import EventKit

let eventStore = EKEventStore()

func createEvent(
    title: String,
    startDate: Date,
    endDate: Date,
    calendar: EKCalendar? = nil
) throws {
    let event = EKEvent(eventStore: eventStore)
    event.title = title
    event.startDate = startDate
    event.endDate = endDate
    event.calendar = calendar ?? eventStore.defaultCalendarForNewEvents

    try eventStore.save(event, span: .thisEvent)
}

// List writable calendars
let calendars = eventStore.calendars(for: .event)
    .filter { $0.allowsContentModifications }

// Use the first writable calendar, or the default
let targetCalendar = calendars.first ?? eventStore.defaultCalendarForNewEvents
event.calendar = targetCalendar

import CoreLocation

let location = EKStructuredLocation(title: "Apple Park")
location.geoLocation = CLLocation(latitude: 37.3349, longitude: -122.0090)
event.structuredLocation = location

func fetchEvents(from start: Date, to end: Date) -> [EKEvent] {
    let predicate = eventStore.predicateForEvents(
        withStart: start,
        end: end,
        calendars: nil  // nil = all calendars
    )
    return eventStore.events(matching: predicate)
        .sorted { $0.startDate < $1.startDate }
}

if let event = eventStore.event(withIdentifier: savedEventID) {
    print(event.title ?? "No title")
}

func createReminder(title: String, dueDate: Date) throws {
    let reminder = EKReminder(eventStore: eventStore)
    reminder.title = title
    reminder.calendar = eventStore.defaultCalendarForNewReminders()

    let dueDateComponents = Calendar.current.dateComponents(
        [.year, .month, .day, .hour, .minute],
        from: dueDate
    )
    reminder.dueDateComponents = dueDateComponents

    try eventStore.save(reminder, commit: true)
}

func fetchIncompleteReminders() async -> [EKReminder] {
    let predicate = eventStore.predicateForIncompleteReminders(
        withDueDateStarting: nil,
        ending: nil,
        calendars: nil
    )

    return await withCheckedContinuation { continuation in
        eventStore.fetchReminders(matching: predicate) { reminders in
            continuation.resume(returning: reminders ?? [])
        }
    }
}

func completeReminder(_ reminder: EKReminder) throws {
    reminder.isCompleted = true
    try eventStore.save(reminder, commit: true)
}

// Every week, indefinitely
let weeklyRule = EKRecurrenceRule(
    recurrenceWith: .weekly,
    interval: 1,
    end: nil
)
event.addRecurrenceRule(weeklyRule)

// Every 2 weeks, ending after 10 occurrences
let biweeklyRule = EKRecurrenceRule(
    recurrenceWith: .weekly,
    interval: 2,
    end: EKRecurrenceEnd(occurrenceCount: 10)
)

// Monthly, ending on a specific date
let monthlyRule = EKRecurrenceRule(
    recurrenceWith: .monthly,
    interval: 1,
    end: EKRecurrenceEnd(end: endDate)
)

// Every Monday and Wednesday
let days = [
    EKRecurrenceDayOfWeek(.monday),
    EKRecurrenceDayOfWeek(.wednesday)
]

let complexRule = EKRecurrenceRule(
    recurrenceWith: .weekly,
    interval: 1,
    daysOfTheWeek: days,
    daysOfTheMonth: nil,
    monthsOfTheYear: nil,
    weeksOfTheYear: nil,
    daysOfTheYear: nil,
    setPositions: nil,
    end: nil
)
event.addRecurrenceRule(complexRule)

// Change only this occurrence
try eventStore.save(event, span: .thisEvent)

// Change this and all future occurrences
try eventStore.save(event, span: .futureEvents)

// 15 minutes before
let alarm = EKAlarm(relativeOffset: -15 * 60)
event.addAlarm(alarm)

// At an absolute date
let absoluteAlarm = EKAlarm(absoluteDate: alertDate)
event.addAlarm(absoluteAlarm)

import EventKitUI

class EventEditorCoordinator: NSObject, EKEventEditViewDelegate {
    let eventStore = EKEventStore()

    func presentEditor(from viewController: UIViewController) {
        let editor = EKEventEditViewController()
        editor.eventStore = eventStore
        editor.editViewDelegate = self
        viewController.present(editor, animated: true)
    }

    func eventEditViewController(
        _ controller: EKEventEditViewController,
        didCompleteWith action: EKEventEditViewAction
    ) {
        switch action {
        case .saved:
            // Event saved
            break
        case .canceled:
            break
        case .deleted:
            break
        @unknown default:
            break
        }
        controller.dismiss(animated: true)
    }
}

import EventKitUI

let viewer = EKEventViewController()
viewer.event = existingEvent
viewer.allowsEditing = true
navigationController?.pushViewController(viewer, animated: true)

let chooser = EKCalendarChooser(
    selectionStyle: .multiple,
    displayStyle: .allCalendars,
    entityType: .event,
    eventStore: eventStore
)
chooser.showsDoneButton = true
chooser.showsCancelButton = true
chooser.delegate = self
present(UINavigationController(rootViewController: chooser), animated: true)

NotificationCenter.default.addObserver(
    forName: .EKEventStoreChanged,
    object: eventStore,
    queue: .main
) { [weak self] _ in
    self?.refreshEvents()
}

// WRONG: Legacy request API on current systems
eventStore.requestAccess(to: .event) { granted, error in }

// CORRECT: Use the granular async methods
let granted = try await eventStore.requestFullAccessToEvents()

// WRONG: No check -- will throw if calendar is read-only
event.calendar = someCalendar
try eventStore.save(event, span: .thisEvent)

// CORRECT: Verify the calendar allows modifications
guard someCalendar.allowsContentModifications else {
    event.calendar = eventStore.defaultCalendarForNewEvents
    return
}
event.calendar = someCalendar
try eventStore.save(event, span: .thisEvent)

// WRONG: Event appears at wrong time for traveling users
event.startDate = Date()
event.endDate = Date().addingTimeInterval(3600)

// CORRECT: Set the timezone explicitly for location-specific events
event.timeZone = TimeZone(identifier: "America/New_York")
event.startDate = startDate
event.endDate = endDate

// WRONG: Changes never persisted
try eventStore.save(event1, span: .thisEvent, commit: false)
try eventStore.save(event2, span: .thisEvent, commit: false)
// Missing commit!

// CORRECT: Commit after batching
try eventStore.save(event1, span: .thisEvent, commit: false)
try eventStore.save(event2, span: .thisEvent, commit: false)
try eventStore.commit()

// WRONG: Event fetched from storeA, saved to storeB
let event = storeA.event(withIdentifier: id)!
try storeB.save(event, span: .thisEvent) // Undefined behavior

// CORRECT: Use the same store throughout
let event = eventStore.event(withIdentifier: id)!
try eventStore.save(event, span: .thisEvent)