Starboard Module Reference: ui_navigation.h

API to allow applications to take advantage of the platform's native UI engine. This is mainly to drive the animation of visual elements and to signal which of those elements have focus. The implementation should not render any visual elements; instead, it will be used to guide the app in where these elements should be drawn.

When the application creates the user interface, it will create SbUiNavItems for interactable elements. Additionally, the app must specify the position and size of these navigation items. As the app's user interface changes, it will create and destroy navigation items as appropriate.

For each render frame, the app will query the local transform for each SbUiNavItem in case the native UI engine moves individual items in response to user interaction. If the navigation item is a container, then the content offset will also be queried to determine the placement of its content items.



Well-defined value for an invalid navigation item.



Navigation items may be one of the following types. This must be specified upon item creation and may not change during the item's lifespan.


  • kSbUiNavItemTypeFocus

    This is a single focusable item.

  • kSbUiNavItemTypeContainer

    This is a container of navigation items which can also be containers themselves or focusable items. Containers themselves cannot be focused.



An opaque handle to an implementation-private structure representing a navigation item.


typedef struct SbUiNavItemPrivate* SbUiNavItem



This structure specifies all the callbacks which the platform UI engine should invoke for various interaction events on navigation items. These callbacks may be invoked from any thread at any frequency. The callback_context is the value that was passed on creation of the relevant SbUiNavItem.


  • void(*onblur)(SbUiNavItem item, void *callback_context)

    Invoke when an item has lost focus. This is only used with focus items.

  • void(*onfocus)(SbUiNavItem item, void *callback_context)

    Invoke when an item has gained focus. This is only used with focus items.

  • void(*onscroll)(SbUiNavItem item, void *callback_context)

    Invoke when an item's content offset is changed. This is only used with container items.


This structure declares the interface to the UI navigation implementation. All function pointers must be specified if the platform supports UI navigation.


  • SbUiNavItem(*create_item)(SbUiNavItemType type, const SbUiNavCallbacks *callbacks, void *callback_context)

    Create a new navigation item. When the user interacts with this item the appropriate SbUiNavCallbacks function will be invoked with the provided callback_context. An item is not interactable until it is enabled.

  • void(*destroy_item)(SbUiNavItem item)

    Destroy the given navigation item. If this is a content of another item, then it will first be unregistered. Additionally, if this item contains other items, then those will be unregistered as well, but they will not be automatically destroyed.

  • void(*set_focus)(SbUiNavItem item)

    This is used to manually force focus on a navigation item of type kSbUiNavItemTypeFocus. Any previously focused navigation item should receive the blur event. If the item is not transitively a content of the root item, then this does nothing.

    Specifying kSbUiNavItemInvalid should remove focus from the UI navigation system.

  • void(*set_item_enabled)(SbUiNavItem item, bool enabled)

    This is used to enable or disable user interaction with the specified navigation item. All navigation items are disabled when created, and they must be explicitly enabled to allow user interaction. If a container is disabled, then all of its contents are not interactable even though they remain enabled. If enabled is false, it must be guaranteed that once this function returns, no callbacks associated with this item will be invoked until the item is re-enabled.

  • void(*set_item_dir)(SbUiNavItem item, SbUiNavItemDir dir)

    This specifies directionality for container items. Containers within containers do not inherit directionality. Directionality must be specified for each container explicitly.

    This should work even if item is disabled.

  • void(*set_item_focus_duration)(SbUiNavItem item, float seconds)

    Set the minimum amount of time the focus item should remain focused once it becomes focused. This may be used to make important focus items harder to navigate over. Focus may still be moved before seconds has elapsed by using the set_focus() function. By default, item focus duration is 0 seconds.

  • void(*set_item_size)(SbUiNavItem item, float width, float height)

    Set the interactable size of the specified navigation item. By default, an item's size is (0,0).

  • void(*set_item_transform)(SbUiNavItem item, const SbUiNavMatrix2x3 *transform)

    Set the transform for the navigation item and its contents if the item is a container. This specifies the placement of the item's center within its container. The transform origin is the center of the item. Distance is measured in pixels with the origin being the top-left of the item's container. By default, an item's transform is identity.

  • bool(*get_item_focus_transform)(SbUiNavItem item, SbUiNavMatrix4 *out_transform)

    Retrieve the focus transform matrix for the navigation item. The UI engine may translate, rotate, and/or tilt focus items to reflect user interaction. This transform should be multiplied with the item's transform to get its position inside its container. The transform origin is the center of the item. Return false if the item position should not be changed (i.e. the transform should be treated as identity).

  • bool(*get_item_focus_vector)(SbUiNavItem item, float *out_x, float *out_y)

    Retrieve a vector representing the focus location within a focused item. This is used to provide feedback about user input that is too small to result in a focus change. If there is no focus vector for the navigation item, then return false and leave out_x and out_y unchanged. Otherwise, return true and set the output values in the range of [-1, +1] with (out_x, out_y) of (-1, -1) being the top-left corner of the navigation item and (0, 0) being the center.

  • void(*set_item_container_window)(SbUiNavItem item, SbWindow window)

    This attaches the given navigation item (which must be a container) to the specified window. Navigation items are only interactable if they are transitively attached to a window.

    The native UI engine should never change this navigation item's content offset. It is assumed to be used as a proxy for the system window.

    A navigation item may only have a SbUiNavItem or SbWindow as its direct container. The navigation item hierarchy is established using set_item_container_item() with the root container attached to a SbWindow using set_item_container_window() to enable interaction with all enabled items in the hierarchy.

    If item is already registered with a different window, then this will unregister it from that window then attach it to the given window. It is an error to register more than one navigation item with any given window. If window is kSbWindowInvalid, then this will unregister the item from its current window if any. Upon destruction of item or window, the item is automatically unregistered from the window.

  • void(*set_item_container_item)(SbUiNavItem item, SbUiNavItem container)

    A container navigation item may contain other navigation items. However, it is an error to have circular containment or for container to not be of type kSbUiNavItemTypeContainer. If item already has a different container, then this first serves that connection. If container is kSbUiNavItemInvalid, then this removes item from its current container. Upon destruction of item or container, the item is automatically removed from the container.

    The position of items within a container are specified relative to the container's position. The position of these content items are further modified by the container's "content offset".

    For example, consider item A with position (5,5) and content offset (0,0). Given item B with position (10,10) is registered as a content of item A.

    1) Item B should be drawn at position (15,15).

    2) If item A's content offset is changed to (10,0), then item B should be drawn at position (5,15).

    Essentially, content items should be drawn at: [container position] + [content position] - [container content offset]

    Content items may overlap within a container. This can cause obscured items to be unfocusable. The only rule that needs to be followed is that contents which are focus items can obscure other contents which are containers, but not vice versa. The caller must ensure that content focus items do not overlap other content focus items and content container items do not overlap other content container items.

  • void(*set_item_content_offset)(SbUiNavItem item, float content_offset_x, float content_offset_y)

    Set the current content offset for the given container. This may be used to force scrolling to make certain content items visible. A container item's content offset helps determine where its content items should be drawn. Essentially, a content item should be drawn at: [container position] + [content position] - [container content offset] If item is not a container, then this does nothing. By default, the content offset is (0,0).

    This should update the values returned by get_item_content_offset() even if the item is disabled.

  • void(*get_item_content_offset)(SbUiNavItem item, float *out_content_offset_x, float *out_content_offset_y)

    Retrieve the current content offset for the navigation item. If item is not a container, then the content offset is (0,0).

    The native UI engine should not change the content offset of a container unless one of its contents (possibly recursively) is focused. This is to allow seamlessly disabling then re-enabling focus items without having their containers change offsets.

  • void(*do_batch_update)(void(*update_function)(void *), void *context)

    Call update_function with context to perform a series of UI navigation changes atomically before returning.


Navigation items of type kSbUiNavItemTypeContainer have directionality. If directionality is not specified for a container, it should default to left-to- right and top-to-bottom.

///   For left-to-right, content offset x = 0 shows the leftmost content.
///                          |<--Container Size-->|
///     +--------------------+--------------------+--------------------+
///     | Not selectable.    | Selectable.        | Selectable.        |
///     | Offscreen.         | Onscreen.          | Offscreen.         |
///     | Negative position. | Positive position. | Positive position. |
///     +--------------------+--------------------+--------------------+
///                          ^
///                  Content Offset X = 0.
///   For right-to-left, content offset x = 0 shows the rightmost content.
///                          |<--Container Size-->|
///     +--------------------+--------------------+--------------------+
///     | Selectable.        | Selectable.        | Not selectable.    |
///     | Offscreen.         | Onscreen.          | Offscreen.         |
///     | Negative position. | Positive position. | Positive position. |
///     +--------------------+--------------------+--------------------+
///                          ^
///                  Content Offset X = 0.
  Top-to-bottom is similar to left-to-right, but for the Y position.
  Bottom-to-top is similar to right-to-left, but for the Y position.


  • bool is_left_to_right
  • bool is_top_to_bottom


This represents a 2x3 transform matrix in row-major order.

///   | a b tx |
///   | c d ty |


  • float m


This represents a 4x4 transform matrix in row-major order.


  • float m



Retrieve the platform's UI navigation implementation. If the platform does not provide one, then return false without modifying out_interface. Otherwise, initialize all members of out_interface and return true. The out_interface pointer must not be NULL.


bool SbUiNavGetInterface(SbUiNavInterface *out_interface)


Returns whether the given navigation item handle is valid.


static bool SbUiNavItemIsValid(SbUiNavItem item)