---
title: callHostBlock
framework: audiotoolbox
role: symbol
role_heading: Instance Property
path: audiotoolbox/aumessagechannel/callhostblock
---

# callHostBlock

A callback for the audio unit to send a message to the host.

## Declaration

```swift
optional var callHostBlock: CallHostBlock? { get set }
```

## Discussion

Discussion The host must set a block on this property to use it.