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# AUMessageChannel

A specification for a bidirectional communication message channel.

```
protocol AUMessageChannel
```

## Overview

Audio units and their hosts have unique communication needs. For example, for better audio processing they can exchange musical context. An audio unit implements a class that conforms to [`AUMessageChannel`](/documentation/AudioToolbox/AUMessageChannel) and returns an instance from [`messageChannel(for:)`](/documentation/AudioToolbox/AUAudioUnit/messageChannel(for:)). A host queries the instance through the channel name.

This protocol offers a method to send messages to an audio unit and a block to send messages to the host.

## Topics

### Sending a Message to an Audio Unit

[`-  callAudioUnit:`](/documentation/AudioToolbox/AUMessageChannel/callAudioUnit(_:))

Sends an audio unit a custom data message.

### Sending a Message to a Host

[`callHostBlock`](/documentation/AudioToolbox/AUMessageChannel/callHostBlock)

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


---

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