How to avoid nesting closures when mapping Combine arrays?

Question

I have an array of strings that I would like to map into different objects. I'm using map to transform the array to another, but it's very nested:

$favorites .map { articleIDs in articleIDs.compactMap { id in state.articles.first { $0.id == id } } } .assign(to: \Self.favorites, on: self) .store(in: &cancellable)

Is there a shorthand to applying a transform to each individual item? I was originally trying to do this but it didn't work:

state.$favorites .mapEach { id in state.articles.first { $0.id == id } } .assign(to: \Self.favorites, on: self) .store(in: &cancellable)

Value is an array of Article ID’s. I’ve updated the code for clarity. — TruMan1
– TruMan1, Commented Nov 24, 2019 at 23:05
If I'm reading this right, favorites is a [[String]], and you're mapping this into [[Article]], and then assign it back to favorites? (How is that possible? What type is favorites?) I usually try to demonstrate these using Just so I can show the entire flow. What's the input and output types? — Rob Napier
– Rob Napier, Commented Nov 25, 2019 at 1:24
I meant I'm trying to map favorites: [String] to [Article] in one flatting closure, but not sure if it's possible without a custom operator. — TruMan1
– TruMan1, Commented Nov 25, 2019 at 2:12

Gil Birman · Accepted Answer · 2019-11-25 16:52:58Z

Here's one way to do it, you:

convert a single array into a pipeline of individual elements with Publishers.Sequence
Process the elements individually.
Convert the elements back to an array with the collect operator.

Here's a contrived example that you can run in an Xcode playground:

import Combine import UIKit class MyStore: ObservableObject { @Published var favorites: [(id: Int, title: String)] = [] let articles = [ (id: 22, title: "foo"), (id: 5, title: "bar"), (id: 13, title: "baz"), ] var cancellable: Set<AnyCancellable> = [] func addFavorites(favorites: AnyPublisher<[Int], Never>) { let articles = self.articles favorites .flatMap(Publishers.Sequence.init) .compactMap { fav in articles.first { fav == $0.id }} .collect() .assign(to: \.favorites, on: self) .store(in: &cancellable) } } let store = MyStore() store.addFavorites(favorites: Just([22, 13]).eraseToAnyPublisher()) print(store.favorites) // => [(id: 22, title: "foo"), (id: 13, title: "baz")]

... however I suspect this isn't actually what you want. A more elegant solution might be to create a bespoke compactMapEach operator. Here's what that looks like:

extension Publisher { public func compactMapEach<T, U>(_ transform: @escaping (T) -> U?) -> Publishers.Map<Self, [U]> where Output == [T] { return map { $0.compactMap(transform) } } }

Thx about the flatMap > compactMap > collect pipeline and suggestion of a custom compactMapEach operator!

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