---
title: HostCallback_GetTransportState
framework: audiotoolbox
role: symbol
role_heading: Type Alias
path: audiotoolbox/hostcallback_gettransportstate
---

# HostCallback_GetTransportState

When called by the system, provides audio transport state and timeline information to an audio unit from a host application.

## Declaration

```swift
typealias HostCallback_GetTransportState = (UnsafeMutableRawPointer?, UnsafeMutablePointer<DarwinBoolean>?, UnsafeMutablePointer<DarwinBoolean>?, UnsafeMutablePointer<Float64>?, UnsafeMutablePointer<DarwinBoolean>?, UnsafeMutablePointer<Float64>?, UnsafeMutablePointer<Float64>?) -> 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

Discussion If you named your callback function MyHostCallback_GetTransportState, you would declare it like this:

## See Also

### Getting Information from the Host

- [HostCallback_GetBeatAndTempo](audiotoolbox/hostcallback_getbeatandtempo.md)
- [HostCallback_GetMusicalTimeLocation](audiotoolbox/hostcallback_getmusicaltimelocation.md)
- [HostCallback_GetTransportState2](audiotoolbox/hostcallback_gettransportstate2.md)
- [AUInputSamplesInOutputCallback](audiotoolbox/auinputsamplesinoutputcallback.md)
- [AUMIDIOutputCallback](audiotoolbox/aumidioutputcallback.md)