# CGEventTapCallBack A client-supplied callback function that’s invoked whenever an associated event tap receives a Quartz event. ``` typealias CGEventTapCallBack = (CGEventTapProxy, CGEventType, CGEvent, UnsafeMutableRawPointer?) -> Unmanaged? ``` ## Parameters `proxy` A proxy for the event tap. See [`CGEventTapProxy`](/documentation/CoreGraphics/CGEventTapProxy). This callback function may pass this proxy to other functions such as the event-posting routines. `type` The event type of this event. See [`CGEventType`](/documentation/CoreGraphics/CGEventType). `event` The incoming event. This event is owned by the caller, and you do not need to release it. `refcon` A pointer to user-defined data. You specify this pointer when you create the event tap. Several different event taps could use the same callback function, each tap with its own user-defined data. ## Discussion If the event tap is an active filter, your callback function should return one of the following: - The (possibly modified) event that is passed in. This event is passed back to the event system. - A newly-constructed event. After the new event has been passed back to the event system, the new event will be released along with the original event. - `NULL` if the event passed in is to be deleted. If the event tap is an passive listener, your callback function may return the event that is passed in, or `NULL`. In either case, the event stream is not affected. --- 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)