Abstract: User engagement is detected and used to control operation of a computing device. User engagement is detected by a sensor such as a camera that identifies if a user's face is oriented towards a display device. If the user is not facing the display device, the sensor determines that the user is unengaged. The computing device is thus able to perform a power-saving operation, such as dimming the display device, when the user is unengaged. The computing device includes an API that abstracts sensor data into a user engagement signal indicating that the user is either engaged or unengaged. The OS and applications running on the computing device act on the user engagement signal provided by the API without communicating directly with the sensor. The user engagement signal may be provided as an input to a state machine.