Exporting WIT

WIT files can be exported from the C# interfaces defined in Attributes. This lets you define the functions and types provided by the host only in C#, and use the exported WIT to generate the bindings on the guest side (such as Rust).

How WIT is generated

When a name in the form of namespace:package/interface is given to [ComponentInterface], the Source Generator generates the WIT definition of the interface and applies it to the interface as the [ComponentWitDefinition] attribute.

[ComponentInterface(@"my-game:events/functions")]
public partial interface IFunctions
{
    [ComponentApi(@"play-sound")]
    ValueTask PlaySound(string @name);
}

The definitions of the interfaces in the same package are combined into the WIT of the package.

package my-game:events;

interface functions {
    play-sound: func(name: string);
}

When an interface refers to types of other interfaces or packages, use is emitted automatically.

info

The code to build WIT from C# is in WaaS.ComponentModel.Binding.WitPackageBuilder. Without Unity, you can get the WIT by passing a list of types to WitPackageBuilder.Build.

Exporting in Unity

In Unity, the packages specified in a WIT Export Settings asset are exported as WIT files.

1. Create a settings asset from Create > WaaS > WIT Export Settings.
2. Set the following items.
   • Output Directory: the directory to export to, relative to the root of the Unity project.
   • Packages: the names of the packages to export, such as my-game:events.

Each package is exported to <Output Directory>/<namespace>-<package>/<namespace>-<package>.wit. In the example above with wit as the Output Directory, wit/my-game-events/my-game-events.wit is created.

The export runs automatically every time the scripts are compiled, and the files are rewritten only when their contents change. To export manually, select Assets > Export WIT for WaaS.

You can create multiple settings assets. The same package can be exported to multiple locations, which is useful when multiple crates include the WIT.

Working with Rust Importer

The exported WIT files are registered as external file dependencies of Rust Importer. When the WIT changes, the Rust source files that read it are reimported automatically.

On the Rust side, specify the exported WIT in path of wit_bindgen::generate! to generate the bindings.

wit_bindgen::generate!({
    inline: r#"
        package my-game:events-inline;

        world imports {
            import my-game:events/functions;
        }
    "#,
    path: ["../wit/my-game-events"],
    world: "my-game:events-inline/imports",
    generate_all,
});