solution for observing changes even across processes, but it has the very annoying issue that keys with periods won't work due to key paths using periods differently. I tried a solution of this nature and failed on the included test with a key containing periods. So I'm not sure what the correct answer is here, but I think it is somewhat expected that @Shared(.appStorage(key)) properties in an extension with a shared app group would update so I'm filing this issue.

Checklist

• [X] I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• [X] If possible, I've reproduced the issue using the main branch of this package.
• [X] This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

@Shared(.appStorage(key)) properties on TCA reducers used in extensions stay in sync with the main app values when defaultAppStorage uses UserDefaults(withSuite:)

Actual behavior

@Shared(.appStorage(key)) properties on TCA reducers used in extensions are correct initially, and then are out of sync with the main app values when defaultAppStorage uses UserDefaults(withSuite:)

Reproducing project

No response

The Composable Architecture version information

main

Destination operating system

iOS 18

Xcode version information

Xcode 16

Swift Compiler version information

swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)