# 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. ``` func addTrackingRect(_ rect: NSRect, owner: Any, userData data: UnsafeMutableRawPointer?, assumeInside flag: Bool) -> NSView.TrackingRectTag ``` ## Parameters `rect` A rectangle that defines a region of the view for tracking mouse-entered and mouse-exited events. `owner` The object that gets sent the event messages. It can be the view itself or some other object (such as an NSCursor or a custom drawing tool object), as long as it responds to both [`mouseEntered(with:)`](/documentation/AppKit/NSResponder/mouseEntered(with:)) and [`mouseExited(with:)`](/documentation/AppKit/NSResponder/mouseExited(with:)). `data` Data stored in the [`NSEvent`](/documentation/AppKit/NSEvent) object for each tracking event. `flag` If , the first event will be generated when the cursor leaves `aRect`, regardless if the cursor is inside `aRect` when the tracking rectangle is added. If the first event will be generated when the cursor leaves `aRect` if the cursor is initially inside `aRect`, or when the cursor enters `aRect` if the cursor is initially outside `aRect`. You usually want to set this flag to . ## Return Value A tag that identifies the tracking rectangle. It is stored in the associated [`NSEvent`](/documentation/AppKit/NSEvent) objects and can be used to remove the tracking rectangle. ## Discussion Tracking rectangles provide a general mechanism that can be used to trigger actions based on the cursor location (for example, a status bar or hint field that provides information on the item the cursor lies over). To simply change the cursor over a particular area, use [`addCursorRect(_:cursor:)`](/documentation/AppKit/NSView/addCursorRect(_:cursor:)). If you must use tracking rectangles to change the cursor, the [`NSCursor`](/documentation/AppKit/NSCursor) class specification describes the additional methods that must be invoked to change cursors by using tracking rectangles. In macOS 10.5 and later, tracking areas provide a greater range of functionality (see [`addTrackingArea(_:)`](/documentation/AppKit/NSView/addTrackingArea(_:))). ## See Also [`addTrackingArea(_:)`](/documentation/AppKit/NSView/addTrackingArea(_:)) Adds a given tracking area to the view. [`userData`](/documentation/AppKit/NSEvent/userData) The data associated with a mouse-tracking event. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)