ReactiveFeedback

Unidirectional reactive architecture. This is a ReactiveSwift implemetation of RxFeedback

Documentation

Motivation

Requirements for iOS apps have become huge. Our code has to manage a lot of state e.g. server responses, cached data, UI state, routing etc. Some may say that Reactive Programming can help us a lot but, in the wrong hands, it can do even more harm to your code base.

The goal of this library is to provide a simple and intuitive approach to designing reactive state machines.

Core Concepts

State

State is the single source of truth. It represents a state of your system and is usually a plain swift type (a type that does not contain any ReactiveSwift primitives). The only way to change from one State to another is to emit an Event

struct Results<T:JSONSerializable> {
    let page: Int
    let totalResults: Int
    let totalPages: Int
    let results: [T]

    static func empty() -> Results<T> {
        return Results<T>(page: 0, totalResults: 0, totalPages: 0, results: [])
    }
}

struct Context {
    var batch: Results<Movie>
    var movies: [Movie]

    static var empty: Context {
        return Context(batch: Results.empty(), movies: [])
    }
}

enum State {
    case initial
    case paging(context: Context)
    case loadedPage(context: Context)
    case refreshing(context: Context)
    case refreshed(context: Context)
    case error(error: NSError, context: Context)
    case retry(context: Context)
}

Event

Represent all possible events that can happen in your system which can transition to a new value of the State

enum Event {
    case startLoadingNextPage
    case response(Results<Movie>)
    case failed(NSError)
    case retry
}

Reducer

A Reducer is a pure function of (State, Event) -> State. While Event represents an action that results in a State change, it's actually not what causes the change. Event is just that, a representation of the intention to change from one state to another. What actually causes the State to change, the embodiment of the corresponding Event, is a Reducer. A Reducer is the only place where a State can be changed.

static func reduce(state: State, event: Event) -> State {
    switch event {
    case .startLoadingNextPage:
        return .paging(context: state.context)
    case .response(let batch):
        var copy = state.context
        copy.batch = batch
        copy.movies += batch.results
        return .loadedPage(context: copy)
    case .failed(let error):
        return .error(error: error, context: state.context)
    case .retry:
        return .retry(context: state.context)
    }
}

Feedback

While State represents where the system is at a given time, Event represents a state change, and Reducer is the pure function that enacts the event causing the state to change, there is not as of yet any type to decide which event should take place given a particular current state. That's the job of the Feedback. It's essentially a "processing engine", listening to changes in the current state and emitting the corresponding next event to take place. It's represented by a pure function of Signal<State, NoError> -> Signal<Event, NoError>. Feebacks don't directly mutate states. Instead, they only emit events which then cause states to change in reducers.

public struct Feedback<State, Event> {
    public let events: (Scheduler, Signal<State, NoError>) -> Signal<Event, NoError>
}

func loadNextFeedback(for nearBottomSignal: Signal<Void, NoError>) -> Feedback<State, Event> {
    return Feedback(predicate: { !$0.paging }) { _ in
        return nearBottomSignal
            .map { Event.startLoadingNextPage }
        }
}

func pagingFeedback() -> Feedback<State, Event> {
    return Feedback<State, Event>(query: { $0.nextPage }) { (nextPage) -> SignalProducer<Event, NoError> in
        return URLSession.shared.fetchMovies(page: nextPage)
            .map(Event.response)
            .flatMapError { (error) -> SignalProducer<Event, NoError> in
                return SignalProducer(value: Event.failed(error))
            }
        }
}

func retryFeedback(for retrySignal: Signal<Void, NoError>) -> Feedback<State, Event> {
    return Feedback<State, Event>(query: { $0.lastError }) { _ -> Signal<Event, NoError> in
        return retrySignal.map { Event.retry }
    }
}

func retryPagingFeedback() -> Feedback<State, Event> {
    return Feedback<State, Event>(query: { $0.retryPage }) { (nextPage) -> SignalProducer<Event, NoError> in
        return URLSession.shared.fetchMovies(page: nextPage)
            .map(Event.response)
            .flatMapError { (error) -> SignalProducer<Event, NoError> in
                return SignalProducer(value: Event.failed(error))
            }
        }
}

The Flow

As you can see from the diagram above we always start with an initial state.
Each changes of the State will be then delivered to all Feedback loops that was added to the system.
Feedback then decides whether we need to perform any action with particular value of the State (e.g calling API, observe UI events) by dispatching an Event or ignoring it by returning SignalProducer.empty
Dispatched Event then goes to the Reducer wich apples it and returnes a new value of the State
And then cycle starts all over (see 2)

Example

let increment = Feedback<Int, Event> { _ in
    return self.plussButton.reactive
        .controlEvents(.touchUpInside)
        .map { _ in Event.increment }
}

let decrement = Feedback<Int, Event> { _ in
    return self.minusButton.reactive
        .controlEvents(.touchUpInside)
        .map { _ in Event.increment }
}

let system = SignalProducer<Int, NoError>.system(initial: 0,
    reduce: { (count, event) -> Int in
        switch event {
        case .increment:
            return count + 1
        case .decrement:
            return count - 1
        }
    },
    feedbacks: [increment, decrement])

label.reactive.text <~ system.map(String.init)

]

Advantages

TODO

p4checo / ReactiveFeedback