Setting alarms for calendar events

One of the features of the EventKit framework is to provide users with the option to set alarms for their calendar items. Alarms appear as a notification and remind users of a scheduled calendar event. They can be time-based, triggering at a specific time.

First of all, you need the user's permission to access the events and create an event on the user's calendar.

The alarm can be created when adding a new event, so the time of creation of the event can be used as a reference for the triggering the notification.

let alarm = EKAlarm(relativeOffset: TimeInterval(-600))

event.addAlarm(alarm)

To create a new alarm use the EKAlarm class. The initializer EKAlarm(absoluteDate:) crates an alarm that triggers on a precise date, while the initializer EKAlarm(relativeOffset:) sets the trigger of the alarm based on the starting date of the event it is associated with.

To associate the alarm with a calendar event use the addAlarm(_:) method. Here is an example of how you can set up an alarm when creating a new event in your application:

import EventKit
import Observation

@Observable
final class CalendarEventManager {

    let eventStore = EKEventStore()
    
    func createEvent(
        title: String,
        startDate: Date,
        endDate: Date,
        alarmLeadMinutes: Int = 10
    ) async {
        // Ensure we have a calendar to save into (user's default for new events).
        guard let targetCalendar = eventStore.defaultCalendarForNewEvents else {
            print("No default calendar available for new events.")
            return
        }
        
        // Create the event and set its primary fields.
        let event = EKEvent(eventStore: eventStore)
        event.title = title
        event.startDate = startDate
        event.endDate = endDate
        event.calendar = targetCalendar
        
        // Configure a relative alarm that fires N minutes *before* the start.
        // Negative offset = before startDate; positive = after.
        let alarmOffset = TimeInterval(-alarmLeadMinutes * 60)
        let alarm = EKAlarm(relativeOffset: alarmOffset)
        event.addAlarm(alarm)
        
        do {
            try eventStore.save(event, span: .thisEvent)
            print("Event with alarm was saved.")
        } catch {
            print("Failed to save event: \(error.localizedDescription)")
        }
    }
    
}