Redirecting Publisher errors

19 Dec 2020 ⇐ Notes archive

(This is an entry in my technical notebook. There will likely be typos, mistakes, or wider logical leaps — the intent here is to “let others look over my shoulder while I figure things out.”)

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Adam Sharp posted a snippet in iOS Folks today that I wanted to highlight (with a couple of light CombineExt-imported edits).

This is a form of materialization that feels more at home in Combine. It tucks away the awkward materialize-share-errors dance and instead focuses on funneling a publisher’s errors directly to a @Published property.

It’s tempting to use Publisher.share at (1) — and Adam avoided it for good reason. share implicitly autoconnects, which could accidentally trigger a subscription too early at the assign call at (2). Instead, he wired up the error-focused publisher to a multicasted PassthroughSubject (which returns a ConnectablePublisher) and then makes sure both subscriptions are properly teed up by autoconnecting the returned publisher at (3).