---
title: IOUSBHostInterestHandler
framework: iousbhost
role: symbol
role_heading: Type Alias
path: iousbhost/iousbhostinteresthandler
---

# IOUSBHostInterestHandler

The callback that handles underlying service-state changes.

## Declaration

```swift
typealias IOUSBHostInterestHandler = (IOUSBHostObject, UInt32, UnsafeMutableRawPointer?) -> Void
```

## Parameters

- `hostObject`: The doc://com.apple.iousbhost/documentation/IOUSBHost/IOUSBHostObject of the interest notification.
- `messageType`: A messageType enumeration that IOKit/IOMessage.h or the doc://com.apple.documentation/documentation/kernel/ioservice family defines.
- `messageArgument`: An argument for the message, dependent on the message type. If the message data is larger than sizeof(void*), messageArgument contains a pointer to the message data; otherwise, messageArgument contains the message data.

## Discussion

Discussion This is the block for the kIOGeneralInterest handler, and handles underlying service-state changes, such as termination. See IOServiceInterestCallback in IOKit for more details. An internal serial queue separate from the input/output queue services all notifications.

## See Also

### Managing the Object Life Cycle

- [IOUSBHostObjectInitOptions](iousbhost/iousbhostobjectinitoptions.md)
- [ioService](iousbhost/iousbhostobject/ioservice.md)
- [queue](iousbhost/iousbhostobject/queue.md)
- [destroy()](iousbhost/iousbhostobject/destroy().md)