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A functional reactive framework for managing state evolution and side-effects [Swift implementation]
.package(url: "https://github.com/spotify/Mobius.swift.git", from: "v0.1-alpha")


CocoaPods Compatible Carthage Compatible codecov License

Mobius is a functional reactive framework for managing state evolution and side-effects. It emphasizes separation of concerns, testability, and isolating stateful parts of the code.

Mobius.swift is the Swift and Apple ecosystem focused implementation of the original Mobius Java framework. To learn more, see the wiki for a user guide. You can also watch a talk from an Android @Scale introducing Mobius.

This repository contains the core Mobius framework and add-ons for common development scenarios and testing.


Environment details
📱 iOS 10.0+
🛠 Xcode 12.0+
🐦 Language Swift 5.0


Mobius.swift supports most popular dependency managers. Choose your preferred method to see the instructions:

Swift Package Manager

Mobius can be built for all Apple platforms using the Swift Package Manager.

Add the following entry to your Package.swift:

.package(url: "https://github.com/spotify/Mobius.swift.git", .upToNextMajor(from: "0.4.0"))

Mobius can only be built for iOS using CocoaPods. For other platforms, please use Swift Package Manager.

Add the following entry in your Podfile:

pod 'MobiusCore', '0.4.0'

Optionally, you can also choose to integrate MobiusExtras, MobiusNimble or MobiusTest:

pod 'MobiusExtras', '0.4.0'
pod 'MobiusNimble', '0.4.0'
pod 'MobiusTest', '0.4.0'

Mobius can only be built for iOS using Carthage. For other platforms, please use Swift Package Manager.

Add the following entry in your Cartfile:

github "spotify/Mobius.swift" "0.4.0"

There are some additional steps to take as explained in the Carthage documentation.

NOTE: At this moment Carthage doesn’t have a way to specify subspecs in a single repo. For this reason, Carthage will automatically pull our dependencies used to provide test helpers in MobiusNimble. You can simply choose not to link this library in your project if you don’t plan to use it.

Mobius in Action - Building a Counter

The goal of Mobius is to give you better control over your application state. You can think of your state as a snapshot of all the current values of the variables in your application. In Mobius, we encapsulate all of the state in a data structure which we call the Model.

The Model can be represented by whatever type you like. In this example we’ll be building a simple counter, so all of our state can be contained in an Int:

typealias CounterModel = Int

Mobius does not let you manipulate the state directly. In order to change the state, you have to send the framework messages saying what you want to do. We call these messages Events. In our case, we’ll want to increment and decrement our counter. Let’s use an enum to define these cases:

enum CounterEvent {
    case increment
    case decrement

Now that we have a Model and some Events, we’ll need to give Mobius a set of rules which it can use to update the state on our behalf. We do this by giving the framework a function which will be sequentially called with every incoming Event and the most recent Model, in order to generate the next Model:

func update(model: CounterModel, event: CounterEvent) -> CounterModel {
    switch event {
    case .increment: return model + 1
    case .decrement: return model - 1

With these building blocks, we can start to think about our applications as transitions between discrete states in response to events. But we believe there still one piece missing from the puzzle – namely the side effects which are associated with moving between states. For instance, pressing a “refresh” button might put our application into a “loading” state, with the side effect of also fetching the latest data from our backend.

In Mobius, we aptly call these side effects Effects. In the case of our counter, let’s say that when the user tries to decrement below 0, we play a sound effect instead. Let’s create an enum that represents all the possible effects (which in this case is only one):

enum CounterEffect {
    case playSound

We’ll now need to augment our update function to also return a set of effects associated with certain state transitions. This looks like:

func update(model: CounterModel, event: CounterEvent) -> Next<CounterModel, CounterEffect> {
    switch event {
    case .increment: 
        return .next(model + 1)
    case .decrement:
        if model == 0 {
            return .dispatchEffects([.playSound])
        } else {
            return .next(model - 1)

Mobius sends each of the effects you return in any state transition to something called an Effect Handler. Let’s make one of those now:

import AVFoundation

private func beep() {

let effectHandler = EffectRouter<CounterEffect, CounterEvent>()
    .routeCase(CounterEffect.playSound).to { beep() }

Now that we have all the pieces in place, let’s tie it all together:

let application = Mobius.loop(update: update, effectHandler: effectHandler)
    .start(from: 0)

Let’s start using our counter:

application.dispatchEvent(.increment) // Model is now 1
application.dispatchEvent(.decrement) // Model is now 0
application.dispatchEvent(.decrement) // Sound effect plays! Model is still 0

This covers the fundamentals of Mobius. To learn more, head on over to our wiki.


Mobius.swift is nearing a 1.0 release. We use the framework internally in deployed features, but have recently made a number of breaking changes. Release 0.3.0 breaks compatibility with the previous 0.2.0 release and contains deprecated backwards-compatbility wrappers for some of the smaller changes. These deprecated versions have been removed in the 0.4.0 release. Other additive changes will be made to form Mobius 1.0.


  1. Clone
  2. Bootstrap the project
  3. Open Mobius.xcodeproj using Xcode.
  4. ????
  5. Create a PR

Code of Conduct

This project adheres to the Open Code of Conduct. By participating, you are expected to honor this code.


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Last commit: 5 weeks ago

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Release Notes

1 year ago

0.3.0 makes many changes from 0.2.0. Where possible, old names and types are available with deprecation attributes; these will soon be removed.

  • Updated threading model:
    • MobiusLoop is now single-threaded
    • MobiusController runs a loop on a single background queue.
    • Fixed several issues around hard-to-avoid assertions in loop teardown.
  • New EffectRouter and EffectHandler replace EffectRouterBuilder (which is deprecated along with several helpers).
  • Effects in First and Next are now an array rather than a set. This doesn’t imply an ordering guarantee, but does mean that effects don’t have to be Hashable.
  • Various things renamed or changed from methods to properties to better conform to Swift API Guidelines and for internal consistency:
    • Initiator is now Initiate, and is only used with MobiusController.
    • MobiusLoop.getMostRecentModel() becomes latestModel.
    • MobiusController.getModel() becomes model.
    • Connectable.InputType and OutputType become Input and Output; Connection.ValueType becomes Value.
  • Update is now a struct. This isn’t leveraged by Mobius itself at the moment, but makes it easier to write transformations on updates in a fluent style.
  • For consistency, MobiusController is created through a makeController() method on Mobius.Builder instead of being initialized with a builder argument.
  • All methods on MobiusLogger now have default do-nothing implementations.
  • ConsoleLogger has been replaced with SimpleLogger, which can take a consumer function to use instead of print.
  • NoEffect and BrokenConnection are deprecated.
  • MobiusHooks.ErrorHandler now returns Never rather than Void.
  • Mobius no longer adds a public extension to NSRecursiveLock.
  • There are more documentation comments than there used to be.
  • Tooling updated to Swift 5.0 and Xcode 11.0.
  • Swift Package Manager is explicitly supported for all Apple platforms; Carthage and CocoaPods are supported for iOS only.

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