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  "identifier" : "/documentation/AppKit/NSEvent/trackingNumber",
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  "title" : "trackingNumber"
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# trackingNumber

The identifier of a mouse-tracking event.

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

## Discussion

This property contains either an [`NSTrackingArea`](/documentation/AppKit/NSTrackingArea) object or an [`NSView.TrackingRectTag`](/documentation/AppKit/NSView/TrackingRectTag) constant, depending on how AppKit generated the event. Valid mouse-tracking event types are [`NSMouseEntered`](/documentation/AppKit/NSMouseEntered), [`NSMouseExited`](/documentation/AppKit/NSMouseExited), and [`NSCursorUpdate`](/documentation/AppKit/NSCursorUpdate). For other types of events, accessing this property raises <doc://com.apple.documentation/documentation/Foundation/NSExceptionName/internalInconsistencyException>.

## See Also

[`+  enterExitEventWithType:location:modifierFlags:timestamp:windowNumber:context:eventNumber:trackingNumber:userData:`](/documentation/AppKit/NSEvent/enterExitEvent(with:location:modifierFlags:timestamp:windowNumber:context:eventNumber:trackingNumber:userData:))

Creates and returns a new event object that describes a tracking-rectangle or cursor-update event.

[`-  addTrackingRect:owner:userData:assumeInside:`](/documentation/AppKit/NSView/addTrackingRect(_:owner:userData:assumeInside:))

Establishes an area for tracking mouse-entered and mouse-exited events within the view and returns a tag that identifies the tracking rectangle.


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