Unidirectional Data Flow in Swift - Inspired by Redux
# ReSwift

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**Supported Swift Versions:** Swift 4.2, 5.x

For Swift 3.2 or 4.0 Support use [Release 5.0.0](https://github.com/ReSwift/ReSwift/releases/tag/5.0.0) or earlier.
For Swift 2.2 Support use [Release 2.0.0](https://github.com/ReSwift/ReSwift/releases/tag/2.0.0) or earlier.

# Introduction

ReSwift is a [Redux](https://github.com/reactjs/redux)-like implementation of the unidirectional data flow architecture in Swift. ReSwift helps you to separate three important concerns of your app's components:

- **State**: in a ReSwift app the entire app state is explicitly stored in a data structure. This helps avoid complicated state management code, enables better debugging and has many, many more benefits...
- **Views**: in a ReSwift app your views update when your state changes. Your views become simple visualizations of the current app state.
- **State Changes**: in a ReSwift app you can only perform state changes through actions. Actions are small pieces of data that describe a state change. By drastically limiting the way state can be mutated, your app becomes easier to understand and it gets easier to work with many collaborators.

The ReSwift library is tiny - allowing users to dive into the code, understand every single line and [hopefully contribute](#contributing).

ReSwift is quickly growing beyond the core library, providing experimental extensions for routing and time traveling through past app states!

Excited? So are we 🎉

Check out our [public gitter chat!](https://gitter.im/ReSwift/public)

# Table of Contents

- [About ReSwift](#about-reswift)
- [Why ReSwift?](#why-reswift)
- [Getting Started Guide](#getting-started-guide)
- [Installation](#installation)
- [Checking Out Source Code](#checking-out-source-code)
- [Demo](#demo)
- [Extensions](#extensions)
- [Example Projects](#example-projects)
- [Contributing](#contributing)
- [Credits](#credits)
- [Get in touch](#get-in-touch)

# About ReSwift

ReSwift relies on a few principles:
- **The Store** stores your entire app state in the form of a single data structure. This state can only be modified by dispatching Actions to the store. Whenever the state in the store changes, the store will notify all observers.
- **Actions** are a declarative way of describing a state change. Actions don't contain any code, they are consumed by the store and forwarded to reducers. Reducers will handle the actions by implementing a different state change for each action.
- **Reducers** provide pure functions, that based on the current action and the current app state, create a new app state

![](Docs/img/reswift_concept.png)

For a very simple app, that maintains a counter that can be increased and decreased, you can define the app state as following:

```swift
struct AppState {
    var counter: Int = 0
}
```

You would also define two actions, one for increasing and one for decreasing the counter. In the [Getting Started Guide](http://reswift.github.io/ReSwift/master/getting-started-guide.html) you can find out how to construct complex actions. For the simple actions in this example we can define empty structs that conform to action:

```swift
struct CounterActionIncrease: Action {}
struct CounterActionDecrease: Action {}
```

Your reducer needs to respond to these different action types, that can be done by switching over the type of action:

```swift
func counterReducer(action: Action, state: AppState?) -> AppState {
    var state = state ?? AppState()

    switch action {
    case _ as CounterActionIncrease:
        state.counter += 1
    case _ as CounterActionDecrease:
        state.counter -= 1
    default:
        break
    }

    return state
}
```
In order to have a predictable app state, it is important that the reducer is always free of side effects, it receives the current app state and an action and returns the new app state.

To maintain our state and delegate the actions to the reducers, we need a store. Let's call it `mainStore` and define it as a global constant, for example in the app delegate file:

```swift
let mainStore = Store<AppState>(
	reducer: counterReducer,
	state: nil
)

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
	[...]
}
```


Lastly, your view layer, in this case a view controller, needs to tie into this system by subscribing to store updates and emitting actions whenever the app state needs to be changed:

```swift
class CounterViewController: UIViewController, StoreSubscriber {

    @IBOutlet var counterLabel: UILabel!

    override func viewWillAppear(_ animated: Bool) {
        mainStore.subscribe(self)
    }

    override func viewWillDisappear(_ animated: Bool) {
        mainStore.unsubscribe(self)
    }

    func newState(state: AppState) {
        counterLabel.text = "\(state.counter)"
    }

    @IBAction func increaseButtonTapped(_ sender: UIButton) {
        mainStore.dispatch(
            CounterActionIncrease()
        )
    }

    @IBAction func decreaseButtonTapped(_ sender: UIButton) {
        mainStore.dispatch(
            CounterActionDecrease()
        )
    }

}
```

The `newState` method will be called by the `Store` whenever a new app state is available, this is where we need to adjust our view to reflect the latest app state.

Button taps result in dispatched actions that will be handled by the store and its reducers, resulting in a new app state.

This is a very basic example that only shows a subset of ReSwift's features, read the Getting Started Guide to see how you can build entire apps with this architecture. For a complete implementation of this example see the [CounterExample](https://github.com/ReSwift/CounterExample) project.

### Create a subscription of several substates combined

Just create a struct representing the data model needed in the subscriber class, with a constructor that takes the whole app state as a param. Consider this constructor as a mapper/selector from the app state to the subscriber state. Being `MySubState` a struct and conforming to `Equatable`, ReSwift (by default) will not notify the subscriber if the computed output hasn't changed. Also, Swift will be able to infer the type of the subscription.

```swift
struct MySubState: Equatable {
    // Combined substate derived from the app state.
    
    init(state: AppState) {
        // Compute here the substate needed.
    }
}
```

```swift
store.subscribe(self) { $0.select(MySubState.init) }
    
func newState(state: MySubState) {
    // Profit!
}
```

# Why ReSwift?

Model-View-Controller (MVC) is not a holistic application architecture. Typical Cocoa apps defer a lot of complexity to controllers since MVC doesn't offer other solutions for state management, one of the most complex issues in app development.

Apps built upon MVC often end up with a lot of complexity around state management and propagation. We need to use callbacks, delegations, Key-Value-Observation and notifications to pass information around in our apps and to ensure that all the relevant views have the latest state.

This approach involves a lot of manual steps and is thus error prone and doesn't scale well in complex code bases.

It also leads to code that is difficult to understand at a glance, since dependencies can be hidden deep inside of view controllers. Lastly, you mostly end up with inconsistent code, where each developer uses the state propagation procedure they personally prefer. You can circumvent this issue by style guides and code reviews but you cannot automatically verify the adherence to these guidelines.

ReSwift attempts to solve these problem by placing strong constraints on the way applications can be written. This reduces the room for programmer error and leads to applications that can be easily understood - by inspecting the application state data structure, the actions and the reducers.

This architecture provides further benefits beyond improving your code base:

- Stores, Reducers, Actions and extensions such as ReSwift Router are entirely platform independent - you can easily use the same business logic and share it between apps for multiple platforms (iOS, tvOS, etc.)
- Want to collaborate with a co-worker on fixing an app crash? Use [ReSwift Recorder](https://github.com/ReSwift/ReSwift-Recorder) to record the actions that lead up to the crash and send them the JSON file so that they can replay the actions and reproduce the issue right away.
- Maybe recorded actions can be used to build UI and integration tests?

The ReSwift tooling is still in a very early stage, but aforementioned prospects excite me and hopefully others in the community as well!

[You can also watch this talk on the motivation behind ReSwift](https://academy.realm.io/posts/benji-encz-unidirectional-data-flow-swift/).

# Getting Started Guide

[A Getting Started Guide that describes the core components of apps built with ReSwift lives here](http://reswift.github.io/ReSwift/master/getting-started-guide.html).

To get an understanding of the core principles we recommend reading the brilliant [redux documentation](http://redux.js.org/).

# Installation

## CocoaPods

You can install ReSwift via [CocoaPods](https://cocoapods.org/) by adding it to your `Podfile`:
```
use_frameworks!

source 'https://github.com/CocoaPods/Specs.git'
platform :ios, '8.0'

pod 'ReSwift'
```

And run `pod install`.

## Carthage

You can install ReSwift via [Carthage](https://github.com/Carthage/Carthage) by adding the following line to your `Cartfile`:

```
github "ReSwift/ReSwift"
```

## Accio

You can install ReSwift via [Accio](https://github.com/JamitLabs/Accio) by adding the following line to your `Package.swift`:

```swift
.package(url: "https://github.com/ReSwift/ReSwift.git", .upToNextMajor(from: "5.0.0")),
```

Next, add `ReSwift` to your App targets dependencies like so:

```swift
.target(
    name: "App",
    dependencies: [
        "ReSwift",
    ]
),
```

Then run `accio update`.

## Swift Package Manager

You can install ReSwift via [Swift Package Manager](https://swift.org/package-manager/) by adding the following line to your `Package.swift`:

```swift
import PackageDescription

let package = Package(
    [...]
    dependencies: [
        .package(url: "https://github.com/ReSwift/ReSwift.git", from: "5.0.0"),
    ]
)
```

# Checking out Source Code

After checking out the project run `pod install` to get the latest supported version of [SwiftLint](https://github.com/realm/SwiftLint), which we use to ensure a consistent style in the codebase.

# Demo

Using this library you can implement apps that have an explicit, reproducible state, allowing you, among many other things, to replay and rewind the app state, as shown below:

![](Docs/img/timetravel.gif)

# Extensions

This repository contains the core component for ReSwift, the following extensions are available:

- [ReSwift-Thunk](https://github.com/ReSwift/ReSwift-Thunk): Provides a ReSwift middleware that lets you dispatch thunks (action creators) to encapsulate processes like API callbacks.
- [ReSwift-Router](https://github.com/ReSwift/ReSwift-Router): Provides a ReSwift compatible Router that allows declarative routing in iOS applications
- [ReSwift-Recorder](https://github.com/ReSwift/ReSwift-Recorder): Provides a `Store` implementation that records all `Action`s and allows for hot-reloading and time travel

# Example Projects

- [CounterExample](https://github.com/ReSwift/CounterExample): A very simple counter app implemented with ReSwift.
- [CounterExample-Navigation-TimeTravel](https://github.com/ReSwift/CounterExample-Navigation-TimeTravel): This example builds on the simple CounterExample app, adding time travel with [ReSwiftRecorder](https://github.com/ReSwift/ReSwift-Recorder) and routing with [ReSwiftRouter](https://github.com/ReSwift/ReSwift-Router).
- [GitHubBrowserExample](https://github.com/ReSwift/GitHubBrowserExample): A real world example, involving authentication, network requests and navigation. Still WIP but should be the best resource for starting to adapt `ReSwift` in your own app.
- [ReduxMovieDB](https://github.com/cardoso/ReduxMovieDB): A simple App that queries the tmdb.org API to display the latest movies. Allows searching and viewing details.
- [Meet](https://github.com/Ben-G/Meet): A real world application being built with ReSwift - currently still very early on. It is not up to date with the latest version of ReSwift, but is the best project for demonstrating time travel.
- [Redux-Twitter](https://github.com/Goktug/Redux-Twitter): A basic Twitter search implementation built with ReSwift and RxSwift, involing Twitter authentication, network requests and navigation.

## Production Apps with Open Source Code

- [Persephone](https://github.com/danbee/persephone), a MPD music player daemon controller for macOS
- [Product Hunt for OS X](https://github.com/producthunt/producthunt-osx) Official Product Hunt client for macOS

# Contributing

There's still a lot of work to do here! We would love to see you involved! You can find all the details on how to get started in the [Contributing Guide](/CONTRIBUTING.md).

# Credits

- Thanks a lot to [Dan Abramov](https://github.com/gaearon) for building [Redux](https://github.com/reactjs/redux) - all ideas in here and many implementation details were provided by his library.

# Get in touch

If you have any questions, you can find the core team on twitter:

- [@benjaminencz](https://twitter.com/benjaminencz)
- [@karlbowden](https://twitter.com/karlbowden)
- [@ARendtslev](https://twitter.com/ARendtslev)
- [@ctietze](https://twitter.com/ctietze)
- [@mjarvis](https://twitter.com/mjarvis)

We also have a [public gitter chat!](https://gitter.im/ReSwift/public)

