Converting Publisher's error type to Never in Combine

Question

I have a Publisher

var subject = PassthroughSubject<Int, Error>()

I want to convert it to

PassthroughSubject<Int, Never>

Is there anyway to achieve this ?

Edit - More Details

I do not want the Publisher to complete and the linked answer did not work because catch still completes the publisher.

Why a `PassthroughSubject` specifically? Does an `AnyPublisher` not work for you? If it does, see [this](https://stackoverflow.com/questions/58227096/set-a-given-publishers-failure-type-to-never-in-combine). — Sweeper, Sep 08 '21 at 00:44
Because my code is using PassthroughSubject but of course that can be changed. I have already tried this link. The problem is that the catch still requires another publisher. What i want is just to somehow ignore all the error and convert Error to Never so that it can be used downstream. — dev_ios999, Sep 08 '21 at 00:49
Catch the error if one comes. Now you can turn the downstream type into whatever you like. https://www.apeth.com/UnderstandingCombine/operators/operatorsErrorHandlers/operatorscatch.html — matt, Sep 08 '21 at 01:19
@matt From your link "The Output type of the publisher must match the Output type of the upstream pipeline." If i have understood it correctly that means cannot be converted to for downstream. However this is exactly what i want or am i misunderstanding. — dev_ios999, Sep 08 '21 at 01:24
I want to convert to so the downstream subscribers only see and the publisher continues publishing. — dev_ios999, Sep 08 '21 at 01:26
The output type is the same (`Int` in your example), not the error type. You may need to combine catch with `share` https://www.apeth.com/UnderstandingCombine/operators/operatorsErrorHandlers/operatorsretry.html — Paulw11, Sep 08 '21 at 01:54
I really don't know what you mean "cannot be converted to ``". That is exactly what you _can_ do. — matt, Sep 08 '21 at 02:18

score 2 · Accepted Answer · answered Sep 08 '21 at 03:47

Here's an example of using catch to turn an <Int, Error> into an <Int, Never>:

import UIKit
import Combine
class ViewController: UIViewController {
    let subject = PassthroughSubject<Int, Error>()
    var storage = Set<AnyCancellable>()
    override func viewDidLoad() {
        super.viewDidLoad()
        self.subject
            .catch { what in
                Empty(completeImmediately:false)
            }
            .sink {print($0)}
            .store(in: &self.storage)
    }
}

If you send 1 to the passthrough subject now, you get 1 out the end. But if you send an error to the passthrough subject, no error arrives at the end; the pipeline end type is <Int, Never>.

However, do note that you can then never send another value. This has nothing to do with the pipeline; it's because once you send an error through a Subject, that Subject is dead. There's nothing you can do about that.

Hmm I thought that somehow there might be possibility that i can prevent the death of subject. This means that now i have redesign my subject. Probably need to add a dedicated subject for errors so that i do not have to send an error through a subject but instead sending error as an event. — dev_ios999, Sep 08 '21 at 09:12

Converting Publisher's error type to Never in Combine

1 Answers1