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🐌 snail Carthage compatible Cocoapods SwiftPM Compatible


A lightweight observables framework, also available in Kotlin



You can install Carthage with Homebrew using the following command:

brew update
brew install carthage

To integrate Snail into your Xcode project using Carthage, specify it in your Cartfile where "x.x.x" is the current release:

github "UrbanCompass/Snail" "x.x.x"

Swift Package Manager

To install using Swift Package Manager have your Swift package set up, and add Snail as a dependency to your Package.swift.

dependencies: [
    .Package(url: "", majorVersion: 0)


Add all the files from Snail/Snail to your project

Developing Locally

  1. Run the setup script to install required dependencies ./scripts/

Creating Observables

let observable = Observable<thing>()


What the Disposer IS

The disposer is used to maintain reference to many subscriptions in a single location. When a disposer is deinitialized, it removes all of its referenced subscriptions from memory. A disposer is usually located in a centralized place where most of the subscriptions happen (ie: UIViewController in an MVVM architecture). Since most of the subscriptions are to different observables, and those observables are tied to type, all the things that are going to be disposed need to comform to Disposable.

What the Disposer IS NOT

The disposer is not meant to prevent retain cycles. A common example is a UIViewController that has reference to a Disposer object. A subscription definition might look something like this:

extension MyViewController {
  button.tap.subscribe(onNext: { [weak self] in
  }).add(to: disposer)

Without specifying a [weak self] capture list in a scenario like this, a retain cycle is created between the subscriber and the view controller. In this example, without the capture list, the view controller will not be deallocated as expected, causing its disposer object to stay in memory as well. Since the Disposer removes its referenced subscribers when it is deinitialized, these subscribers will stay in memory as well.

See for more details on memory management in Swift.

Closure Wrapper

The main usage for the Disposer is to get rid of subscription closures that we create on Observables, but the other usage that we found handy, is the ability to dispose of regular closures. As part of the library, we created a small Closure wrapper class that complies with Disposable. This way you can wrap simple closures to be disposed.

let closureCall = Closure {
    print("We ❤️ Snail")
}.add(to: Disposer)

Please note that this would not dispose of the closureCall reference to closure, it would only Dispose the content of the Closure.

Subscribing to Observables

    onNext: { thing in ... }, // do something with thing
    onError: { error in ... }, // do something with error
    onDone: { ... } //do something when it's done
).add(to: disposer)

Closures are optional too...

    onNext: { thing in ... } // do something with thing
).add(to: disposer)
    onError: { error in ... } // do something with error
).add(to: disposer)

Creating Observables Variables

let variable = Variable<whatever>(some initial value)
let optionalString = Variable<String?>(nil)
    onNext: { string in ... } // do something with value changes
).add(to: disposer)

optionalString.value = "something"
let int = Variable<Int>(12)
    onNext: { int in ... } // do something with value changes
).add(to: disposer)

int.value = 42

Combining Observable Variables

let isLoaderAnimating = Variable<Bool>(false)
isLoaderAnimating.bind(to: viewModel.isLoading) // forward changes from one Variable to another

viewModel.isLoading = true
print(isLoaderAnimating.value) // true
Observable.merge([userCreated, userUpdated]).subscribe(
  onNext: { user in ... } // do something with the latest value that got updated
}).add(to: disposer)

userCreated.value = User(name: "Russell") // triggers 
userUpdated.value = User(name: "Lee") // triggers 
Observable.combineLatest((isMapLoading, isListLoading)).subscribe(
  onNext: { isMapLoading, isListLoading in ... } // do something when both values are set, every time one gets updated
}).add(to: disposer)

isMapLoading.value = true
isListLoading.value = true // triggers

Miscellaneous Observables

let just = Just(1) // always returns the initial value (1 in this case)

enum TestError: Error {
  case test
let failure = Fail(TestError.test) // always fail with error

let n = 5
let replay = Replay(n) // replays the last N events when a new observer subscribes


Snail provides some basic operators in order to transform and operate on observables.

  • map: This operator allows to map the value of an obsverable into another value. Similar to map on Collection types.

    let observable = Observable<Int>()
    let subject = { "Number: \($0)" }
    // -> subject emits `String` whenever `observable` emits.
  • filter: This operator allows filtering out certain values from the observable chain. Similar to filter on Collection types. You simply return true if the value should be emitted and false to filter it out.

    let observable = Observable<Int>()
    let subject = observable.filter { $0 % 2 == 0 }
    // -> subject will only emit even numbers.
  • flatMap: This operator allows mapping values into other observables, for example you may want to create an observable for a network request when a user tap observable emits.

    let fetchTrigger = Observable<Void>()
    let subject = fetchTrigger.flatMap { Variable(100).asObservable() }
    // -> subject is an `Observable<Int>` that is created when `fetchTrigger` emits.

Subscribing to Control Events

let control = UIControl()
  onNext: { ... }  // do something with thing
).add(to: disposer)

let button = UIButton()
  onNext: { ... }  // do something with thing
).add(to: disposer)


You can specify which queue an observables will be notified on by using .subscribe(queue: <desired queue>). If you don't specify, then the observable will be notified on the same queue that the observable published on.

There are 3 scenarios:

  1. You don't specify the queue. Your observer will be notified on the same thread as the observable published on.

  2. You specified main queue AND the observable published on the main queue. Your observer will be notified synchronously on the main queue.

  3. You specified a queue. Your observer will be notified async on the specified queue.


Subscribing on DispatchQueue.main

observable.subscribe(queue: .main,
    onNext: { thing in ... }
).add(to: disposer)

In Practice

Subscribing to Notifications

  .subscribe(queue: .main, onNext: { notification in
  }).add(to: disposer)

Subscribing to Gestures

let panGestureRecognizer = UIPanGestureRecognizer()
  .subscribe(queue: .main, onNext: { sender in
    // Your code here
  }).add(to: disposer)

Subscribing to UIBarButton Taps

  .subscribe(onNext: {
    self.dismiss(animated: true, completion: nil)
  }).add(to: disposer)