---
title: AUHostTransportStateBlock
framework: audiotoolbox
role: symbol
role_heading: Type Alias
path: audiotoolbox/auhosttransportstateblock
---

# AUHostTransportStateBlock

A block through which hosts provide information about their transport state.

## Declaration

```swift
typealias AUHostTransportStateBlock = (UnsafeMutablePointer<AUHostTransportStateFlags>?, UnsafeMutablePointer<Double>?, UnsafeMutablePointer<Double>?, UnsafeMutablePointer<Double>?) -> Bool
```

## Discussion

Discussion If the host app provides this block to an audio unit, via the 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 true if the transport state was able to be retrieved from the host; it returns false otherwise. The block takes the following parameters:

## See Also

### Providing Data to the Host

- [musicalContextBlock](audiotoolbox/auaudiounit/musicalcontextblock.md)
- [transportStateBlock](audiotoolbox/auaudiounit/transportstateblock.md)
- [contextName](audiotoolbox/auaudiounit/contextname.md)
- [supportsMPE](audiotoolbox/auaudiounit/supportsmpe.md)
- [AUHostMusicalContextBlock](audiotoolbox/auhostmusicalcontextblock.md)