---
title: "addObserver(_:forKeyPath:options:context:)"
framework: foundation
role: symbol
role_heading: Instance Method
path: "foundation/nsset/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. The observer must implement the key-value observing method doc://com.apple.documentation/documentation/ObjectiveC/NSObject-swift.class/observeValue(forKeyPath:of:change:context:).
- `keyPath`: The key path, relative to the set, of the property to observe. This value must not be nil.
- `options`: A combination of the doc://com.apple.foundation/documentation/Foundation/NSKeyValueObservingOptions values that specifies what is included in observation notifications. For possible values, see doc://com.apple.foundation/documentation/Foundation/NSKeyValueObservingOptions.
- `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 NSSet objects are not observable, so this method raises an exception when invoked on an NSSet object. Instead of observing a set, observe the unordered to-many relationship for which the set is the collection of related objects.

## See Also

### Key-Value Observing

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