# MTLCopyAllDevicesWithObserver Returns an array of all the Metal GPU devices in the system and registers a notification handler that Metal calls when the device list changes. ``` extern NSArray> *MTLCopyAllDevicesWithObserver(id*observer, MTLDeviceNotificationHandler handler); ``` ## Parameters `observer` A pointer to an object instance the method sets to a new observer — which Metal retains — before returning. `handler` A notification handler you implement that Metal calls when the system adds or removes a GPU device from the system. ## Discussion Keep a copy of `observer` in your app after this function returns in case you want to stop receiving notifications. You can stop receiving notifications by passing `observer` to the [`MTLRemoveDeviceObserver(_:)`](/documentation/Metal/MTLRemoveDeviceObserver(_:)) function. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)