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Cross-platform app development in Rust

Crux helps you share your app’s business logic and behavior across mobile (iOS and Android) and web, as a single, reusable core built with Rust.

Unlike React Native, the user interface layer is built natively, with modern declarative UI frameworks such as Swift UI, Jetpack Compose and React/Vue or a WASM based framework on the web.

The UI layer is as thin as it can be, and all other work is done by the shared core. The interface with the core has static type checking across languages.

§Getting Started

Crux applications are split into two parts: a Core written in Rust and a Shell written in the platform native language (e.g. Swift or Kotlin). It is also possible to use Crux from Rust shells. The Core architecture is based on Elm architecture.

Quick glossary of terms to help you follow the example:

  • Core - the shared core written in Rust

  • Shell - the native side of the app on each platform handling UI and executing side effects

  • App - the main module of the core containing the application logic, especially model changes and side-effects triggered by events. App can be composed from modules, each resembling a smaller, simpler app.

  • Event - main input for the core, typically triggered by user interaction in the UI

  • Model - data structure (typically tree-like) holding the entire application state

  • View model - data structure describing the current state of the user interface

  • Effect - A side-effect the core can request from the shell. This is typically a form of I/O or similar interaction with the host platform. Updating the UI is considered an effect.

  • Capability - A user-friendly API used to request effects and provide events that should be dispatched when the effect is completed. For example, a HTTP client is a capability.

Below is a minimal example of a Crux-based application Core:

// src/
use crux_core::{render::Render, App, macros::Effect};
use serde::{Deserialize, Serialize};

// Model describing the application state
struct Model {
   count: isize,

// Event describing the actions that can be taken
#[derive(Serialize, Deserialize)]
pub enum Event {

// Capabilities listing the side effects the Core
// will use to request side effects from the Shell
#[cfg_attr(feature = "typegen", derive(crux_core::macros::Export))]
pub struct Capabilities {
   pub render: Render<Event>,

struct Hello;

impl App for Hello {
   // Use the above Event
   type Event = Event;
   // Use the above Model
   type Model = Model;
   type ViewModel = String;
   // Use the above Capabilities
   type Capabilities = Capabilities;

   fn update(&self, event: Event, model: &mut Model, caps: &Capabilities) {
       match event {
           Event::Increment => model.count += 1,
           Event::Decrement => model.count -= 1,
           Event::Reset => model.count = 0,

       // Request a UI update

   fn view(&self, model: &Model) -> Self::ViewModel {
       format!("Count is: {}", model.count)

§Integrating with a Shell

To use the application in a user interface shell, you need to expose the core interface for FFI. This “plumbing” will likely be simplified with macros in the future versions of Crux.

// src/
pub mod app;

use lazy_static::lazy_static;
use wasm_bindgen::prelude::wasm_bindgen;

pub use crux_core::bridge::{Bridge, Request};
pub use crux_core::Core;
pub use crux_http as http;

pub use app::*;


lazy_static! {
    static ref CORE: Bridge<Effect, App> = Bridge::new(Core::new::<Capabilities>());

pub fn process_event(data: &[u8]) -> Vec<u8> {

pub fn handle_response(id: u32, data: &[u8]) -> Vec<u8> {
    CORE.handle_response(id, data)

pub fn view() -> Vec<u8> {

You will also need a hello.udl file describing the foreign function interface:

// src/hello.udl
namespace hello {
  sequence<u8> process_event([ByRef] sequence<u8> msg);
  sequence<u8> handle_response([ByRef] sequence<u8> res);
  sequence<u8> view();

Finally, you will need to set up the type generation for the Model, Message and ViewModel types. See [typegen] for details.



  • Capabilities provide a user-friendly API to request side-effects from the shell.
  • A capability which can spawn tasks which orchestrate across other capabilities. This is useful for orchestrating a number of different effects into a single transaction.
  • Built-in capability used to notify the Shell that a UI update is necessary.
  • Testing support for unit testing Crux apps.


  • Panics if the pattern doesn’t match an Effect from the specified Update


  • The Crux core. Create an instance of this type with your effect type, and your app type as type parameters
  • Request represents an effect request from the core to the shell.


  • Implement App on your type to make it into a Crux app. Use your type implementing App as the type argument to Core or Bridge.
  • Implemented automatically with the Effect macro from crux_macros. This is used by the Bridge to serialize effects going across the FFI boundary.