Latest 0.1.2
License Apache 2.0
Platforms osx 10.14
Dependencies Tokamak
Frameworks AppKit


React-like framework for native UI written in pure Swift 🛠⚛️📲

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Tokamak provides a declarative, testable and scalable API for building UI
components backed by fully native views. You can use it for your new iOS apps or
add to existing apps with little effort and without rewriting the rest of the
code or changing the app’s overall architecture.

Tokamak recreates React Hooks API
improving it with Swift’s strong type system, high performance and efficient
memory management thanks to being compiled to a native binary.

Why yet another React-like library?

One of the strong points of React is that in general it makes app architecture
more declarative, but still preserves a smooth learning curve when compared to
more complex FRP
. In
addition, its cross-platform reconciler
can be reused across many
different platforms: on the web, for mobile apps, and even
. The downside is that it
requires you to use JavaScript, which causes all
of different
. It’s also
not very easy to integrate React Native with existing iOS apps written in Swift.
On the other hand, none of the available libraries similar to React written in
Swift provided a concise API for building component hierarchies, or allowed
building stateful function-based components similar to what’s possible with
React Hooks. Some only port Redux, which requires a lot
of boilerplate, some bake in assumptions about the usage of UIKit, which makes
them restricted to a single platform.

In short, both plain UIKit MVC and React have advantages and disadvantages. The
goal of Tokamak is to provide the best of both worlds for Swift: declarative
architecture, cross-platform core, easy to learn and to integrate into existing

When compared to standard UIKit MVC or other patterns built on top of
it (MVVM, MVP, VIPER etc), Tokamak provides:

  • Declarative DSL
    for native UI
    : no more conflicts caused by Storyboards, no template languages
    or XML. Describe UI of your app concisely in Swift and get views native to
    iOS with full support for accessibility, auto layout and native navigation gestures.

  • Easy to use one-way data binding: tired of didSet, delegates,
    notifications or KVO? UI components automatically update in response to state

  • Clean composable architecture: components can be passed to other
    components as children with an established API focused on code reuse. You can
    easily embed Tokamak components within your existing UIKit code or vice versa:
    expose that code as Tokamak components. No need to decide whether you should
    subclass UIView or UIViewController to make your UI composable.

  • Off-screen rendering for unit-tests: no need to maintain slow and flaky UI
    tests that render everything on a simulator screen and simulate actual touch
    events to just test UI logic. Components written with Tokamak can be tested
    off-screen with tests completing in a fraction of a second. If your UI doesn’t
    require any code specific to UIKit (and Tokamak provides helpers to achieve
    that) you can even run your UI-related unit-tests on Linux!

  • Platform-independent core: our main goal is to eventually support as many
    platforms as possible. Starting with iOS/UIKit and basic support for
    macOS/AppKit, we plan to add renderers for WebAssembly/DOM and native Android
    in future versions. As the core API is cross-platform, UI components written
    with Tokamak won’t need to change to become available on newly added platforms
    unless you need UI logic specific to a device or OS. And if they do, you can
    still cleanly separate platform-specific components thanks to easy

  • Architecture proven to work: React has been available for years and gained
    a lot of traction and is still growing. We’ve seen so many apps successfully
    rebuilt with it and heard positive feedback on React itself, but we also see
    a lot of complaints about its overreliance on JavaScript. Tokamak makes
    architecture of React with its established patterns available to you in Swift.

Important: Tokamak is relatively stable at this point, as in not having
any blocking or critical bugs that the maintainers are aware of. The core API of
Component and Hooks types is frozen, and there’s a plenty of standard
to start building useful apps on iOS. The
macOS/AppKit renderer has support for only the most basic components and
improving its feature parity with the iOS renderer is the top priority. If in
the future there’s a breaking change that’s absolutely needed, we aim to
deprecate old APIs in a source-compatible way and will introduce any
replacements gradually. It’s important to note that source breaking
changes can’t always be avoided, but they would be reflected with
appropriate version number change and migration guides.

Don’t forget to check out Tokamak community on
and leave your feedback, comments and

Table of contents

Example code

An example of a Tokamak component that binds a button to a label looks like

import Tokamak

struct Counter: LeafComponent {
  struct Props: Equatable {
    let countFrom: Int

  static func render(props: Props, hooks: Hooks) -> AnyNode {
    let count = hooks.state(props.countFrom)

    return StackView.node(.init(
      Edges.equal(to: .parent),
      axis: .vertical,
      distribution: .fillEqually), [
          onPress: Handler { count.set { $0 + 1 } },
          text: "Increment"
        Label.node(.init(alignment: .center, text: "(count.value)"))

Then you can add this component to any iOS app as a view controller this way:

import TokamakUIKit

final class ViewController: TokamakViewController {
  override var node: AnyNode {
    return Counter.node(.init(countFrom: 1))

Counter component

Or similarly it can be added to a macOS app:

import TokamakAppKit

final class ViewController: TokamakViewController {
  override var node: AnyNode {
    return View.node(
        Edges.equal(to: .parent),
        Width.equal(to: 200),
        Height.equal(to: 100),
      Counter.node(.init(countFrom: 1))

Note that we added explicit constraints to use this as a window’s root view
controller, and windows don’t have a fixed predefined size by default.

Counter component

Example project

The best way to try Tokamak in action is to run the example project:

  1. Verify that you have CocoaPods and
    Xcode 10.1 or later installed:
pod --version
xcode-select -p
  1. Clone the repository
git clone
  1. Install the dependencies in the example project:
cd Tokamak/Example
pod install
  1. Open the Example workspace from Finder or from Terminal:
open -a Xcode *.xcworkspace
  1. Build executable target TokamakDemo-iOS for iOS and TokamakDemo-macOS for

Standard components

Tokamak provides a few basic components that you can reuse in your apps. On iOS
these components are rendered to corresponding UIView subclasses that you’re
already used to, e.g. Button component is rendered as UIButton, Label as
UILabel etc. Check out the complete up to date

for more info.

Quick introduction

We try to keep Tokamak’s API as simple as possible and the core algorithm with
supporting protocols/structures currently fit in only ~600 lines of code. It’s
all built upon a few basic concepts:


Props describe a "configuration" of what you’d like to see on user’s screen.
An example could be a struct describing background color, layout, initial
value etc. Props are immutable and
Equatable, which
allows us to observe when they change. You always use struct or enum and
never use class for props so that immutability is guaranteed. You wouldn’t
ever need to provide your own Equatable implementation for Props as Swift
compiler is able to generate one for you automatically behind the
Here’s a simple Props struct you could use for your own component like
Counter from the example above:

struct Props: Equatable {
  let countFrom: Int


Sometimes "configuration" is described in a tree-like fashion. For example, a
list of views contains an array of subviews, which themselves can contain other
subviews. In Tokamak this is called Children, which behave similar to
Props but are important enough to be treated separately. Children
are also immutable and Equatable, which allows us to observe those for changes


Component is a protocol, which couples given Props and
Children on screen and provides some declaration how these are
rendered on screen:

protocol Component {
  associatedtype Props: Equatable
  associatedtype Children: Equatable

(Don’t worry if you don’t know what associatedtype means, it’s only a simple
requirement for components to provide these types and make them Equatable. If
you do know what a PAT is, you
also shouldn’t worry. 😄 Tokamak’s API is built specifically to hide "sharp edges"
of PATs from the public API and to make it easy to use without requiring
advanced knowledge of Swift. This is similar to what Swift standard

has done, which is built on top of PATs but stays flexible and ergonomic).


A node is a container for Props, Children and a type
conforming to Component rendering this "configuration". If
you’re familiar with React, nodes in Tokamak correspond to elements in
. When Children is an
array of nodes, we can indirectly form a tree describing the app’s UI.
Corollary, nodes are immutable and Equatable. You’d only need to use the
standard AnyNode type provided by Tokamak:

struct AnyNode: Equatable {
  // ... `Props` and `Children` stored here by Tokamak as private properties 

Here’s an example of an array of nodes used as Children in the standard
StackView component provided by Tokamak, which describe subviews of the stack

struct StackView: Component {
  struct Props: Equatable {
    // ...
  typealias Children = [AnyNode]

For every component Tokamak provides an easy way to create a node for it
coupled with given props and children:

// this extension and its `node` function are defined for you by Tokamak
extension Component {
  static func node(_ props: Props, _ children: Children) -> AnyNode {
    // ...

For example, an empty vertical stack view is created like this:

StackView.node(.init(axis: .vertical), [])

Render function

The most simple component is a pure
taking Props
and Children as an argument and returning a node tree as a

protocol PureComponent: Component {
  // this is the function you define for your own components, 
  // Tokamak takes care of the rest
  static func render(props: Props, children: Children) -> AnyNode

Tokamak calls render on your components when their Props or Children passed
from parent components change. You don’t ever need to call render yourself,
pass different values as props or children to nodes returned from parent
render and Tokamak will update only those views on screen that need to be

Note that render function does not return other components, it returns
nodes that describe other components
. It’s a very important distiction,
which allows Tokamak to stay efficient and to avoid updating deep trees of

Here’s an example of a simple component that renders its child in a vertical
stack as many times as were passed via its Props:

struct StackRepeater: PureComponent {
  typealias Props = UInt
  typealias Children = AnyNode

  static func render(props x: UInt, children: AnyNode) -> AnyNode {
    return StackView.node(
      .init(axis: .vertical),
      (0..<x).map { _ in children }

You can then use StackRepeater in any other component by creating its node
and passing any other node as a child this way:

StackRepeater.node(5, Label.node("repeated"))

which will present a label on screen with text "repeated" 5 times.

Leaf components

Some of your components wouldn’t need Children at all, for those Tokamak
provides a PureLeafComponent helper protocol that allows you to implement only
a single function with a simpler signature:

// Helpers provided by Tokamak:

struct Null: Equatable {}

protocol PureLeafComponent: PureComponent where Children == Null {
  static func render(props: Props) -> AnyNode

extension PureLeafComponent {
  static func render(props: Props, children: Children) -> AnyNode {
    return render(props: props)

Thus your components can conform to PureLeafComponent instead of
PureComponent, which allows you to avoid children argument in a render
function when you don’t need it.


Quite frequently you need components that are stateful or cause some other
side effects.
Hooks provide a clear separation between declarative components and other
imperative code, such as state management, file I/O, networking etc.

The standard protocol CompositeComponent in Tokamak gets Hooks injected into
render function as an argument.

protocol CompositeComponent: Component {
  static func render(
    props: Props,
    children: Children,
    hooks: Hooks
  ) -> AnyNode

In fact, the standard PureComponent is a special case of a
CompositeComponent that doesn’t use Hooks during rendering:

// Helpers provided by Tokamak:

protocol PureComponent: CompositeComponent {
  static func render(props: Props, children: Children) -> AnyNode

extension PureComponent {
  static func render(
    props: Props,
    children: Children,
    hooks: Hooks
  ) -> AnyNode {
    return render(props: props, children: children)

One of the simplest hooks is state. It allows a component to have its own
state and to be updated when the state changes. We’ve seen it used in the
Counter example:

struct Counter: LeafComponent {
  // ...
  static func render(props: Props, hooks: Hooks) -> AnyNode {
    // type signature for this constant is inferred automatically
    // and is only added here for documentation purposes
    let count: State<Int> = hooks.state(1)
    // ...

It returns a very simple state container, which on initial call of render
contains 1 as a value and values passed to count.set(_: Int) on subsequent

// values of this type are returned by `hooks.state`
struct State<T> {
  let value: T

  // set the state to a value you already have
  func set(_ value: T)

  // or update the state with a pure function
  func set(_ transformer: @escaping (T) -> T)

  // or efficiently update the state in place with a mutating function
  // (helps avoiding expensive memory allocations when state contains 
  // large arrays/dictionaries or other copy-on-write value)
  func set(_ updater: @escaping (inout T) -> ())

Note that set functions are not mutating, they never update the component’s
state in-place synchronously, but only schedule an update with Tokamak at a later
time. A call to render is only scheduled on the component that obtained this
state with hooks.state.

When you need state changes to update any of the descendant components, you can
pass the state value within props or children of nodes returned from render.
In Counter component the label’s content is "bound" to count this way:

struct Counter: LeafComponent {
  static func render(props: Null, hooks: Hooks) -> AnyNode {
    let count = hooks.state(1)

    return StackView.node([
          onPress: Handler { count.set { $0 + 1 } }, 
          text: "Increment"

Hooks provide a great way to compose side effects and also to keep them separate
from your component code. You can always create your own hook reusing existing
ones: just add it to your extension Hooks wherever works best for you.


When mapping Tokamak’s architecture to what’s previosly been established in iOS,
Component corresponds to a "view-model" layer, while
Hooks provide a reusable "controller" layer. A Renderer is a
"view" layer in these terms, but it’s fully managed by Tokamak. Not only this
greatly simplifies the code of your components and allows you to make it
declarative, it also completely decouples platform-specific code.

Note that Counter component above doesn’t contain a single
type from UIKit module, although the component itself is passed to a specific
UIKitRenderer (via its TokamakViewController public API) to make it
available in an app that uses UIKit. On other platforms you could use a
different renderer, while the component code could stay the same if its behavior
doesn’t need to change for that environment. Otherwise you can adjust
component’s behavior via Props and pass different "initializing" props
depending on the renderer’s platform.

Providing renderers for other platforms in the future is one of our top
priorities. Tokamak already provides basic support for macOS apps in
TokamakAppKit module that allows you to render same standard components on iOS
and macOS without any changes applied to the component code and without
requiring Marzipan!


  • iOS 11.0 or later for TokamakUIKit
  • macOS 10.14 for TokamakAppKit
  • Xcode 10.1 or later
  • Swift 4.2



CocoaPods is a dependency manager for Swift and
Objective-C Cocoa projects. You can install it with the following command:

$ gem install cocoapods

Navigate to the project directory and create Podfile with the following command:

$ pod install

Inside of your Podfile, specify the Tokamak pod:

# Uncomment the next line to define a global platform for your project
# platform :ios, '11.0'

target 'YourApp' do
  # Comment the next line if you don't want to use dynamic frameworks

  # Pods for YourApp
  pod 'TokamakUIKit', '~> 0.1'

Then, run the following command:

$ pod install

Open the the YourApp.xcworkspace file that was created. This should be the
file you use everyday to create your app, instead of the YourApp.xcodeproj


What are "Rules of Hooks"?

Hooks are a great way to inject state and other side effects into pure
functions. In some sense, you could consider Hooks an emulation of indexed
algebraic effects, which served as inspiration for
Hooks in
Unfortunately, due to Swift’s current limitations, we can’t express monads or
algebraic effects natively, so Hooks need a few restrictions applied to make it
work. Similar restrictions are also applied to Hooks in

  1. You can call Hooks from render function of any component. 👍
  2. You can call Hooks from your custom Hooks (defined by you in an extension
    of Hooks). 🙌
  3. Don’t call Hooks from a loop, condition or nested function/closure. 🚨
  4. Don’t call Hooks from any function that’s not a static func render on a
    component, or not a custom Hook. ⚠️

In a future version Tokamak will provide a linter able to catch violations of
Rules of Hooks at compile time.

Why do Rules of Hooks exist?

Same as
Tokamak maintains an array of "memory cells" for every stateful component to hold
the actual state. It needs to distinguish one Hooks call from another to map
those to corresponding cells during execution of a render function of your
component. Consider this:

struct ConditionalCounter: LeafComponent {
  typealias Props = Null

  static func render(props: Props, hooks: Hooks) -> AnyNode {
    // this code won't work as expected as it violates Rule 3:
    // > Don't call Hooks from a condition

    // state stored in "memory cell" 1
    let condition = hooks.state(false) 
    if condition {
      // state stored in "memory cell" 2
      count = hooks.state(0) 
    } else {
      // state, which should be stored in "memory cell" 3, 
      // but will be actually queried from "memory cell" 2
      count = hooks.state(42) 

    return StackView.node([
      Switch.node(.init(value: condition.value,
                        valueHandler: Handler(condition.set)))
        onPress: Handler { count.set { $0 + 1 } },
        text: "Increment"

How does Tokamak renderer know on subsequent calls to
ConditionalCounter.render which state you’re actually addressing? It relies on
the order of those calls, so if the order dynamically changes from one rendering
to another, you could unexpectedly get a value of the one state cell, when you
expected a value of a different state cell.

We encourage you to keep any hooks logic at the top level of a render
definition, which makes all side effects of a component clear upfront and is a
good practice anyway. If you do need conditions or loops applied, you can always
create a separate component and return a node conditionally or an array of nodes
for this new child component from render of a parent component. The fixed
version of ConditionalCounter would look like this:

struct ConditionalCounter: LeafComponent {
  typealias Props = Null

  static func render(props: Props, hooks: Hooks) -> AnyNode {
    // this works as expected
    let condition = hooks.state(false)
    let count1 = hooks.state(0)
    let count2 = hooks.state(42)

    let value = (condition ? count1 : count2).value

    return StackView.node([
      Switch.node(.init(value: condition.value,
                        valueHandler: Handler(condition.set)))
        onPress: Handler { count.set { $0 + 1 } },
        text: "Increment"

Why does Tokamak use value types and protocols instead of classes?

Swift developers focused on GUI might be used to classes thanks to abundance of
class hierarchies in UIKit and AppKit (although benefits of composition

were previously highlighted by Apple). Unfortunately, while UIKit is a
relatively fresh development, it still closely follows many patterns used in
AppKit, which was itself built in late
. Both of these were developed with
Objective-C in mind, years before Swift became public and protocol-oriented

One of the main goals of Tokamak is to build a UI framework that feels native to
Swift. Tokamak’s API brings these benefits when compared to class-based APIs:

  • no need to subclass NSObject to conform to commonly used
  • no need to use override and to remember to call super;
  • no need for required init, convenience init or to be concerned with strict
    class initialization rules;
  • you can’t create a reference cycle with immutable values, no need for

    when using callbacks;
  • you don’t need to worry about modifying an object in a different scope
    accidentaly captured by reference: immutable values are implicitly copied
    and most of the copies are removed by the compiler during optimization;
  • focus on composition over inheritance: no need to subclass UIViewController
    or UIView and to worry about all of the above when you only need simple
  • focus on functional and declarative programming, while still allowing to use
    imperative code when needed: value types guarantee lack of unexpected
    side effects in pure functions.

Is there anything like JSX available for Tokamak?

At the moment the answer is no, but we find that Tokamak’s API allows you to
create nodes much more concisely when compared to React.createElement
. In fact, with Tokamak’s
.node API you don’t need closing element tags you’d have to write with JSX.
E.g. compare this:


to this:


We do agree that there’s an overhead of .init for props and a requirement of
props initializer arguments to be ordered. For the latter, we have a helpful
convention in Tokamak that all named arguments to props initializers should be
ordered alphabetically.

The main problem is that currently there’s no easily extensible Swift parser or
a macro system available that would allow something like JSX to be used for
Tokamak. As soon is it becomes easy to implement, we’d definitely consider it as
an option.

Why is render function static on Component protocol?

With an alternative approach to API design of a framework like this we could
define components as plain functions, which wouldn’t need to be static:

func counter(hooks: Hooks) -> AnyNode {
  // ...

The problem here is that we need equality comparison on components to be able
to define Equatable on AnyNode. This isn’t available for plain functions:

let x = counter
let y = counter

// won't compile
x == y

// won't compile: reference equality is also not defined on functions,
// even though functions are reference types ¯_(ツ)_/¯ 
x === y

Protocols and structs with static functions allow us to work around this and
to formalise an hierarchy of different kinds of components with protocols and
Equatable constraints:

// equality comparison is available for types
struct Counter {
  static func render(hooks: Hooks) -> AnyNode { 
    // ...

// Tokamak does something like this internally for your components,
// consider following a pseudocode:
let xComponent = Counter.self
let yComponent = OtherComponent.self

var rendered: AnyNode?
if xComponent != yComponent {
  rendered = xComponent.render()

We could remove static from render on Component protocol, but this makes
possible adding and referencing instance properties from a non-static version
of render. Components could become inadvertently stateful that way, hiding the
fact that components are actually functions, not instances. Consider this
hypothetical API:

struct Counter {
  // this makes `Counter` component stateful,
  // but prevents observing state changes
  var count = 0

  // no `static` here, which makes `var` above accessible
  func render() -> AnyNode {
    return Label.node("(count)")

Now there’s direct access to component’s state, but we aren’t able to easily
schedule updates of the component tree when this state changes. We could require
authors of components to implement
on every instance property, but this is cumbersome and hard to enforce.
Marking render as static makes it harder to introduce unobservable local
state, while intended local state is managed with Hooks.


  • Thanks to the Swift community for
    building one of the best programming languages available!
  • Thanks to React

    for building a UI framework that is practical and elegant, while keeping it
    usable with JavaScript at the same time. 😄
  • Thanks to Render,
    ReSwift, Katana
    Komponents for inspiration!


This project adheres to the Contributor Covenant Code of
By participating, you are expected to uphold this code. Please report
unacceptable behavior to [email protected]


Max Desiatov, Matvii


Tokamak is available under the Apache 2.0 license. See the
LICENSE file for
more info.

Latest podspec

    "name": "TokamakAppKit",
    "version": "0.1.2",
    "summary": "AppKit support for Tokamak: React-like UI framework",
    "description": "Tokamak provides a declarative, testable and scalable API for building UI ncomponents backed by fully native views. You can use it for your new macOS apps nor add to existing apps with little effort and without rewriting the rest of nthe code or changing the app's overall architecture.nnTokamak recreates React Hooks API improving it with Swift's strong type nsystem, high performance and efficient memory management thanks to being ncompiled to a native binary.",
    "homepage": "",
    "license": {
        "type": "Apache 2.0",
        "file": "LICENSE"
    "authors": {
        "Max Desiatov": "[email protected]"
    "source": {
        "git": "",
        "tag": "0.1.2"
    "social_media_url": "",
    "platforms": {
        "osx": "10.14"
    "swift_version": "4.2",
    "source_files": "Sources/TokamakAppKit/**/*",
    "frameworks": "AppKit",
    "dependencies": {
        "Tokamak": [
            "~> 0.1"

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