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A (potentially) cross-platform, unidirectional data flow framework to build applications using a declarative and immutable UI API.


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Build Status Swift GitHub release Carthage compatible Platform License

A (potentially) cross-platform, unidirectional data flow framework to build applications using a declarative and immutable UI API.

WARNING!: This is still a work-in-progress, although the minimum features are available to create real world applications the API is still under design and some key optimizations are still missing. Use at your own risk.


  • An application framework heavily inspired by the Elm architecture.
  • Declarative API inspired by Elm and React.
  • 100% in Swift and decoupled from UIKit which makes it (potentially) cross-platform.
  • Uses facebook's Yoga. A cross-platform layout engine that implements Flexbox which is used by ReactNative.
  • Leverage the Swift compiler in order to have a strongly type-safe API.

All you need to do to have a working application is to implement the Application protocol.

public protocol Application {

    associatedtype MessageType
    associatedtype StateType
    associatedtype CommandType
    associatedtype RouteType: Route
    associatedtype SubscriptionType: Equatable
    associatedtype NavigatorType: Equatable

    var initialState: StateType { get }

    var initialRoute: RouteType { get }

    func translateRouteChange(from currentRoute: RouteType, to nextRoute: RouteType) -> MessageType?

    func update(state: StateType, message: MessageType) -> (StateType, CommandType?)?

    func view(for state: StateType) -> View<RouteType, MessageType, NavigatorType>

    func subscriptions(for state: StateType) -> [Subscription<MessageType, RouteType, SubscriptionType>]


The application state is updated in a centralized place every time a new message arrives. Messages can be triggered by user actions or as the result of a computation or access to an external system (like a web service o database).

Every time a new messages arrives, the update function is called. This function responsibility is to provide the next application's state and return a command to be executed in case side-effects are needed, like fetching data from a web service.

Once a new state has been computed the view function will be called to render a new view.

Here is a sneak peak of the API but you can also check this examples or read the library documentation to learn more about the main concepts.

enum Message {

  case like
  case goToDetailScreen


let component: Component<Message> = container(
  children: [
      text: "Hello Portal!",
      style: labelStyleSheet() { base, label in
          base.backgroundColor = .white
          label.textColor = .red
          label.textSize = 12
      layout: layout() {
          $0.flex = flex() {
              $0.grow = .one
          $0.justifyContent = .flexEnd
      properties: properties() {
          $0.text = "Tap to like!"
          $0.onTap = .like
      properties: properties() {
          $0.text = "Tap to got to detail screen"
          $0.onTap = .goToDetailScreen



Install Carthage first by either using the official .pkg installer for the latest release or If you use Homebrew execute the following commands:

brew update
brew install carthage

Once Carthage is installed add the following entry to your Cartfile

github "guidomb/Portal" "master"




For some examples on how the API looks like and how to use this library check

  • The example target in this repository.
  • The Voices Twitter client application.


Portal is still a work-in-progress. Documentation will be added as the library matures inside the Documentation directory. You can read the library overview to learn more about the main concepts.



Install Carthage first, then run

git clone
cd Portal
open Portal.xcodeproj

If you want to know how the project is doing, what features are in the pipeline for the next milestone and whare are the ideas that already in the backlog, check this repo's project


A (potentially) cross-platform, unidirectional data flow framework to build applications using a declarative and immutable UI API.









  • Swift 69.1%
  • C 25.0%
  • Objective-C 3.2%
  • Ruby 2.3%
  • Shell 0.4%