Why does Combine assign crash when writing to an optional property?

Question

This code crashes ("Unexpectedly found nil while unwrapping an Optional value")

Code Block import Combine
class Receiver {
    var value: Int!
    var cancellables = Set<AnyCancellable>([])
    init(_ p: AnyPublisher<Int,Never>) {
        p.assign(to: \.value, on: self).store(in: &cancellables)
    }
}
let receiver = Receiver(Just(5).eraseToAnyPublisher())

It does not crash if I use

Code Block p.sink { self.value = $0 }.store(in: &cancellables)

instead of the assign, and it does not crash if I do not use an optional for the value-property.

To me this looks like a bug in Swift's constructor code, but maybe I am overlooking something?

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Posted by

kristofv

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Answer 1

I believe it is because you are using an implicitly unwrapped optional value with Int!, which means that you are guaranteeing it will have a value before value is accessed.

To prevent the crash you should just assign an initial value to it.

var value: Int! = 0

That may or may not work for your situation.

Posted by

psolt-gpsw

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Why does Combine assign crash when writing to an optional property?

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