`RandomNumberViewModel` sketch.

random_number_view_model.swift

final class RandomNumberViewModel: ObservableObject {
	private var subscriptions = Set<AnyCancellable>()

	// MARK: - Inputs

	let randomNumberPing = PassthroughSubject<Void, Never>() /// (1) `randomNumberPing` would
	/// receive `send()` calls from a `Button`’s `action` closure.

	// MARK: - Outputs

	@Published var randomNumber: Int? /// (2) On the output side, `randomNumber` could power a modal that
	/// displays the returned value.

	init() {
		randomNumberPing
			.setFailureType(to: RandomDotOrgError.self) /// (3) To line up our subject’s error with `randomNumberPublisher`’s.
			/// I have some [more detailed writing](https://jasdev.me/set-output-type) on `.setFailureType(to:)`, for the curious.
			.map(randomNumberPublisher)
			.switchToLatest() /// (4) Switching to latest here cancels any in-flight requests.
			.print() /// (5) To help with downstream debugging.
			.receive(on: DispatchQueue.main) /// (6) Generally, we’ll need to dispatch to the main queue when exposing outputs to views.
			.assign(to: \.randomNumber, onWeak: self)
			.store(in: &subscriptions)
	}
}

// MARK: - Helpers

extension Publisher where Failure == Never {
	func assign<Root: AnyObject>( /// (7) A `weak` analog to
		/// [`Publisher.assign(to:on:)`](https://developer.apple.com/documentation/combine/publisher/3235801-assign).
		/// This operator is helpful in scenarios like the above where we’re piping a sequence to a `@Published`
		/// property on the containing instance and need to avoid a retain cycle.
		///
		/// Additional notes: https://jasdev.me/notes/weak-assignment.
		to keyPath: ReferenceWritableKeyPath<Root, Output>,
		onWeak object: Root
	) -> AnyCancellable {
		sink { [weak object] value in
			object?[keyPath: keyPath] = value
		}
	}
}