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Session roles and responsibilities, by example

The session component encapsulate a product experience, within a component topology. It launches all of the product-specific components (directly or indirectly), manages the lifecycle of those components, and manages the flow of control and information between the product-specific components (such as user-facing apps) and device-specific components (such as input devices, audio, and a display, if any).

The following sections demonstrate how a session author might implement some of these responsibilities.

Presenting an element's view

In the following example, a session forwards an Element's view to a GraphicalPresenter, by calling PresentView() with a ViewSpec. The ViewSpec includes the duplicated ViewRef (a sharable handle to the Element's View), and an optional set of initial, product-specific Annotations.

The component that implements the Graphical Presenter role knows how to open the view on a connected display.

    fn present_view_for_element(
        graphical_presenter: &GraphicalPresenterProxy,
        element: &Element
    ) -> Result<ViewControllerProxy, Error> {
        let view_provider = element.connect_to_service::<ViewProviderMarker>()?;
        let token_pair = scenic::ViewTokenPair::new()?;
        let scenic::ViewRefPair {
            mut control_ref,
            mut view_ref
        } = scenic::ViewRefPair::new()?;
        let view_ref_dup = fuchsia_scenic::duplicate_view_ref(&view_ref)?;

            &mut control_ref,
            &mut view_ref,

        let annotations = element.get_annotations()?;

        let view_spec = ViewSpec {
            view_holder_token: Some(token_pair.view_holder_token),
            view_ref: Some(view_ref_dup),
            annotations: Some(annotations),

        let (view_controller_proxy, server_end) = create_proxy::<ViewControllerMarker>()?;
        graphical_presenter.present_view(view_spec, Some(server_end))?;


Handling input

In the following example, if the MouseHandler detects a mouse-typed input event, the handler sends the event to Scenic and returns an empty vector. On all other types of input events, the MouseHandler returns a vector containing the InputEvent for the next InputHandler to process.

impl InputHandler for MouseHandler {
   async fn handle_input_event(
       &mut self,
       input_event: InputEvent,
   ) -> Vec<InputEvent> {
       match input_event {
           InputEvent {
               device_event: InputDeviceEvent::Mouse(mouse_event),
               device_descriptor: InputDeviceDescriptor::Mouse(mouse_descriptor),
           } => {
               // ... Handler specific details
               vec![] // InputEvent is consumed because it was sent to Scenic
           _ => vec![input_event], // InputEvent is returned for the next InputHanlder