# MIDINotifyProc A callback function for notifying clients of state changes. ``` typealias MIDINotifyProc = (UnsafePointer, UnsafeMutableRawPointer?) -> Void ``` ## Parameters `message` A structure that contains information about what changed. `refCon` The client’s `refCon`, passed to [`MIDIClientCreate(_:_:_:_:)`](/documentation/CoreMIDI/MIDIClientCreate(_:_:_:_:)). ## 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(_:_:_:_:)`](/documentation/CoreMIDI/MIDIClientCreate(_:_:_:_:)) on. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)