SwiftUI-compatible framework for building browser apps with WebAssembly and native apps for other platforms

What's New



This release adds support for more SwiftUI types and modifiers, and fixes bugs. Including, but not limited to:

  • Toolbar type and toolbar modifier
  • ProgressView type
  • Animation and related types and modifiers
  • opacity, scaleEffect, aspectRatio, and controlSize modifiers
  • Material and Gradient types
  • HierarchicalShapeStyle (.primary/.secondary/.tertiary/.quaternary) type
  • ContainerRelativeShape type
  • spacing argument support for initializers of HStack and VStack
  • support for standard Foundation types, such as CGRect, CGSize (we previously used our own implementation of those, which weren't fully compatible with Foundation)
  • ability to sort HTML attributes when generating static HTML, which is essential for end-to-end tests that cover generated output.

Many thanks to @carson-katri, @ezraberch, and @yonihemi for their contributions to this release!

Closed issues:

  • Is there anyway to compile this from Xcode? (#406)
  • Xcode doesn't compile — gtk/gtk.h not found (#405)
  • Use NSGeometry types from Foundation (#404)
  • Adding padding to a view contained in a Button causes the Button to disappear (#403)
  • .background modifier with contained shape causes view to expand to full vertical size of the screen (#402)
  • Multi-line string handling in Text views (#400)
  • Content with spacer jumps when blurring and focusing the page (#395)
  • Frame sizes do not match expected behavior. (#387)
  • URL hash change demo crashes (#369)
  • Infinite loops w/ 100% CPU usage caused by stack overflows (#367)
  • TokamakDemo breaks after use of _domRef (#326)
  • Add support for toolbar modifier and related types (#316)

Merged pull requests:

Tokamak logo

SwiftUI-compatible framework for building browser apps with WebAssembly

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At the moment Tokamak implements a very basic subset of SwiftUI. Its DOM renderer supports a few view types and modifiers (you can check the current list in the progress document), and a new HTML view for constructing arbitrary HTML. The long-term goal of Tokamak is to implement as much of SwiftUI API as possible and to provide a few more helpful additions that simplify HTML and CSS interactions.

If there's some SwiftUI API that's missing but you'd like to use it, please review the existing issues and PRs to get more details about the current status, or create a new issue to let us prioritize the development based on the demand. We also try to make the development of views and modifiers easier (with the help from the HTML view, see the example below), so pull requests are very welcome! Don't forget to check the "Contributing" section first.

If you'd like to participate in the growing SwiftWasm community, you're also very welcome to join our Discord server, or the #webassembly channel in the SwiftPM Slack.

Example code

Tokamak API attempts to resemble SwiftUI API as much as possible. The main difference is that you use import TokamakShim instead of import SwiftUI in your files. The former makes your views compatible with Apple platforms, as well as platforms supported by Tokamak (currently only WebAssembly/WASI with more coming in the future):

import TokamakShim

struct Counter: View {
  @State var count: Int
  let limit: Int

  var body: some View {
    if count < limit {
      VStack {
        Button("Increment") { count += 1 }
      .onAppear { print("Counter.VStack onAppear") }
      .onDisappear { print("Counter.VStack onDisappear") }
    } else {
      VStack { Text("Limit exceeded") }

struct CounterApp: App {
  var body: some Scene {
    WindowGroup("Counter Demo") {
      Counter(count: 5, limit: 15)

// @main attribute is not supported in SwiftPM apps.
// See for more details.

Arbitrary HTML

With the HTML view you can also render any HTML you want, including inline SVG:

struct SVGCircle: View {
  var body: some View {
    HTML("svg", ["width": "100", "height": "100"]) {
      HTML("circle", [
        "cx": "50", "cy": "50", "r": "40",
        "stroke": "green", "stroke-width": "4", "fill": "yellow",

HTML doesn't support event listeners, and is declared in the TokamakStaticHTML module, which TokamakDOM re-exports. The benefit of HTML is that you can use it for static rendering in libraries like TokamakVapor and TokamakPublish.

Another option is the DynamicHTML view provided by the TokamakDOM module, which has a listeners property with a corresponding initializer parameter. You can pass closures that can handle onclick, onmouseover and other DOM events for you in the listeners dictionary. Check out MDN docs for the full list.

Arbitrary styles and scripts

While JavaScriptKit is a great option for occasional interactions with JavaScript, sometimes you need to inject arbitrary scripts or styles, which can be done through direct DOM access:

import JavaScriptKit

let document =
let script = document.createElement("script")
script.setAttribute("src", "")

_ = document.head.insertAdjacentHTML("beforeend", #"""

This way both Semantic UI styles and moment.js localized date formatting (or any arbitrary style/script/font added that way) are available in your app.

Requirements for app developers

  • macOS 10.15 and Xcode 11.4 or later. macOS 11.0 and Xcode 12.0 or later are required if you're building a multi-platform app with Tokamak that also needs to support SwiftUI on macOS.
  • Swift 5.2 or later and Ubuntu 18.04 if you'd like to use Linux. Other Linux distributions are currently not supported.

Requirements for app users

Any browser that supports WebAssembly should work, which currently includes:

  • Edge 16+
  • Firefox 61+
  • Chrome 66+
  • (Mobile) Safari 12+

Not all of these were tested though, compatibility reports are very welcome!

Getting started

Tokamak relies on carton as a primary build tool. As a part of these steps you'll install carton via Homebrew on macOS (unfortunately you'll have to build it manually on Linux). Assuming you already have Homebrew installed, you can create a new Tokamak app by following these steps:

  1. Install carton:
brew install swiftwasm/tap/carton

If you had carton installed before this, make sure you have version 0.9.0 or greater:

carton --version
  1. Create a directory for your project and make it current:
mkdir TokamakApp && cd TokamakApp
  1. Initialize the project from a template with carton:
carton init --template tokamak
  1. Build the project and start the development server, carton dev can be kept running during development:
carton dev
  1. Open in your browser to see the app running. You can edit the app source code in your favorite editor and save it, carton will immediately rebuild the app and reload all browser tabs that have the app open.

You can also clone this repository and run carton dev --product TokamakDemo in its root directory. This will build the demo app that shows almost all of the currently implemented APIs.


unable to find utility "xctest" error when building

This error can only happen on macOS, so make sure you have Xcode installed as listed in the requirements. If you do have Xcode installed but still get the error, please refer to this StackOverflow answer.

Syntax highlighting and autocomplete don't work in Xcode

Open Package.swift of your project that depends on Tokamak with Xcode and build it for macOS. As Xcode currently doesn't support cross-compilation for non-Apple platforms, your project can't be indexed if it doesn't build for macOS, even if it isn't fully function on macOS when running. If you need to exclude some WebAssembly-specific code in your own app that doesn't compile on macOS, you can rely on #if os(WASI) compiler directives.

All relevant modules of Tokamak (including TokamakDOM) should compile on macOS. You may see issues with TokamakShim on macOS Catalina, where relevant SwiftUI APIs aren't supported, but replacing import TokamakShim with import TokamakDOM should resolve the issue until you're able to update to macOS Big Sur.

If you stumble upon code in Tokamak that doesn't build on macOS and prevents syntax highlighting or autocomplete from working in Xcode, please report it as a bug.

Syntax highlighting and autocomplete don't work in VSCode

Make sure you have the SourceKit LSP extension installed. If you don't trust this unofficial release, please follow the manual building and installation guide. Apple currently doesn't provide an official build of the extension on the VSCode Marketplace unfortunately.


Modular structure

Tokamak is built with modularity in mind, providing a multi-platform TokamakCore module and separate modules for platform-specific renderers. Currently, the only available renderer modules are TokamakDOM and TokamakStaticHTML, the latter can be used for static websites and server-side rendering. If you'd like to implement your own custom renderer, please refer to our renderers guide for more details.

Tokamak users only need to import a renderer module they would like to use, while TokamakCore is hidden as an "internal" Tokamak package target. Unfortunately, Swift does not allow us to specify that certain symbols in TokamakCore are private to a package, but they need to stay public for renderer modules to get access to them. Thus, the current workaround is to mark those symbols with underscores in their names to indicate this. It can be formulated as these "rules":

  1. If a symbol is restricted to a module and has no public access control, no need for an underscore.
  2. If a symbol is part of a public renderer module API (e.g. TokamakDOM), no need for an underscore, users may use those symbols directly, and it is re-exported from TokamakCore by the renderer module via public typealias.
  3. If a function or a type have public on them only by necessity to make them available in TokamakDOM, but unavailable to users (or not intended for public use), underscore is needed to indicate that.

The benefit of separate modules is that they allow us to provide separate renderers for different platforms. Users can pick and choose what they want to use, e.g. purely static websites would use only TokamakStaticHTML, single-page apps would use TokamakDOM, maybe in conjuction with TokamakStaticHTML for pre-rendering. As we'd like to try to implement a native renderer for Android at some point, probably in a separate TokamakAndroid module, Android apps would use TokamakAndroid with no need to be aware of any of the web modules.

Coding Style

This project uses SwiftFormat and SwiftLint to enforce formatting and coding style. SwiftFormat 0.45.3 and SwiftLint 0.39.2 or later versions are recommended. We encourage you to run SwiftFormat and SwiftLint within a local clone of the repository in whatever way works best for you. You can do that either manually, or automatically with VSCode extensions for SwiftFormat and SwiftLint respectively, or with the Xcode extension, or build phase.

To guarantee that these tools run before you commit your changes on macOS, you're encouraged to run this once to set up the pre-commit hook:

brew bundle # installs SwiftLint, SwiftFormat and pre-commit
pre-commit install # installs pre-commit hook to run checks before you commit

Refer to the pre-commit documentation page for more details and installation instructions for other platforms.

SwiftFormat and SwiftLint also run on CI for every PR and thus a CI build can fail with inconsistent formatting or style. We require CI builds to pass for all PRs before merging.

Code of Conduct

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


If this library saved you any amount of time or money, please consider sponsoring the work of its maintainers on their sponsorship pages: @carson-katri, @kateinoigakukun, and @MaxDesiatov. While some of the sponsorship tiers give you priority support or even consulting time, any amount is appreciated and helps in maintaining the project.


In alphabetical order: Carson Katri, David Hunt, Jed Fox, Max Desiatov, Morten Bek Ditlevsen, Yuta Saito.


  • Thanks to the Swift community for building one of the best programming languages available!
  • Thanks to everyone who developed React with its reconciler/renderer architecture that inspired Tokamak in the first place.
  • Thanks to the designers of the SwiftUI API who showed us how to write UI apps in Swift declaratively (arguably even in a better way than React did).
  • Thanks to SwiftWebUI for reverse-engineering some of the bits of SwiftUI and kickstarting the front-end Swift ecosystem for the web.
  • Thanks to Render, ReSwift, Katana UI and Komponents for inspiration!

SwiftUI is a trademark owned by Apple Inc. Software maintained as a part of the Tokamak project is not affiliated with Apple Inc.


Tokamak is available under the Apache 2.0 license. Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the LICENSE file for more info.


  • Swift Tools 5.3.0
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Last updated: Wed Oct 20 2021 04:04:58 GMT-0500 (GMT-05:00)