Advanced example showing how to control an external application from the Rerun viewer, by extending the viewer UI. > [!NOTE] > [#2337](https://github.com/rerun-io/rerun/issues/2337): In order to spawn a web viewer with these customizations applied, you have to build the web viewer of the version yourself. This is currently not supported outside of the Rerun repository. ## Overview This example is divided into two parts: - **Viewer** ([`src/viewer.rs`](src/viewer.rs)): Wraps the Rerun viewer inside an [`eframe`](https://github.com/emilk/egui/tree/master/crates/eframe) app. - **App** ([`src/app.rs`](src/app.rs)): The application that uses the Rerun SDK. In the `app`, an additional gRPC server is opened to allow the `viewer` to send messages to the `app`. Similar to the [`extend_viewer_ui`](../extend_viewer_ui/) example, the `viewer` is wrapped in an `eframe` app, which allows us to handle the extra communication logic and define our own control UI using [`egui`](https://github.com/emilk/egui). The communication between the `viewer` and the `app` is implemented in the [`comms`](src/comms/) module. It defines a simple protocol to send messages between the `viewer` and the `app` using [`bincode`](https://github.com/bincode-org/bincode). The protocol supports basic commands that the `viewer` can send to the `app`, such as logging a [`Boxes3D`](https://www.rerun.io/docs/reference/types/archetypes/boxes3d) or [`Point3D`](https://www.rerun.io/docs/reference/types/archetypes/points3d) to an entity, or changing the radius of a set of points that is being logged. ## Usage First start the Rerun SDK app with `cargo run -p custom_callback --bin custom_callback_app`, and then start the extended viewer with `cargo run -p custom_callback --bin custom_callback_viewer`. ## Relationship with Viewer callbacks The [`re_viewer`] crate also exposes some baseline Viewer events through the [`StartupOptions.on_event`](https://docs.rs/re_viewer/latest/re_viewer/struct.StartupOptions.html#structfield.on_event) field, which can exist alongside your own events from widgets added by extending the UI.