Running In A Browser Using WebAssembly¶

Warning

Only Rust supports using Slint with WebAssembly.

If you’re using Rust, the tutorial so far used cargo run to build and run the code as a native application. Native applications are the primary target of the Slint framework, but it also supports WebAssembly for demonstration purposes. This section uses the standard rust tool wasm-bindgen and wasm-pack to run the game in the browser. Read the wasm-bindgen documentation for more about using wasm and rust.

Install wasm-pack using cargo:

cargo install wasm-pack

Edit the Cargo.toml file to add the dependencies.

[target.'cfg(target_arch = "wasm32")'.dependencies]
wasm-bindgen = { version = "0.2" }
getrandom = { version = "0.2.2", features = ["js"] }

'cfg(target_arch = "wasm32")' ensures that these dependencies are only active when compiling for the wasm32 architecture. Note that the rand dependency is now duplicated, to enable the "wasm-bindgen" feature.

While you are editing the Cargo.toml, make one last change. To turn the binary into a library by adding the following:

[lib]
path = "src/main.rs"
crate-type = ["cdylib"]

This is because wasm-pack requires Rust to generate a "cdylib".

You also need to change main.rs by adding the wasm_bindgen(start) attribute to the main function and export it with the pub keyword:

#[cfg_attr(target_arch = "wasm32",
           wasm_bindgen::prelude::wasm_bindgen(start))]
pub fn main() {
    //...
}

Compile the program with wasm-pack build --release --target web. This creates a pkg directory containing several files, including a .js file named after the program name that you need to import into an HTML file.

Create a minimal index.html in the top level of the project that declares a <canvas> element for rendering and loads the generated wasm file. The Slint runtime expects the <canvas> element to have the id id = "canvas". (Replace memory.js with the correct file name).

<html>
    <body>
        <!-- canvas required by the Slint runtime -->
        <canvas id="canvas"></canvas>
        <script type="module">
            // import the generated file.
            import init from "./pkg/memory.js";
            init();
        </script>
    </body>
</html>

Unfortunately, loading ES modules isn’t allowed for files on the file system when accessed from a file:// URL, so you can’t load the index.html. Instead, you need to serve it through a web server. For example, using Python, by running:

python3 -m http.server

Now you can access the game at http://localhost:8000.