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# IOUSBHostInterestHandler

The callback that handles underlying service-state changes.

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

## Parameters

`hostObject`

The [`IOUSBHostObject`](/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

This is the block for the `kIOGeneralInterest` handler, and handles underlying service-state changes, such as termination. See <doc://com.apple.documentation/documentation/iokit/ioserviceinterestcallback> in <doc://com.apple.documentation/documentation/iokit> for more details. An internal serial queue separate from the input/output queue services all notifications.

---

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