Shared in memory value update race condition

[jiledaa]

Hi, I subscribe to in memory shared value like:

Reduce { state, action in
    switch action {
    case .initializeCallLog:
        return .publisher {
            state.$callLogs.publisher
                .dropFirst()
                .map(to: .callLogsChanged)

            case .callLogsChanged:
                @SharedReader(.callLogs) var callLogs: IdentifiedArrayOf<CallLog>
                state.localCallLogs = callLogs
                return .none
        }
    }
}

and update it in async context like:

@Shared(.callLogs) var callLogs: IdentifiedArrayOf<CallLogEntry>
for await callLogs in my2NManager.dataSource.observeCallLogs() {
    $callLogs.withLock { $0 = IdentifiedArrayOf(uniqueElements: callLogs) }
}

When callLogs is changed, it triggers ".callLogsChanged" in reducer, but there is still previous value (in my case empty array) in the shared call logs.
Is there some race condition possible? Because when I add .receive(on: mainQueue). It starts working.

[pyrtsa]

I think that's because publisher has willSet semantics, as is common with Combine. The docs could explain that better.

i haven't tried it, but the new Observations (or Perceptions) async sequence probably gets what you want, i.e. didSet.

[mbrandonw]

In addition to what @pyrtsa mentioned, the $callLogs.publisher publisher emits the actual current value (not the previous value available in the willSet), and so if you delivered that to your action you would have the freshest value:

return .publisher {
    state.$callLogs.publisher
        .dropFirst()
        .map(Action.callLogsChanged)

    case .callLogsChanged(let callLogs):
        state.localCallLogs = callLogs
        return .none
}

[jiledaa]

ok, thanks for the answer