---
title: "BacklightService"
canonical_url: https://developer.repebble.com/docs/c/Foundation/Event_Service/BacklightService/
last_updated: 2026-11-05
sdk_version: 4.43
---
# BacklightService

Source: https://developer.repebble.com/docs/c/Foundation/Event_Service/BacklightService/

The APIs on this page will only work with SDK 4.9+.

Notifies your app when the backlight turns on or off.

The BacklightService lets your app react to the backlight turning on. The handler is invoked whenever the backlight transitions between off and on (any non-off state — fully on, on with timeout, or fading out is treated as "on"). This means you get a single edge per wake, not one event per fade step.

## Function Documentation

void backlight\_service\_subscribe([BacklightHandler](https://developer.repebble.com/docs/c/Foundation/Event_Service/BacklightService.md#BacklightHandler) handler)

Subscribe to the backlight event service. Once subscribed, the handler is called every time the backlight transitions between off and on.

#### Parameters
 handler

A callback to be executed on backlight on/off events.

void backlight\_service\_unsubscribe(void)

Unsubscribe from the backlight event service. Once unsubscribed, the previously registered handler will no longer be called.

To read the current backlight state at any time, use [light\_is\_on()](https://developer.repebble.com/docs/c/User_Interface/Light.md#light_is_on).

## Typedef Documentation

 typedef void(\* BacklightHandler)(bool on) 

Callback type for backlight on/off events.

#### Parameters
 on

true when the backlight has just turned on, false when it has just turned fully off.