# CGScreenRefreshCallback A client-supplied callback function that’s invoked when an area of the display is modified or refreshed. ``` typealias CGScreenRefreshCallback = (UInt32, UnsafePointer, UnsafeMutableRawPointer?) -> Void ``` ## Parameters `count` The number of rectangles in the `rectArray` parameter. `rectArray` A list of the rectangles in the refreshed areas, specified in the global display coordinate space. The origin is the upper-left corner of the main display. You should not modify or deallocate memory pointed to by `rectArray`. `userParameter` The user data you specify when you register this callback. ## Discussion To register a screen-refresh callback function, you call the function [`CGRegisterScreenRefreshCallback(_:_:)`](/documentation/CoreGraphics/CGRegisterScreenRefreshCallback(_:_:)). Quartz invokes your callback function when operations such as drawing, window movement, scrolling, or display reconfiguration occur on local displays. When you finish using a callback registration, you should call the function [`CGUnregisterScreenRefreshCallback(_:_:)`](/documentation/CoreGraphics/CGUnregisterScreenRefreshCallback(_:_:)) to remove it. Note that a single rectangle may occupy multiple displays, either by overlapping the displays or by residing on coincident displays when mirroring is active. You can use the function [`CGGetDisplaysWithRect(_:_:_:_:)`](/documentation/CoreGraphics/CGGetDisplaysWithRect(_:_:_:_:)) to determine the displays a rectangle occupies. --- 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)