Skip to content
Swift µframework implementing the Observer pattern 📡
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.


codecov Build Status Swift 4.1 License MIT

  1. Intro
  2. 🌈 Enter Receiver! 🌈
  3. Adding as a Dependency 🚀
  4. Basic usage 😎
  5. Operators 🤖
  6. Strategies
  7. Opinionated, in what way? 🤓
  8. Ok, so why would I use this? 🤷‍♀️


As a ReactiveSwift user myself, most of time, it's difficult to convince someone to just simply start using it. The reality, for better or worse, is that most projects/teams are not ready to adopt it:

  1. The intrinsic problems of adding a big dependency.
  2. The learning curve.
  3. Adapting the current codebase to a FRP mindset/approach.

Nevertheless, a precious pattern can still be used, even without such an awesome lib like ReactiveSwift. 😖

🌈 Enter Receiver! 🌈

Receiver is nothing more than an opinionated micro framework implementation of the Observer pattern (~120 LOC). Or, if you prefer, FRP without the F and a really small R (rP 🤔).

Adding as a Dependency 🚀


If you use Carthage to manage your dependencies, simply add Receiver to your Cartfile:

github "RuiAAPeres/Receiver" ~> 0.0.1

If you use Carthage to build your dependencies, make sure you have added Receiver.framework to the "Linked Frameworks and Libraries" section of your target, and have included them in your Carthage framework copying build phase.


If you use CocoaPods to manage your dependencies, simply add Receiver to your Podfile:

pod 'Receiver', '~> 0.0.1'

Basic usage 😎

Let's begin with the basics. There are three methods in total. Yup, that's right.

1. Creating the Receiver

let (transmitter, receiver) = Receiver<Int>.make()

A receiver can never be created without an associated transmitter (what good would that be?)

2. Listening to an event 📡

This is how you observe events:

receiver.listen { cheezburgers in print("Can I haz \(cheezburgers) cheezburger. 🐈") }

As expected, you can do so as many times as you want:

receiver.listen { cheezburgers in print("Can I haz \(cheezburgers) cheezburger. 🐈") }

receiver.listen { cheezburgers in print("I have \(cheezburgers) cheezburgers and you have none!")}

And both handlers will be called, when an event is broadcasted. ⚡️

3. Broadcasting an event 📻

This is how you send events:


Operators 🤖

Receiver provides a set of operators akin to ReactiveSwift:


If you are familiar with FRP, you must have heard about cold and hot semantics (if not don't worry! ☺️). Receiver provides all three flavours explicitly, when you initialize it, via make(strategy:). By default, the Receiver is .hot.

.cold ❄️:

let (transmitter, receiver) = Receiver<Int>.make(with: .cold)

receiver.listen { wave in
    // This will be called with `wave == 1`
    // This will be called with `wave == 2`
    // This will be called with `wave == 3`
    // This will be called with `wave == 4`


Internally, the Receiver will keep a buffer of the previous sent values. Once there is a new listener, all the previous values are sent. When the 4 is sent, it will be "listened to" as expected.

.warm(upTo: Int) 🌈:

This strategy allows you to specify how big the buffer should be:

let (transmitter, receiver) = Receiver<Int>.make(with: .warm(upTo: 1))

receiver.listen { wave in
    // This will be called with `wave == 2`
    // This will be called with `wave == 3`


In this case 1 will never be called, because the limit specified (upTo: 1) is too low, so only 2 is kept in the buffer.

.hot 🔥:

let (transmitter, receiver) = Receiver<Int>.make(with: .hot) // this is the default strategy

receiver.listen { wave in
    // This will be called with `wave == 3`


Anything broadcasted before listening is discarded.

Opinionated, in what way? 🤓

Initializer. 🌳

The make method, follows the same approach used in ReactiveSwift, with pipe. Since a receiver only makes sense with a transmitter, it's only logical for them to be created together.

Separation between the reader and the writer. ⬆️ ⬇️

A lot of libs have the reader and the writer bundled within the same entity. For the purposes and use cases of this lib, it makes sense to have these concerns separated. It's a bit like a UITableView and a UITableViewDataSource: one fuels the other, so it might be better for them to be split into two different entities.

Ok, so why would I use this? 🤷‍♀️

Well, to make your codebase awesome of course. There are a lot of places where the observer pattern can be useful. In the most simplistic scenario, when delegation is not good enough and you have an 1-to-N relationship.

A good use case for this would in tracking an UIApplication's lifecycle:

enum ApplicationLifecycle {
    case didFinishLaunching
    case didBecomeActive
    case didEnterBackground

class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    private var transmitter: Receiver<ApplicationLifecycle>.Transmitter!

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

        let (transmitter, receiver) = Receiver<ApplicationLifecycle>.make()
        self.transmitter = transmitter
        // Pass down the `receiver` to where it's needed (e.g. ViewModel, Controllers)

        return true

    func applicationDidEnterBackground(_ application: UIApplication) {

    func applicationDidBecomeActive(_ application: UIApplication) {

Similar to the ApplicationLifecycle, the same approach could be used for MVVM:

class MyViewController: UIViewController {
    private let viewModel: MyViewModel
    private let transmitter: Receiver<UIViewControllerLifecycle>.Transmitter

    init(viewModel: MyViewModel, transmitter: Receiver<UIViewControllerLifecycle>.Transmitter) {
        self.viewModel = viewModel
        self.transmitter = transmitter
        super.init(nibName: nil, bundle: nil)

    override func viewDidLoad() {

    override func viewDidAppear(_ animated: Bool) {

    override func viewDidDisappear(_ animated: Bool) {

The nice part is that the UIViewController is never aware of the receiver, as it should be.

At initialization time:

let (transmitter, receiver) = Receiver<UIViewControllerLifecycle>.make()
let viewModel = MyViewModel(with: receiver)
let viewController = MyViewController(viewModel: viewModel, transmitter: transmitter)
You can’t perform that action at this time.
You signed in with another tab or window. Reload to refresh your session. You signed out in another tab or window. Reload to refresh your session.