SwiftUI-compatible framework for building browser apps with WebAssembly

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 }
        Text("\(count)")
      }
      .onAppear { print("Counter.VStack onAppear") }
      .onDisappear { print("Counter.VStack onDisappear") }
    } else {
      VStack { Text("Limit exceeded") }
    }
  }
}

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

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.

An example of mouse events handling with DynamicHTML would look like this:

struct MouseEventsView: View {
  @State var position: CGPoint = .zero
  @State var isMouseButtonDown: Bool = false

  var body: some View {
    DynamicHTML(
      "div",
      ["style": "width: 200px; height: 200px; background-color: red;"],
      listeners: [
        "mousemove": { event in
          guard
            let x = event.offsetX.jsValue.number,
            let y = event.offsetY.jsValue.number
          else { return }

          position = CGPoint(x: x, y: y)
        },
        "mousedown": { _ in isMouseButtonDown = true },
        "mouseup": { _ in isMouseButtonDown = false },
      ]
    ) {
      Text("position is \(position), is mouse button down? \(isMouseButtonDown)")
    }
  }
}

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 = JSObject.global.document
let script = document.createElement("script")
script.setAttribute("src", "https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.27.0/moment.min.js")
document.head.appendChild(script)

_ = document.head.insertAdjacentHTML("beforeend", #"""
<link
  rel="stylesheet"
  href="https://cdnjs.cloudflare.com/ajax/libs/semantic-ui/2.4.1/semantic.min.css">
"""#)

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.

Fiber renderers

A new reconciler modeled after React’s Fiber reconciler is optionally available. It can provide faster updates and allow for larger View hierarchies. It also includes layout steps that can match SwiftUI layouts closer than CSS approximations.

You can specify which reconciler to use in your App‘s configuration:

struct CounterApp: App {
  static let _configuration: _AppConfiguration = .init(
    // Specify `useDynamicLayout` to enable the layout steps in place of CSS approximations.
    reconciler: .fiber(useDynamicLayout: true)
  )

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

Note: Not all Views and ViewModifiers are supported by Fiber renderers yet.

Requirements

For app developers

• macOS 11 and Xcode 13.2 or later when using VS Code. macOS 12 and Xcode 13.3 or later are recommended if you’d like to use Xcode for auto-completion, or when developing multi-platform apps that target WebAssembly and macOS at the same time.
• Swift 5.6 or later and Ubuntu 18.04/20.04 if you’d like to use Linux. Other Linux distributions are currently not supported.
• carton 0.15.x (carton is our build tool, see the “Getting started” section for installation steps)

For users of apps depending on Tokamak

Any recent browser that supports WebAssembly and required JavaScript features should work, which currently includes:

• Edge 84+
• Firefox 79+
• Chrome 84+
• Desktop Safari 14.1+
• Mobile Safari 14.8+

If you need to support older browser versions, you’ll have to build with JAVASCRIPTKIT_WITHOUT_WEAKREFS flag, passing -Xswiftc -DJAVASCRIPTKIT_WITHOUT_WEAKREFS flags when compiling. This should lower browser requirements to these versions:

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

Not all of these versions are tested on regular basis 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 0.15.0 or greater:

carton --version

2. Create a directory for your project and make it current:

mkdir TokamakApp && cd TokamakApp

3. Initialize the project from a template with carton:

carton init --template tokamak

4. Build the project and start the development server, carton dev can be kept running during development:

carton dev

5. Open http://127.0.0.1:8080/ 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.

If you have any questions, pleaes check out the FAQ document, and/or join the #tokamak channel on the SwiftWasm Discord server.

Security

By default, the DOM renderer will escape HTML control characters in Text views. If you wish to override this functionality, you can use the _domTextSanitizer modifier:

Text("<font color='red'>Unsanitized Text</font>")
  ._domTextSanitizer(Sanitizers.HTML.insecure)

You can also use custom sanitizers; the argument to _domTextSanitizer is simply a String -> String closure. If _domTextSanitizer is applied to a non-Text view, it will apply to all Text in subviews, unless overridden.

If you use user-generated or otherwise unsafe strings elsewhere, make sure to properly sanitize them yourself.

Troubleshooting

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.

Contributing

All contributions, no matter how small, are very welcome. You don’t have to be a web developer or a SwiftUI expert to meaningfully contribute. In fact, by checking out how some of the simplest views are implemented in Tokamak you may learn more how SwiftUI may work under the hood.

Updating our documentation and taking on the starter bugs is also appreciated. Don’t forget to join our Discord server to get in touch with the maintainers and other users. See CONTRIBUTING.md for more details.

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 conduct@tokamak.dev.

Sponsorship

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.

Maintainers

In alphabetical order: Carson Katri, Ezra Berch, Jed Fox, Morten Bek Ditlevsen, Yuta Saito.

Acknowledgments

• 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.

License

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.