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# CFRunLoopObserverCallBack

Callback invoked when a CFRunLoopObserver object is fired.

```
typealias CFRunLoopObserverCallBack = (CFRunLoopObserver?, CFRunLoopActivity, UnsafeMutableRawPointer?) -> Void
```

## Parameters

`observer`

The run loop observer that is firing.

`activity`

The current activity stage of the run loop.

`info`

The `info` member of the [`CFRunLoopObserverContext`](/documentation/CoreFoundation/CFRunLoopObserverContext) structure that was used when creating the run loop observer.

## Discussion

You specify this callback when you create the run loop observer with [`CFRunLoopObserverCreate(_:_:_:_:_:_:)`](/documentation/CoreFoundation/CFRunLoopObserverCreate(_:_:_:_:_:_:)).

---

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