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# CFMessagePortSetInvalidationCallBack(_:_:)

Sets the callback function invoked when a CFMessagePort object is invalidated.

```
func CFMessagePortSetInvalidationCallBack(_ ms: CFMessagePort!, _ callout: CFMessagePortInvalidationCallBack!)
```

## Parameters

`ms`

The message port to examine.

`callout`

The callback function to invoke when `ms` is invalidated. Pass `NULL` to remove a callback.

## Discussion

If `ms` is already invalid, `callout` is invoked immediately.

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