PassthroughSubject + flatMap cannot be called more than once?

Question

If I send to PassthroughSubject<Void, Error>, this process will run once, but if I send it more than once, the process in the flatMap will not run. Why is this?

var testTappedSubject = PassthroughSubject<Void, Error>()

testTappedSubject
    .print("testTappedSubject")
    .flatMap({ () -> AnyPublisher<Int, Error> in
        print("called test")
        return Fail(error: LoginError.someError(error: .unknown))
                .eraseToAnyPublisher()
    })            
    .sink { error in
        print("error", error)
    } receiveValue: { value in
        print("pressed", value)
    }
    .store(in: &cancellables)

Answer 1

That's the way Combine (and other Reactive frameworks) work.

you are setting up a subscription to a publisher, and the only thing that subscription does is emit an error:

return Fail(error: LoginError.someError(error: .unknown))
                .eraseToAnyPublisher()

When a publisher emits an error. the subscription completes and does not receive any more events.

I put a version of your code in a playground:

import Combine

var cancellables = Set<AnyCancellable>()

var testTappedSubject = PassthroughSubject<Void, Error>()

enum LoginError: Error {
    case unknown
}

testTappedSubject
    .print("testTappedSubject")
    .flatMap({ () -> AnyPublisher<Int, Error> in
        print("called test")
        return Fail(error: LoginError.unknown)
            .eraseToAnyPublisher()
    })

    .sink { error in
        print("error", error)
    } receiveValue: { value in
        print("pressed", value)
    }
    .store(in: &cancellables)

testTappedSubject.send()
testTappedSubject.send()
testTappedSubject.send()

and the results in the console were:

testTappedSubject: receive subscription: (PassthroughSubject)
testTappedSubject: request unlimited
testTappedSubject: receive value: (())
called test
error failure(__lldb_expr_72.LoginError.unknown)
testTappedSubject: receive value: (())
testTappedSubject: receive value: (())

This shows that an unlimited subscription was received, and then, after sending a value, your print("called test") is called and then an error is received.

The subscription is now complete.

Sending more values just shows that a value was received, but nothing was sent to the subscriber.