FlatMapLatestInCombine.swift

import Combine

extension Publisher {

    func flatMapLatest<T: Publisher>(_ transform: @escaping (Output) -> T) -> Publishers.SwitchToLatest<T, Publishers.Map<Self, T>> where T.Failure == Failure {
        map(transform).switchToLatest()
    }
}

enum Pattern {
    case one
    case two
}

let trigger = PassthroughSubject<Pattern, Never>()

let subject1 = PassthroughSubject<Int, Never>()
let subject2 = PassthroughSubject<Int, Never>()

let cancellable = trigger
    .flatMapLatest { pattern -> AnyPublisher<Int, Never> in
        switch pattern {
        case .one:
            return subject1.eraseToAnyPublisher()
        case .two:
            return subject2.eraseToAnyPublisher()
        }
    }
    .sink(receiveValue: { intValue in
        print(intValue)
    })

subject1.send(1)
subject2.send(2)

trigger.send(.one)
subject1.send(3)
subject2.send(4)

trigger.send(.two)
subject1.send(5)
subject2.send(6)

trigger.send(.one)
subject1.send(7)
subject2.send(8)

cancellable.cancel()