# MIDIReceiveBlock A block receiving MIDI input that includes the incoming messages and a refCon to identify the source. ``` typealias MIDIReceiveBlock = (UnsafePointer, UnsafeMutableRawPointer?) -> Void ``` ## Parameters `evtlist` The incoming MIDI message(s). `srcConnRefCon` The refCon that identifies the source of the data, which is the value that you pass for the `connRefCon` parameter to [`MIDIPortConnectSource(_:_:_:)`](/documentation/CoreMIDI/MIDIPortConnectSource(_:_:_:)). This value is always  when receiving a MIDI event on a virtual input. ## Discussion A client receives incoming MIDI messages through this callback block. The [`MIDIInputPortCreateWithProtocol(_:_:_:_:_:)`](/documentation/CoreMIDI/MIDIInputPortCreateWithProtocol(_:_:_:_:_:)) and [`MIDIDestinationCreateWithProtocol(_:_:_:_:_:)`](/documentation/CoreMIDI/MIDIDestinationCreateWithProtocol(_:_:_:_:_:)) functions receive a [`MIDIReceiveBlock`](/documentation/CoreMIDI/MIDIReceiveBlock). The system creates a high-priority receive thread on your client’s behalf, and from that thread it calls your [`MIDIReceiveBlock`](/documentation/CoreMIDI/MIDIReceiveBlock) when incoming MIDI messages arrive. --- 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)