Reactor

Reactor is a framework for making more reactive applications inspired by Elm, Redux, and recent work on ReSwift. It's small and simple (just one file), so you can either use Carthage to stay up to date, or just drag and drop into your project and go. Or you can look through it and roll your own.

Reactor encourages unidirectional data flow from a single source of truth—i.e., there is one global object responsible for managing application data, and all UI is derived and updated from it. This way your UI is always in sync with your data, and your data is sync with itself since there are not multiple copies of it floating around your app.

Architecture

                                                     ┌──────────────────┐
                                                     │                  │
                                                     │                  │
                                                     │     Command      │
                                                 ┌───│     (Async)      │
                                                 │   │                  │
                                                 │   │                  │
                                                 │   └──────────────────┘
                                                 │
┌──────────────────┐                             │   ┌──────────────────┐
│                  │                             │   │                  │
│                  │         ┌───────────┐       │   │                  │
│                  │◀────────┤   Event   ├────◀──┴───┤                  │
│                  │         └───────────┘           │                  │
│                  │                                 │                  │
│       Core       │                                 │    Subscriber    │
│                  │                                 │                  │
│                  │                                 │                  │
│    ┌───────┐     │         ┌───────────┐           │                  │
│    │ State │     ├─────────┤   State   ├───────┬──▶│                  │
│    └───────┘     │         └───────────┘       │   │                  │
│                  │                             │   │                  │
└──────────────────┘                             │   └──────────────────┘
                                                 │
                                                 │   ┌──────────────────┐
                                                 │   │                  │
                                                 │   │                  │
                                                 └──▶│    Middleware    │
                                                     │                  │
                                                     │                  │
                                                     └──────────────────┘

There are six objects in the Reactor architecture:

1. The State object - A struct with properties representing application data.
2. The Event - Can trigger a state update.
3. The Core - Holds the application state and responsible for firing events.
4. The Subscriber - Often a view controller, listens for state updates.
5. The Command - A task that can asynchronously fire events. Useful for networking, working with databases, or any other asynchronous task.
6. Middleware - Receives every event and corresponding state. Useful for analytics, error handling, and other side effects.

State

State is anything that conforms to State. Here is an example:

struct Player: State {
    var name: String
    var level: Int

    mutating func react(to event: Event) {
        switch event {
        case let _ as LevelUp:
            level += 1
        default:
            break
        }
    }
}

Here we have a simple Player model, which is state in our application. Obviously most application states are more complicated than this, but this is where composition comes into play: we can create state by composing states.

struct RPGState: State {
    var player: Player
    var monsters: Monsters

    mutating func react(to event: Event) {
        player.react(to: event)
        monsters.react(to: event)
    }
}

Parent states can react to events however they wish, although this will in most cases involve delegating to substates default behavior.

Side note: does the sight of mutating make you feel impure? Have no fear, mutating semantics on value types here are actualy very safe in Swift, and it gives us an imperative look and feel, with the safety of functional programming.

Events

We've seen that an Event can change state. What does an Event look like? In it's most basic form, an event might look like this:

struct LevelUp: Event {}

In other situations, you might want to pass some data along with the event. For example, in an application with more than one player we need to know which player is leveling up.

struct LevelUp: Event {
    var playerID: Int
}

For many events, generics work very nicely.

struct Update<T>: Event {
    var newValue: T
}

Tying it Together with the Core

So, how does the state get events? Since the Core is responsible for all State changes, you can send events to the core which will in turn update the state by calling react(to event: Event) on the root state. You can create a shared global Core used by your entire application (my suggestion), or tediously pass the reference from object to object if you're a masochist.

Here is an example of a simple view controller with a label displaying our intrepid character's level, and a "Level Up" button.

class PlayerViewController: UIViewController {
    var core = App.sharedCore
    @IBOutlet weak var levelLabel: UILabel!

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        core.add(subscriber: self)
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        core.remove(subscriber: self)
    }

    @IBAction func didPressLevelUp() {
        core.fire(event: LevelUp())
    }
}

extension ViewController: Reactor.Subscriber {
    func update(with state: State) {
        levelLabel?.text = String(state.count)
    }
}

By subscribing and subscribing in viewDidAppear/viewDidDisappear respectively, we ensure that whenever this view controller is visible it is up to date with the latest application state. Upon initial subscription, the core will send the latest state to the subscriber's update function. Button presses forward events back to the core, which will then update the state and result in subsequent calls to update. (note: the Core always dispatches back to the main thread when it updates subscribers, so it is safe to perform UI updates in update.)

Commands

Sometimes you want to fire an Event at a later point, for example after a network request, database query, or other asynchronous operation. In these cases, Command helps you interact with the Core in a safe and consistent way.

struct CreatePlayer: Command {
    var session = URLSession.shared
    var player: Player

    func execute(state: RPGState, core: Core<RPGState>) {
        let task = session.dataTask(with: player.createRequest()) { data, response, error in
            // handle response appropriately
            // then fire an update back to the Core
            core.fire(event: AddPlayer(player: player))
        }
        task.resume()
    }
}

// to fire a command
core.fire(command: CreatePlayer(player: myNewPlayer))

Commands get a copy of the current state, and a reference to the Core which allows them to fire Events as necessary.

Middleware

Sometimes you want to do something with an event besides just update application state. This is where Middleware comes into play. When you create a Core, along with the initial state, you may pass in an array of middleware. Each middleware gets called every time an event is passed in. Middleware is not allowed to mutate the state, but it does get a copy of the state along with the event. Middleware makes it easy to add things like logging, analytics, and error handling to an application.

struct LoggingMiddleware: Middleware {
    func process(event: Event, state: State) {
        switch event {
        case _ as LevelUp:
            print("Leveled Up!")
        default:
            break
        }
    }
}

Installation

Follow the installation guides to integrate Reactorin your App.

Swift Package Manager

To integrate Reactor in your App using Swift Package Manager, specify it in your Package.swift file:

import PackageDescription

let package = Package(
    [...]
    dependencies: [
        .Package(url: "https://github.com/ReactorSwift/Reactor.git", majorVersion: XYZ)
    ]
)

Carthage

To integrate Reactor in your App using Carthage, specify it in your Cartfile:

github "ReactorSwift/Reactor" ~> X.Y.Z

Run carthage update to build the framework and drag the built Reactor.framework into your Xcode project.