iOS 14 Widget - Update widget when calendar changed or receive local notification

I have a widget that need to to be refreshed (Reload timeline) every time a calendar changed (EKEventStore changed). How can I achieve this? I am researching on the background task refresh but has no idea how to listen to the notification.

func onBackgroundURLSessionEvents(matching matchingString: String, _urlSessionEvent: @escaping ((String, @escaping () -> Void) -> Void)) -> someWidgetConfiguration

This func is recommended in Apples's how to keep widget up to date. But cannot understand how to apply this to my case.

Thank you for any suggestions.

iOS 14 Widget - Update widget when calendar changed or receive local notification
 
 
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