# MIDINotifyBlock A callback block for notifying clients of state changes. ``` typealias MIDINotifyBlock = (UnsafePointer) -> Void ``` ## Parameters `message` A structure that contains information about what changed. ## Discussion The system calls this block when some aspect of the current MIDI setup changes. The system calls it on an arbitrary thread chosen by the implementation; thread safety is the responsibility of the block. --- 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)