---
title: "CFRunLoopAddObserver(_:_:_:)"
framework: corefoundation
role: symbol
role_heading: Function
path: "corefoundation/cfrunloopaddobserver(_:_:_:)"
---

# CFRunLoopAddObserver(_:_:_:)

Adds a CFRunLoopObserver object to a run loop mode.

## Declaration

```swift
func CFRunLoopAddObserver(_ rl: CFRunLoop!, _ observer: CFRunLoopObserver!, _ mode: CFRunLoopMode!)
```

## Parameters

- `rl`: The run loop to modify.
- `observer`: The run loop observer to add.
- `mode`: The run loop mode to which to add observer. Use the constant doc://com.apple.corefoundation/documentation/CoreFoundation/CFRunLoopMode/commonModes to add observer to the set of objects monitored by all the common modes.

## Discussion

Discussion A run loop observer can be registered in only one run loop at a time, although it can be added to multiple run loop modes within that run loop. If rl already contains observer in mode, this function does nothing.

## See Also

### Managing Observers

- [CFRunLoopContainsObserver(_:_:_:)](corefoundation/cfrunloopcontainsobserver(_:_:_:).md)
- [CFRunLoopRemoveObserver(_:_:_:)](corefoundation/cfrunloopremoveobserver(_:_:_:).md)