Propagate events with state for feedbacks

[sergdort]

Per #5

Resurrecting old proposal from @inamiy babylonhealth/ReactiveFeedback#41

This is an additive change to add Optional<Event> argument in Feedback so that unnecessary intermediate states will no longer be required.

Event-driven feedback will be useful for following scenarios, without needing to add a new state and then transit (and transit back again):

• Logging
• Analytics
• Routing
• Image loader (but not managing its internal states)

This is a change from Moore model to (kind of) Mealy model as discussed in babylonhealth/ReactiveFeedback#32 (review) .

Please note that reducer and feedback are still in sequence, not parallel.

Also, please note that Optional<Event> is used here as a workaround since it requires more breaking changes to minimize into non-optional Event.

[andersio]

Could you add unit tests to assert that events are flowing in the order as expected?

[sergdort]

@andersio I believe I addressed your comments

[andersio]

@sergdort

We need to test this:

Loop/Loop/Public/FeedbackLoop.swift

Lines 82 to 87 in ef4a208

	public init(
	events: @escaping (
	_ state: SignalProducer<(State, Event?), Never>,
	_ output: FeedbackEventConsumer<Event>
	) -> SignalProducer<Never, Never>
	) {

that subscribing to state (p.s. need renaming?) does yield nil and subsequent events in the order we expected them to be.