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Mark observer as nonisolated and unsafe in MainActor #1613

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Mark observer as nonisolated and unsafe in MainActor #1613

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8 changes: 7 additions & 1 deletion Sources/FoundationEssentials/NotificationCenter/MainActorMessage.swift
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Expand Up @@ -225,7 +225,13 @@ extension NotificationCenter {
}
})
#else
return ObservationToken(center: self, token: _addObserver(Message.name, object: subject, using: observer))
nonisolated(unsafe) let observer = observer
return ObservationToken(center: self, token: _addObserver(Message.name, object: subject) { (message: Message) in
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@cthielen cthielen Nov 21, 2025

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I'm uncertain about the tradeoff of introducing the runtime cost of a closure to silence a false positive concurrency warning. (We know @MainActor closures are called via @MainActor post()s, so we're not actually sending or losing main actor as the warnings imply, but the compiler doesn't know that.)

Two other options come to mind:

  1. Use unsafeBitCast to simply change the signature of the closure, e.g. let unsafeObserver = unsafeBitCast(observer, to: (@Sendable (Message) -> Void).self), then pass unsafeObserver instead of observer. I think this is safe because we're only modifying annotations with no runtime representation.
  2. A cleaner solution might be for the internal registrar that _addObserver() ultimately works with to take an enum of the two possible closure types instead. That, though, has a little bit of a memory cost that could become substantial in extreme cases where NotificationCenter has 10,000+ observers, though I don't think that's the typical case.

nonisolated(unsafe) let message = message
MainActor.assumeIsolated {
observer(message)
}
})
#endif
}

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