# CMBufferQueueInstallTrigger(_:callback:refcon:condition:time:triggerTokenOut:) Installs a trigger with a callback on a buffer queue. ``` func CMBufferQueueInstallTrigger(_ queue: CMBufferQueue, callback: CMBufferQueueTriggerCallback?, refcon: UnsafeMutableRawPointer?, condition: CMBufferQueueTriggerCondition, time: CMTime, triggerTokenOut: UnsafeMutablePointer?) -> OSStatus ``` ## Parameters `queue` `CMBufferQueue` on which the trigger is being set. `callback` Callback to be called when the trigger condition becomes true. Can be `NULL`, if client intends only to explicitly test the condition. if `triggerTokenOut` is `NULL` this parameter cannot be `NULL` otherwise the trigger would be meaningless. `refcon` Refcon to be passed to the triggerCallback. Can be `NULL` if the callback doesn’t need it, or is itself `NULL`. `condition` The condition to be tested when evaluating the trigger. `time` The time value to compare against when evaluating the trigger. Must be numeric (ie. not invalid, indefinite, or infinite), except for certain trigger conditions which ignores it (eg, kCMBufferQueueTrigger_WhenMinPresentationTimeStampChanges). `triggerTokenOut` Address where created trigger token will be written. Can be `NULL`, if client has no need to explicitly test or remove the trigger. Cannot be `NULL` when triggerCallback is `NULL`, since the trigger would be meaningless then. ## Return Value A result code. See `Result Codes` ## Discussion The returned trigger token can be passed to `CMBufferQueueTestTrigger` and `CMBufferQueueRemoveTrigger`. The `triggerTokenOut` parameter can be `NULL` (client doesn’t need to test or remove trigger), and the `triggerCallback` parameter can be `NULL` (client doesn’t need callbacks, but rather will explicitly test the trigger). One of these two parameters must be non-NULL, however, since an untestable trigger that does not perform a callback is meaningless. If the trigger condition is already true, `CMBufferQueueInstallTrigger` will call the triggerCallback and will first write the trigger token to *triggerTokenOut. --- 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)