Swiftpack.co - Package - iZettle/Flow

Build Status Platforms Carthage Compatible Swift Package Manager Compatible

Modern applications often contain complex asynchronous flows and life cycles. Flow is a Swift library aiming to simplify building these by solving three main problems:

Flow was carefully designed to be:

  • Easy to use: APIs are carefully designed for readability and ease of use.
  • Pragmatic: Evolved and designed to solve real problems.
  • Composable: Types compose nicely making building complex flows easy.
  • Performant: Flow has been highly tuned for performance.
  • Concurrent: Flow is thread safe and uses a scheduler model that is easy to reason about.
  • Extensible: Flow was designed to be extensible.
  • Strongly typed: Flow makes use of Swift strong typing to better express intention.
  • Correct: Backed by hundreds of unit tests and field tested for years.

Example usage

In flow the Disposable protocol is used for lifetime management:

extension UIView {
  func showSpinnerOverlay() -> Disposable { 
    let spinner = ...
    return Disposer {

let disposable = view.showSpinnerOverlay()

disposable.dispose() // Hide spinner

And the Signal<T> type is used for event handling:

let bag = DisposeBag() // Bag of disposables

// UIButton provides a Signal<()>
let loginButton = UIButton(...)

bag += loginButton.onValue { 
  // Log in user if tapped

// UITextField provides a ReadSignal<String>
let emailField = UITextField(...)
let passwordField = UITextField(...)

// Combine and transform signals
let enableLogin = combineLatest(emailField, passwordField)
  .map { email, password in
    email.isValidEmail && password.isValidPassword
  } // -> ReadSignal<Bool>

// Use bindings and key-paths to update your UI on changes
bag += enableLogin.bindTo(loginButton, \.isEnabled)

And finally the Future<T> type handles asynchronous operations:

func login(email: String, password: String) -> Future<User> {
  let request = URLRequest(...)
  return URLSession.shared.data(for: request).map { data in
    User(data: data)

login(...).onValue { user in
  // Handle successful login
}.onError { error in
  // Handle failed login

These three types come with many extensions that allow us to compose complex UI flows:

class LoginController: UIViewController {
  let emailField: UITextField
  let passwordField: UITextField
  let loginButton: UIButton 
  let cancelButton: UIBarButtonItem 
  var enableLogin: ReadSignal<Bool> { // Introduced above }
  func login() -> Future<User> { // Introduced above }
  func showSpinnerOverlay() -> Disposable { // Introduced above }
  // Returns future that completes with true if user chose to retry
  func showRetryAlert(for error: Error) -> Future<Bool> { ... }
  // Will setup UI observers and return a future completing after a successful login 
  func runLogin() -> Future<User> {
    return Future { completion in // Completion to call with the result  
      let bag = DisposeBag() // Resources to keep alive while executing 
      // Make sure to signal at once to set up initial enabled state
      bag += self.enableLogin.atOnce().bindTo(self.loginButton, \.isEnabled)  

      // If button is tapped, initiate potentially long running login request
      bag += self.loginButton.onValue {
          .performWhile { 
            // Show spinner during login request
          }.onErrorRepeat { error in
	          // If login fails with an error show an alert...
	          // ...and retry the login request if the user chose to
            self.showRetryAlert(for: error)
          }.onValue { user in
            // If login is successful, complete runLogin() with the user
      // If cancel is tapped, complete runLogin() with an error
      bag += self.cancelButton.onValue { 
      return bag // Return a disposable to dispose once the future completes


Introductions to the main areas of Flow can be found at:

To learn even more about available functionality you are encouraged to explore the source files that are extensively documented. Code-completion should also help you to discover many of the transformations available on signals and futures.


  • Xcode 9.3+
  • Swift 4.1
  • Platforms:
    • iOS 9.0+
    • macOS 10.11+
    • tvOS 9.0+
    • watchOS 2.0+
    • Linux



github "iZettle/Flow" >= 1.0

Cocoa Pods

platform :ios, '9.0'

target 'Your App Target' do
  pod 'FlowFramework', '~> 1.0'

Swift Package Manager

import PackageDescription

let package = Package(
  name: "Your Package Name",
  dependencies: [
      .Package(url: "https://github.com/iZettle/Flow.git",
               majorVersion: 1)

Field tested

Flow was developed, evolved and field-tested over the course of several years, and is pervasively used in iZettle's highly acclaimed point of sales app.


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1.0.0 - Apr 17, 2018