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# addObserver(_:forKeyPath:options:context:)

Registers the observer object to receive KVO notifications for the key path relative to the object receiving this message.

```
func addObserver(_ observer: NSObject, forKeyPath keyPath: String, options: NSKeyValueObservingOptions = [], context: UnsafeMutableRawPointer?)
```

## Parameters

`observer`

The object to register for KVO notifications. The observer must implement the key-value observing method [`observeValue(forKeyPath:of:change:context:)`](/documentation/ObjectiveC/NSObject-swift.class/observeValue(forKeyPath:of:change:context:)).

`keyPath`

The key path, relative to the object receiving this message, of the property to observe. This value must not be `nil`.

`options`

A combination of the `NSKeyValueObservingOptions` values that specifies what is included in observation notifications. For possible values, see <doc://com.apple.documentation/documentation/Foundation/NSKeyValueObservingOptions>.

`context`

Arbitrary data that is passed to `observer` in [`observeValue(forKeyPath:of:change:context:)`](/documentation/ObjectiveC/NSObject-swift.class/observeValue(forKeyPath:of:change:context:)).

## Discussion

Neither the object receiving this message, nor `observer`, are retained. An object that calls this method must also eventually call either the [`removeObserver(_:forKeyPath:)`](/documentation/ObjectiveC/NSObject-swift.class/removeObserver(_:forKeyPath:)) or [`removeObserver(_:forKeyPath:context:)`](/documentation/ObjectiveC/NSObject-swift.class/removeObserver(_:forKeyPath:context:)) method to unregister the observer when participating in KVO.

---

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