# AUHostTransportStateBlock A block through which hosts provide information about their transport state. ``` typealias AUHostTransportStateBlock = (UnsafeMutablePointer?, UnsafeMutablePointer?, UnsafeMutablePointer?, UnsafeMutablePointer?) -> Bool ``` ## Discussion If the host app provides this block to an audio unit, via the [`transportStateBlock`](/documentation/AudioToolbox/AUAudioUnit/transportStateBlock) property, then the block may be called at the beginning of each render cycle to obtain information about the current transport state. Any of the provided parameters may be null to indicate that the audio unit is not interested in that particular piece of information. This block returns if the transport state was able to be retrieved from the host; it returns otherwise. The block takes the following parameters: - transportStateFlags: The current state of the audio transport. - currentSamplePosition: The current position in the host’s timeline, in samples at the audio unit’s output sample rate. - cycleStartBeatPosition: If cycling, the starting beat position of the cycle. - cycleEndBeatPosition: If cycling, the ending beat position of the cycle. --- 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)