CombineExt provides a collection of operators, publishers and utilities for Combine, that are not provided by Apple themselves, but are common in other Reactive Frameworks and standards.
withLatestFrom doesn't seem to be thread safe when subscribing to two different publishers that emit elements from different threads.
This is part of the current implementation of withLatestFrom. Check the comments in uppercase.
private func trackLatestFromSecond(onInitialValue: @escaping () -> Void) {
var gotInitialValue = false
let subscriber = AnySubscriber<Other.Output, Other.Failure>(
receiveSubscription: { [weak self] subscription in
self?.otherSubscription = subscription
subscription.request(.unlimited)
},
receiveValue: { [weak self] value in
guard let self = self else { return .none }
// THIS IS CALLED FROM THREAD B
self.latestValue = value
if !gotInitialValue {
// When getting initial value, start pulling values
// from upstream in the main sink
self.sink = Sink(upstream: self.upstream,
downstream: self.downstream,
transformOutput: { [weak self] value in
guard let self = self,
// THIS IS CALLED FROM THREAD A, ACCESSING THE SAME VAR BUT IT IS NOT SYNCHRONIZED
let other = self.latestValue else { return nil }
return self.resultSelector(value, other)
},
transformFailure: { $0 })
// Signal initial value to start fulfilling downstream demand
gotInitialValue = true
onInitialValue()
}
return .unlimited
},
receiveCompletion: nil)
self.second.subscribe(subscriber)
}
Should a lock over latestValue be added to ensure thread safety? Are there reasons not to do it (for example, performance)? Should CombineExt at least update the Swift docs saying it is not supposed to be used from multiple threads?
withLatestFrom doesn't seem to be thread safe when subscribing to two different publishers that emit elements from different threads. This is part of the current implementation of withLatestFrom. Check the comments in uppercase.
Should a lock over latestValue be added to ensure thread safety? Are there reasons not to do it (for example, performance)? Should CombineExt at least update the Swift docs saying it is not supposed to be used from multiple threads?
Thank you.