Applicable XR devices This guidance helps you build experiences for these types of XR devices. [Learn about XR device types →](https://developer.android.com/develop/xr/devices)  Audio \& Display Glasses [](https://developer.android.com/develop/xr/devices#audio-display) [Learn about XR device types →](https://developer.android.com/develop/xr/devices)
Different types of audio glasses and display glasses have different capabilities. For example, while all audio glasses and display glasses feature audio experiences through voice, only display glasses have a display where your app can show [UIs built with Jetpack Compose Glimmer](https://developer.android.com/develop/xr/jetpack-xr-sdk/jetpack-compose-glimmer). To create a seamless user experience across a range of devices, plan your [`Activity`](https://developer.android.com/reference/kotlin/android/app/Activity) for glasses to check for different device capabilities. This approach simplifies development by letting you build one activity that adapts its behavior, rather than multiple activities that target specific devices. ## Understand the lifecycle of projected activities The activity that you build for audio glasses and display glasses doesn't run directly on the device, but is instead projected to the device from a host device (such as user's phone). Dedicated activities that you build for this purpose are *projected activities* . The lifecycle of projected activities is built off the [standard activity lifecycle](https://developer.android.com/guide/components/activities/activity-lifecycle), but it also includes several key differences that support the capabilities of different types of audio glasses and display glasses.  **Figure 1**. The key events in the lifecycle of projected activities. Here is a breakdown of the key events, with callouts for interactions that are specific to activities that are projected to audio glasses and display glasses: - [`onCreate()`](https://developer.android.com/reference/kotlin/android/app/Activity#onCreate(android.os.Bundle)) - Called when the projected activity is created. - Initialize your app's Jetpack Compose Glimmer UI and other components here. - [`onStart()`](https://developer.android.com/reference/kotlin/android/app/Activity#onStart()) - Called when the projected activity is starting and the user is [aware](https://developer.android.com/develop/xr/jetpack-xr-sdk/glasses/support-different-types#awareness) of the app. - [`onResume()`](https://developer.android.com/reference/kotlin/android/app/Activity#onResume()) - Called when the projected activity regains focus. While the activity is in focus, it is interactable and can consume touchpad or button input. - Called when the glasses are put back on (donned), after they were previously taken off the head (doffed). - [`onPause()`](https://developer.android.com/reference/kotlin/android/app/Activity#onPause()) - Called when the projected activity loses focus, but the user is still [aware](https://developer.android.com/develop/xr/jetpack-xr-sdk/glasses/support-different-types#awareness) of your app. While the activity is out of focus, it isn't interactable stops consuming input. - [`onStop()`](https://developer.android.com/reference/kotlin/android/app/Activity#onStop()) - Called when the system believes the user is no longer [aware](https://developer.android.com/develop/xr/jetpack-xr-sdk/glasses/support-different-types#awareness) of your app. - Called when the glasses are removed from the head (doffed). - [`onDestroy()`](https://developer.android.com/reference/kotlin/android/app/Activity#onDestroy()) - Called when the projected activity is about to be destroyed. When this is called, the system releases all resources that are tied to the activity. ### Understand how display state affects the projected activity lifecycle In a standard [`Activity`](https://developer.android.com/reference/kotlin/android/app/Activity), the lifecycle state changes when the device's screen turns off, typically moving to [`onPause()`](https://developer.android.com/reference/kotlin/android/app/Activity#onPause()) or [`onStop()`](https://developer.android.com/reference/kotlin/android/app/Activity#onStop()). In contrast, the projected activity lifecycle doesn't change when the glasses' display turns on or off. This behavior means your projected activity continues to run in the Started or Resumed state even when the display is off, which lets your app's audio experiences continue without interruption. Other [activity state changes](https://developer.android.com/guide/components/activities/state-changes) triggered by different system and user events behave the usual way. ### Understand how user awareness affects your projected activity A user can be aware of your projected activity even if it's not visible. *Awareness* refers to all the ways a user can sense and interact with your app's experiences, including the following: - Listening to audio, audible feedback, or other sound cues. - App actions that trigger a user-facing LED, such as a privacy indicator if your app accesses the camera or microphone. In these situations, the user is aware that their audio glasses or display glasses are active and responding, even if they aren't looking at a display: - The [`onStart()`](https://developer.android.com/reference/kotlin/android/app/Activity#onStart()) state for projected activities means the activity is active. - The [`onResume()`](https://developer.android.com/reference/kotlin/android/app/Activity#onResume()) state means the activity is interactable and can receive touchpad input, or is receiving primary input dispatch. So long as the user is aware of your app, your activity remains active and in the foreground. If the system doesn't detect any signals of awareness for a short period of time, the system removes the activity from the foreground and eventually triggers [`onStop()`](https://developer.android.com/reference/kotlin/android/app/Activity#onStop()). ## Understand projected activities and projected contexts Audio glasses and display glasses are treated as a connected device that extends the capabilities of a user's phone. A *projected context* is a device-aware [`Context`](https://developer.android.com/reference/kotlin/android/content/Context) that lets apps interact with the hardware on a connected glasses device---such as its sensors or camera---rather than the phone's hardware. As you develop experiences for audio glasses and display glasses, your app must use a projected context to access the glasses' hardware. A projected context can be automatically granted to your app depending on the calling activity's context: - **For projected activities**: If your app's code is running from within your projected activity, its own activity context is already a projected context. In this scenario, calls made within that activity can already access the glasses' hardware. - **For phone apps or services** : If a part of your app outside of your projected activity (such as a phone activity or a service) needs to access the glasses' hardware, it must explicitly obtain a projected context. To do this, use the [`createProjectedDeviceContext()`](https://developer.android.com/develop/xr/jetpack-xr-sdk/access-hardware-projected-context#phone-activity-service) method. For more information, see [Use a projected context to access hardware on audio glasses and display glasses](https://developer.android.com/develop/xr/jetpack-xr-sdk/access-hardware-projected-context). ### Understand device-aware APIs Some standard Android APIs change which device's hardware they access depending on the calling activity's `Context`. When these APIs receive a projected context, they access the glasses' hardware instead of the hardware on the host phone device: - [`CameraManager`](https://developer.android.com/reference/kotlin/android/hardware/camera2/CameraManager): Accesses the camera on the glasses. - [`SensorManager`](https://developer.android.com/reference/kotlin/android/hardware/SensorManager): Retrieves sensor data (for example, gyroscope or accelerometer data) from the glasses. - [`AudioManager`](https://developer.android.com/reference/kotlin/android/media/AudioManager): Manages audio streams, volume, and routing on the glasses.