---
title: trackingNumber
framework: appkit
role: symbol
role_heading: Instance Property
path: appkit/nsevent/trackingnumber
---

# trackingNumber

The identifier of a mouse-tracking event.

## Declaration

```swift
var trackingNumber: Int { get }
```

## Discussion

Discussion This property contains either an NSTrackingArea object or an NSView.TrackingRectTag constant, depending on how AppKit generated the event. Valid mouse-tracking event types are NSMouseEntered, NSMouseExited, and NSCursorUpdate. For other types of events, accessing this property raises internalInconsistencyException.

## See Also

### Related Documentation

- [enterExitEvent(with:location:modifierFlags:timestamp:windowNumber:context:eventNumber:trackingNumber:userData:)](appkit/nsevent/enterexitevent(with:location:modifierflags:timestamp:windownumber:context:eventnumber:trackingnumber:userdata:).md)
- [addTrackingRect(_:owner:userData:assumeInside:)](appkit/nsview/addtrackingrect(_:owner:userdata:assumeinside:).md)

### Getting tracking area information

- [eventNumber](appkit/nsevent/eventnumber.md)
- [trackingArea](appkit/nsevent/trackingarea.md)
- [userData](appkit/nsevent/userdata.md)