Swift Combine: How to create a single publisher from a list of publishers?

问题

Using Apple's new Combine framework I want to make multiple requests from each element in a list. Then I want a single result from a reduction of all the the responses. Basically I want to go from list of publishers to a single publisher that holds a list of responses.

I've tried making a list of publishers, but I don't know how to reduce that list into a single publisher. And I've tried making a publisher containing a list but I can't flat map a list of publishers.

Please look at the "createIngredients" function

    func createIngredient(ingredient: Ingredient) -> AnyPublisher<CreateIngredientMutation.Data, Error> {
        return apollo.performPub(mutation: CreateIngredientMutation(name: ingredient.name, optionalProduct: ingredient.productId, quantity: ingredient.quantity, unit: ingredient.unit))
        .eraseToAnyPublisher()
    }

    func createIngredients(ingredients: [Ingredient]) -> AnyPublisher<[CreateIngredientMutation.Data], Error> {
        // first attempt
        let results = ingredients
            .map(createIngredient)
        // results = [AnyPublisher<CreateIngredientMutation.Data, Error>]

        // second attempt
        return Publishers.Just(ingredients)
            .eraseToAnyPublisher()
            .flatMap { (list: [Ingredient]) -> Publisher<[CreateIngredientMutation.Data], Error> in
                return list.map(createIngredient) // [AnyPublisher<CreateIngredientMutation.Data, Error>]
        }
    }

I'm not sure how to take an array of publishers and convert that to a publisher containing an array.

Result value of type '[AnyPublisher]' does not conform to closure result type 'Publisher'

回答1:

Essentially, in your specific situation you're looking at something like this:

    func createIngredients(ingredients: [Ingredient]) -> AnyPublisher<[CreateIngredientMutation.Data], Error> {
        let publisherOfPublishers = Publishers.Sequence<[AnyPublisher<CreateIngredientMutation.Data, Error>], Error>(sequence: ingredients.map(createIngredient))
        return publisherOfPublishers.flatMap { $0 }.collect().eraseToAnyPublisher()
    }

This 'collects' all the elements produced by the upstream publishers and – once they have all completed – produces an array with all the results and finally completes itself.

Bear in mind, if one of the upstream publishers fails – or produces more than one result – the number of elements may not match the number of subscribers, so you may need additional operators to mitigate this depending on your situation.

The more generic answer, with a way you can test it using the EntwineTest framework:

import XCTest
import Combine
import EntwineTest

final class MyTests: XCTestCase {

    func testCreateArrayFromArrayOfPublishers() {

        typealias SimplePublisher = Publishers.Just<Int>

        // we'll create our 'list of publishers' here
        let publishers: [SimplePublisher] = [
            .init(1),
            .init(2),
            .init(3),
        ]

        // we'll turn our publishers into a sequence of
        // publishers, a publisher of publishers if you will
        let publisherOfPublishers = Publishers.Sequence<[SimplePublisher], Never>(sequence: publishers)

        // we flatten our publisher of publishers into a single merged stream
        // via `flatMap` then we `collect` all the results into a single array,
        // and finally we return the resulting publisher
        let finalPublisher = publisherOfPublishers.flatMap{ $0 }.collect()

        // Let's test what we expect to happen, will happen.
        // We'll create a scheduler to run our test on
        let testScheduler = TestScheduler()

        // Then we'll start a test. Our test will subscribe to our publisher
        // at a virtual time of 200, and cancel the subscription at 900
        let testableSubscriber = testScheduler.start { finalPublisher }

        // we're expecting that, immediately upon subscription, our results will
        // arrive. This is because we're using `just` type publishers which
        // dispatch their contents as soon as they're subscribed to
        XCTAssertEqual(testableSubscriber.sequence, [
            (200, .subscription),            // we're expecting to subscribe at 200
            (200, .input([1, 2, 3])),        // then receive an array of results immediately
            (200, .completion(.finished)),   // the `collect` operator finishes immediately after completion
        ])
    }
}

来源：https://stackoverflow.com/questions/56782078/swift-combine-how-to-create-a-single-publisher-from-a-list-of-publishers

标签

swift

combine