Non-Send Resources

"Non-send" refers to data types that must only be accessed from the "main thread" of the application. Such data is marked by Rust as !Send (lacking the Send trait).

Some (often system) libraries have interfaces that cannot be safely used from other threads. A common example of this are various low-level OS interfaces for things like windowing, graphics, or audio. If you are doing advanced things like creating a Bevy plugin for interfacing with such things, you may encounter the need for this.

Normally, Bevy works by running all your systems on a thread-pool, making use of many CPU cores. However, you might need to ensure that some code always runs on the "main thread", or access data that is not safe to access in a multithreaded way.

Non-Send Systems and Data Access

To do this, you can use a NonSend<T> / NonSendMut<T> system parameter. This behaves just like Res<T> / ResMut<T>, letting you access an ECS resource (single global instance of some data), except that the presence of such a parameter forces the Bevy scheduler to always run the system on the main thread. This ensures that data never has to be sent between threads or accessed from different threads.

One example of such a resource is WinitWindows in Bevy. This is the low-level layer behind the window entities that you typically use for window management. It gives you more direct access to OS window management functionality.

fn setup_raw_window(
    q_primary: Query<Entity, With<PrimaryWindow>>,
    mut windows: NonSend<WinitWindows>
) {
    let raw_window = windows.get_window(q_primary.single());
    // do some special things using `winit` APIs
}

// just add it as a normal system;
// Bevy will notice the NonSend parameter
// and ensure it runs on the main thread
app.add_systems(Startup, setup_raw_window);

Custom Non-Send Resources

Normally, to insert resources, their types must be Send.

Bevy tracks non-Send resources separately, to ensure that they can only be accessed using NonSend<T> / NonSendMut<T>.

It is not possible to insert non-send resources using Commands, only using direct World access. This means that you have to initialize them in an exclusive system, FromWorld impl, or from the app builder.

fn setup_platform_audio(world: &mut World) {
    // assuming `OSAudioMagic` is some primitive that is not thread-safe
    let instance = OSAudioMagic::init();

    world.insert_non_send_resource(instance);
}

app.add_systems(Startup, setup_platform_audio);

Or, for simple things, if you don't need a full-blown system:

app.world.insert_non_send_resource(OSAudioMagic::init());