Visit Monitoring

As Covid-19 started to spread, we tried to find a way to help as developers, with the tools at our disposal. One idea was to use out of the box Visit Monitoring service proposed by Apple to match locations with infected people. The idea at the time was the same as the ExposureNotification framework, but using location service instead of bluetooth.

That’s a good opportunity to focus on visit monitoring here and see if it’s a good candidate for this use case.

The service


Visit monitoring requires the Always authorization for location. The service is not available when When In Use is requested. You can find all the location services and their needed authorizations here.

Visit monitoring is meant to be used in background. Like the other background services, it will continue to run even when the app is suspended, and will launch the app in the background to deliver events. Make sure to enable Location Updates in the application background modes capabilities and to set allowsBackgroundLocationUpdates to true for the location manager.

Start monitoring visits

The use is really straightforward, as for other location services.

First create your CLLocationManager and call the method startMonitoringVisits(). If the service is authorized and running, you will receive callbacks on the delegate method locationManager(_:didVisit:).

Here is a skeleton of implementation of such a feature.

import CoreLocation

class LocationService: NSObject, CLLocationManagerDelegate {

    private let locationManager: CLLocationManager
    private var pendingAuthorizationCompletion: ((Bool) -> Void)?

    init(locationManager: CLLocationManager = CLLocationManager()) {
        self.locationManager = locationManager
        locationManager.allowsBackgroundLocationUpdates = true
        locationManager.delegate = self

    // MARK: - LocationService

    func start() {
        switch CLLocationManager.authorizationStatus() {
        case .notDetermined:
        case .authorizedAlways,
            // Handle unauthorized
        @unknown default:

    // MARK: - CLLocationManagerDelegate

    func locationManager(_ manager: CLLocationManager,
                         didChangeAuthorization status: CLAuthorizationStatus) {
        switch status {
        case .authorizedAlways:
        case .notDetermined,
             // Handle unauthorized
        @unknown default:

    func locationManager(_ manager: CLLocationManager,
                         didVisit visit: CLVisit) {
        // Handle visit


The service runs in background and you have no idea when the system will choose to trigger a visit update. That’s why it’s difficult to debug it.

Something you can do to help debug the application is to trigger a local notification on the device every time the device logs a new visit. That way, when using your device in your daily life, you’ll be able to know precisely when and where the visits occur.

func locationManager(_ manager: CLLocationManager, didVisit visit: CLVisit) {
    postDebugMessage(for: visit)
    // ...

private func postDebugMessage(for visit: CLVisit) {
    let content = UNMutableNotificationContent()
    content.title = "New place registered"
    content.body = "\(visit.coordinate)"
    content.sound = .default
    let trigger = UNTimeIntervalNotificationTrigger(
        timeInterval: 1,
        repeats: false
    let request = UNNotificationRequest(
        identifier: "",
        content: content,
        trigger: trigger
    center.add(request, withCompletionHandler: nil)

Pros and cons



In Swift, though, the visit class contains non optional arrivalDate and departureDate, as we could have expected. These two properties rather contain invalid data (like distantPast or distantFuture):

@available(iOS 8.0, *)
class CLVisit : NSObject, NSSecureCoding, NSCopying {
    // This may be equal to [NSDate distantPast]
    // if the true arrival date isn't available.
    var arrivalDate: Date { get }
    // This is equal to [NSDate distantFuture]
    // if the device hasn't yet left.
    var departureDate: Date { get }
    var coordinate: CLLocationCoordinate2D { get }
    var horizontalAccuracy: CLLocationAccuracy { get }

That means you can’t trust the visit dates and have to discard invalid visits if you rely on both arrival and departure dates.


Visit monitoring is a powerful service that should be considered as an alternative to Significant-change monitoring in some cases. Its minimal impact on battery life makes it perfect to use in situations where you just need to get context from a noteworthy place: restaurant, gym, home, work, etc.

In our use case we wanted to match locations of multiple users during the same period of time. But as we have seen previously, arrival and departure dates are not always present and the location is not precise. So visit monitoring is not well suited for this situation.

Note: This article was also published on Fabernovel blog.