---
title: "addObserver(_:forKeyPath:options:context:)"
framework: foundation
role: symbol
role_heading: Instance Method
path: "foundation/nsorderedset/addobserver(_:forkeypath:options:context:)"
---

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

Raises an exception.

## Declaration

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

## Parameters

- `observer`: The object to register for KVO notifications.
- `keyPath`: The key path, relative to the array, of the property to observe. This value must not be nil.
- `options`: A combination of doc://com.apple.foundation/documentation/Foundation/NSKeyValueObservingOptions values that specifies what is included in observation notifications.
- `context`: Arbitrary data that is passed to observer in doc://com.apple.documentation/documentation/ObjectiveC/NSObject-swift.class/observeValue(forKeyPath:of:change:context:).

## Discussion

Discussion NSOrderedSet objects are not observable, so this method raises an exception when invoked on an NSOrderedSet object. Instead of observing an ordered set, observe the to-many relationship for which the ordered set is the collection of related objects.

## See Also

### Key-Value Observing Support

- [removeObserver(_:forKeyPath:)](foundation/nsorderedset/removeobserver(_:forkeypath:).md)
- [removeObserver(_:forKeyPath:context:)](foundation/nsorderedset/removeobserver(_:forkeypath:context:).md)