Why can't I get passthrough subjects to work for combine?

user_8591381 0
Viewed 98

Just getting into Combine, and for some reason I can't get passthrough subjects to work. Even though I have copy-pasted examples from multiple different sources, they just won't print anything. I have tried with Publishers and CurrentValueSubjects and they work fine, but with PassThroughSubjects; nope. Here's an example that I have tried:

let mySubject = PassthroughSubject<String, Error>()

mySubject.sink(receiveCompletion: { completion in
        print("-- completion", completion)
    }, receiveValue: { value in
        print("-- value", value)
}).cancel()

mySubject.send("one")
mySubject.send("two")
mySubject.send("three")

This is run in viewDidLoad. What am I doing wrong?

Like I said, I have tried Publishers and CurrentValueSubjects with success:

["one", "two", "three"].publisher
    .sink(receiveValue: { v in
        print("-- hello", v)
}).cancel()

let subject = CurrentValueSubject<String, Error>("Initial Value")

subject.send("Hello")

subject.sink(receiveCompletion: { c in
    print("-- completion", c)
}, receiveValue: { v in
    print("-- value", v)
}).cancel()
combine swift ios
2 Answers

The warning that you are seeing that the subscription is unused is a hint to store the token returned by sink like so:

let mySubject = PassthroughSubject<String, Error>()

let cancellable = mySubject
    .sink(
        receiveCompletion: { completion in
            print("-- completion", completion)
        },
        receiveValue: { value in
            print("-- value", value)
        }
    )

then when you call:

mySubject.send("one")
mySubject.send("two")
mySubject.send("three")

you will see this printed out:

-- value one
-- value two
-- value three

you can cancel the subscription if you are not longer interested in receiving updates:

cancellable.cancel()

or you can send a completion:

mySubject.send(completion: .finished)

and then you will see this printed out:

-- completion finished
user_79483720

Coming from rx I imagined .cancel() worked like .dispose(by:)

No, cancel() is like dispose(), not disposed(by:) in rx. You should not cancel first, then send things to the subject. And unlike a CurrentValueSubject, it doesn't remember the value you sent it, so you must send values to it after you sink, but before you cancel.

Just like how you would use a DisposeBag in rx, you should do this with a Set<AnyCancellable> in Combine:

var cancellables: Set<AnyCancellable> = []

The Combine counterpart of disposed(by:) is store(in:):

subject.sink(receiveCompletion: { c in
    print("-- completion", c)
}, receiveValue: { v in
    print("-- value", v)
}).store(in: &cancellables)

subject.send("Hello")
user_51335850
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