# HostCallback_GetTransportState When called by the system, provides audio transport state and timeline information to an audio unit from a host application. ``` typealias HostCallback_GetTransportState = (UnsafeMutableRawPointer?, UnsafeMutablePointer?, UnsafeMutablePointer?, UnsafeMutablePointer?, UnsafeMutablePointer?, UnsafeMutablePointer?, UnsafeMutablePointer?) -> OSStatus ``` ## Parameters `inHostUserData` Custom data that you provided when registering your callback with the audio unit. `outIsPlaying` On output, `TRUE` if audio is playing, or `FALSE` otherwise. `outTransportStateChanged` On output, `TRUE` if the transport state changed since the last time the callback was invoked, or `FALSE` otherwise. `outCurrentSampleInTimeLine` On output, the sample number, indexed from zero from the beginning of the timeline. `outIsCycling` On output, `TRUE` if cycling, or `FALSE` otherwise. `outCycleStartBeat` `outCycleEndBeat` ## Discussion If you named your callback function `MyHostCallback_GetTransportState`, you would declare it like this: --- 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)