Swift 3 iOS code to add event to calendar (http://stackoverflow.com/questions/42754868/save-event-to-users-calendar)

AddEvent.swift

//  ViewController.swift
import UIKit
import EventKit
import EventKitUI

class ViewController: UIViewController {
    
    let store = EKEventStore()
    
    @IBAction func didTapButton(_ sender: AnyObject) {
        checkAuthorization {
            self.createEvent()
        }
    }
    
    /// Check calendar authorization
    ///
    /// - If authorized, just call the provided closure;
    /// - If not yet determined, perform the standard `requestAccess` process;
    /// - If denied/restricted, tell user it's required and ask if they want to go to settings to authorize.
    ///
    /// - Parameter authorizedSuccessBlock: The closure to call once permission is confirmed. If never confirmed, the closure is not called.
    
    private func checkAuthorization(authorized: @escaping () -> ()) {
        let status = EKEventStore.authorizationStatus(for: .event)
        
        switch status {
        case .denied, .restricted:
            promptToOpenSettings()
            
        case .authorized:
            authorized()
            
        case .notDetermined:
            store.requestAccess(to: .event) { granted, error in
                DispatchQueue.main.async {
                    guard granted else {
                        self.promptToOpenSettings()
                        return
                    }

                    authorized()
                }
            }
        }
    }
    
    /// Ask user whether they want to allow access to settings
    
    private func promptToOpenSettings() {
        let alert = UIAlertController(title: nil, message: "This app cannot add event to your calendar until you grant the app permission to do so. Would you like to go to Settings to enable calendar access?", preferredStyle: .actionSheet)
        alert.addAction(UIAlertAction(title: "Go to Settings", style: .default) { action in
            UIApplication.shared.open(URL(string: UIApplicationOpenSettingsURLString)!, options: [:], completionHandler: nil)
        })
        alert.addAction(UIAlertAction(title: "Cancel", style: .cancel))
        self.present(alert, animated: true)
    }
    
    /// Create the event
    
    private func createEvent() {
        // create the event object
        let event = EKEvent(eventStore: store)
        event.title = "Foo"
        event.startDate = Date().addingTimeInterval(60 * 60)          // create event for one hour from now
        event.endDate = event.startDate.addingTimeInterval(60 * 60)   // and it's one hour long
        
        // prompt user to add event (to whatever calendar they want)
        let controller = EKEventEditViewController()
        controller.event = event
        controller.eventStore = store
        controller.editViewDelegate = self
        present(controller, animated: true)
    }
}

extension ViewController: EKEventEditViewDelegate {
    
    func eventEditViewController(_ controller: EKEventEditViewController, didCompleteWith action: EKEventEditViewAction) {
        controller.dismiss(animated: true)
    }
}

Note, this Swift 3 code:

• performs "just in time" calendar authorization (you're more likely to get permission if you ask at the point the user is actively trying to undertake some activity to add something to their calendar ... if you ask at app startup, the user doesn't quite know your intentions and is more likely to deny permissions);
• lets the user go to "Settings" if access was previously denied.