---
title: MIDINotifyProc
framework: coremidi
role: symbol
role_heading: Type Alias
path: coremidi/midinotifyproc
---

# MIDINotifyProc

A callback function for notifying clients of state changes.

## Declaration

```swift
typealias MIDINotifyProc = (UnsafePointer<MIDINotification>, UnsafeMutableRawPointer?) -> Void
```

## Parameters

- `message`: A structure that contains information about what changed.
- `refCon`: The client’s refCon, passed to doc://com.apple.coremidi/documentation/CoreMIDI/MIDIClientCreate(_:_:_:_:).

## Discussion

Discussion The system invokes this callback when some aspect of the current MIDI setup changes. The system calls it on the same thread that you called MIDIClientCreate(_:_:_:_:) on.

## See Also

### Callbacks

- [MIDINotification](coremidi/midinotification.md)