How to Monitor More than 20 Regions in Your IOS App

The region monitoring feature of IOS only provides the possibility to monitor up to 20 regions at a time. If you have more than 20 geofences, then you have to manage these geofences yourself. You have to ensure that the 20 geofences your user is closest to are the 20 active monitored regions.

iOS provides significant location changes that inform your app that the user has made a significant movement. What is significant isn’t formally defined, but it should be enough to replace the existing geofences with the 20 nearest ones. You can calculate the distance to each geofence by using the Haversine formula. Because these significant location changes sometimes occur multiple times within a short period of time, you shouldn’t perform heavy or long running tasks (such as HTTP-requests) every time an update occurs.

You can subscribe to significant location changes just like you subscribe to normal location updates. An important difference between normal location updates and significant location updates is that the significant location updates continue, even when your app is no longer running. When a significant location change occurs when your app is no longer running, your app will be started in the background and you receive the location update directly when you start listening for significant location updates.

In the didUpdateLocations: method you receive an array with locations where iOS thinks you currently are. You can compare these locations based on accuracy and age to determine the best one.

To start the monitoring of a region you call the method [locationManager startMonitoringForRegion:region]. These regions stay active when your app is closed and even when the device is restarted. Therefore you always have to check what fences are active before you start monitoring for new regions. You can get the currently active geofences with the call locationManager.monitoredRegions. That call returns a set with CLRegion objects. Each region has a radius and a center coordinate which you can use to determine whether the geofence is still relevant. When it is no longer relevant, you can tell iOS with the method [locationManager stopMonitoringForRegion:region].

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How to Monitor More than 20 Regions in Your IOS App

Geofencing in iOS with Core Location Swift 3

Setting Up a Location Manager and Permissions

Before any geofence monitoring can happen, though, you need to set up a Location Manager instance and request the appropriate permissions.

Open GeotificationsViewController.swift and declare a constant instance of a CLLocationManager near the top of the class, as shown below:

class GeotificationsViewController: UIViewController {

  @IBOutlet weak var mapView: MKMapView!

  var geotifications = [Geotification]()
  let locationManager = CLLocationManager() // Add this statement

  ...
}

Next, replace viewDidLoad() with the following code:

override func viewDidLoad() {
  super.viewDidLoad()
  // 1
  locationManager.delegate = self
  // 2
  locationManager.requestAlwaysAuthorization()
  // 3
  loadAllGeotifications()
}

Let’s run through this method step by step:

  1. You set the view controller as the delegate of the locationManager instance so that the view controller can receive the relevant delegate method calls.
  2. You make a call to requestAlwaysAuthorization(), which invokes a prompt to the user requesting for Always authorization to use location services. Apps with geofencing capabilities need Always authorization, due to the need to monitor geofences even when the app isn’t running. Info.plist has already been setup with a message to show the user when requesting the user’s location under the key NSLocationAlwaysUsageDescription.
  3. You call loadAllGeotifications(), which deserializes the list of geotifications previously saved to NSUserDefaults and loads them into a local geotifications array. The method also loads the geotifications as annotations on the map view.

When the app prompts the user for authorization, it will show NSLocationAlwaysUsageDescription, a user-friendly explanation of why the app requires access to the user’s location. This key is mandatory when you request authorization for location services. If it’s missing, the system will ignore the request and prevent location services from starting altogether.

Build and run the project, and you’ll see a user prompt with the aforementioned description that’s been set:

You’ve set up your app to request the required permission. Great! Click or tap Allow to ensure the location manager will receive delegate callbacks at the appropriate times.

Before you proceed to implement the geofencing, there’s a small issue you have to resolve: the user’s current location isn’t showing up on the map view! This feature is disabled, and as a result, the zoom button on the top-left of the navigation bar doesn’t work.

Fortunately, the fix is not difficult — you’ll simply enable the current location only after the app is authorized.

In GeotificationsViewController.swift, add the following delegate method to the CLLocationManagerDelegate extension:

extension GeotificationsViewController: CLLocationManagerDelegate {
  func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus) {
    mapView.showsUserLocation = (status == .authorizedAlways)
  }
}

The location manager calls locationManager(_:didChangeAuthorizationStatus:) whenever the authorization status changes. If the user has already granted the app permission to use Location Services, this method will be called by the location manager after you’ve initialized the location manager and set its delegate.

That makes this method an ideal place to check if the app is authorized. If it is, you enable the map view to show the user’s current location.

Build and run the app. If you’re running it on a device, you’ll see the location marker appear on the main map view. If you’re running on the simulator, click Debug\Location\Apple in the menu to see the location marker:

In addition, the zoom button on the navigation bar now works. :]

Registering Your Geofences

With the location manager properly configured, the next order of business is to allow your app to register user geofences for monitoring.

In your app, the user geofence information is stored within your custom Geotification model. However, Core Location requires each geofence to be represented as a CLCircularRegion instance before it can be registered for monitoring. To handle this requirement, you’ll create a helper method that returns a CLCircularRegion from a given Geotification object.

Open GeotificationsViewController.swift and add the following method to the main body:

func region(withGeotification geotification: Geotification) -> CLCircularRegion {
  // 1
  let region = CLCircularRegion(center: geotification.coordinate, radius: geotification.radius, identifier: geotification.identifier)
  // 2
  region.notifyOnEntry = (geotification.eventType == .onEntry)
  region.notifyOnExit = !region.notifyOnEntry
  return region
}

Here’s what the above method does:

  1. You initialize a CLCircularRegion with the location of the geofence, the radius of the geofence and an identifier that allows iOS to distinguish between the registered geofences of a given app. The initialization is rather straightforward, as the Geotification model already contains the required properties.
  2. The CLCircularRegion instance also has two Boolean properties, notifyOnEntry and notifyOnExit. These flags specify whether geofence events will be triggered when the device enters and leaves the defined geofence, respectively. Since you’re designing your app to allow only one notification type per geofence, you set one of the flags to true while you set the other to false, based on the enum value stored in the Geotification object.

Next, you need a method to start monitoring a given geotification whenever the user adds one.

Add the following method to the body of GeotificationsViewController:

func startMonitoring(geotification: Geotification) {
  // 1
  if !CLLocationManager.isMonitoringAvailable(for: CLCircularRegion.self) {
    showAlert(withTitle:"Error", message: "Geofencing is not supported on this device!")
    return
  }
  // 2
  if CLLocationManager.authorizationStatus() != .authorizedAlways {
    showAlert(withTitle:"Warning", message: "Your geotification is saved but will only be activated once you grant Geotify permission to access the device location.")
  }
  // 3
  let region = self.region(withGeotification: geotification)
  // 4
  locationManager.startMonitoring(for: region)
}

Let’s walk through the method step by step:

  1. isMonitoringAvailableForClass(_:) determines if the device has the required hardware to support the monitoring of geofences. If monitoring is unavailable, you bail out entirely and alert the user accordingly. showSimpleAlertWithTitle(_:message:viewController) is a helper function in Utilities.swift that takes in a title and message and displays an alert view.
  2. Next, you check the authorization status to ensure that the app has also been granted the required permission to use Location Services. If the app isn’t authorized, it won’t receive any geofence-related notifications.However, in this case, you’ll still allow the user to save the geotification, since Core Location lets you register geofences even when the app isn’t authorized. When the user subsequently grants authorization to the app, monitoring for those geofences will begin automatically.
  3. You create a CLCircularRegion instance from the given geotification using the helper method you defined earlier.
  4. Finally, you register the CLCircularRegion instance with Core Location for monitoring.

With your start method done, you also need a method to stop monitoring a given geotification when the user removes it from the app.

In GeotificationsViewController.swift, add the following method below startMonitoringGeotificiation(_:):

func stopMonitoring(geotification: Geotification) {
  for region in locationManager.monitoredRegions {
    guard let circularRegion = region as? CLCircularRegion, circularRegion.identifier == geotification.identifier else { continue }
    locationManager.stopMonitoring(for: circularRegion)
  }
}

The method simply instructs the locationManager to stop monitoring the CLCircularRegion associated with the given geotification.

Now that you have both the start and stop methods complete, you’ll use them whenever you add or remove a geotification. You’ll begin with the adding part.

First, take a look at addGeotificationViewController(_:didAddCoordinate) in GeotificationsViewController.swift.

The method is the delegate call invoked by the AddGeotificationViewController upon creating a geotification; it’s responsible for creating a new Geotification object using the values passed from AddGeotificationsViewController, and updating both the map view and the geotifications list accordingly. Then you call saveAllGeotifications(), which takes the newly-updated geotifications list and persists it via NSUserDefaults.

Now, replace the method with the following code:

func addGeotificationViewController(controller: AddGeotificationViewController, didAddCoordinate coordinate: CLLocationCoordinate2D, radius: Double, identifier: String, note: String, eventType: EventType) {
  controller.dismiss(animated: true, completion: nil)
  // 1
  let clampedRadius = min(radius, locationManager.maximumRegionMonitoringDistance)
  let geotification = Geotification(coordinate: coordinate, radius: clampedRadius, identifier: identifier, note: note, eventType: eventType)
  add(geotification: geotification)
  // 2
  startMonitoring(geotification: geotification)
  saveAllGeotifications()
}

You’ve made two key changes to the code:

  1. You ensure that the value of the radius is clamped to the maximumRegionMonitoringDistance property of locationManager, which is defined as the largest radius in meters that can be assigned to a geofence. This is important, as any value that exceeds this maximum will cause monitoring to fail.
  2. You add a call to startMonitoringGeotification(_:) to ensure that the geofence associated with the newly-added geotification is registered with Core Location for monitoring.

At this point, the app is fully capable of registering new geofences for monitoring. There is, however, a limitation: As geofences are a shared system resource, Core Location restricts the number of registered geofences to a maximum of 20 per app.

While there are workarounds to this limitation (See Where to Go From Here? for a short discussion), for the purposes of this tutorial, you’ll take the approach of limiting the number of geotifications the user can add.

 

Finally, let’s deal with the removal of geotifications. This functionality is handled in mapView(_:annotationView:calloutAccessoryControlTapped:), which is invoked whenever the user taps the “delete” accessory control on each annotation.

Add a call to stopMonitoring(geotification:) to mapView(_:annotationView:calloutAccessoryControlTapped:), as shown below:

func mapView(_ mapView: MKMapView, annotationView view: MKAnnotationView, calloutAccessoryControlTapped control: UIControl) {
  // Delete geotification
  let geotification = view.annotation as! Geotification
  stopMonitoring(geotification: geotification)   // Add this statement
  removeGeotification(geotification)
  saveAllGeotifications()
}

The additional statement stops monitoring the geofence associated with the geotification, before removing it and saving the changes to NSUserDefaults.

At this point, your app is fully capable of monitoring and un-monitoring user geofences. Hurray!

Build and run the project. You won’t see any changes, but the app will now be able to register geofence regions for monitoring. However, it won’t be able to react to any geofence events just yet. Not to worry—that will be your next order of business!

Reacting to Geofence Events

You’ll start by implementing some of the delegate methods to facilitate error handling – these are important to add in case anything goes wrong.

In GeotificationsViewController.swift, add the following methods to the CLLocationManagerDelegateextension:

func locationManager(_ manager: CLLocationManager, monitoringDidFailFor region: CLRegion?, withError error: Error) {
  print("Monitoring failed for region with identifier: \(region!.identifier)")
}

func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) {
  print("Location Manager failed with the following error: \(error)")
}

These delegate methods simply log any errors that the location manager encounters to facilitate your debugging.

Note: You’ll definitely want to handle these errors more robustly in your production apps. For example, instead of failing silently, you could inform the user what went wrong.

Next, open AppDelegate.swift; this is where you’ll add code to properly listen and react to geofence entry and exit events.

Add the following line at the top of the file to import the CoreLocation framework:

import CoreLocation

Ensure that the AppDelegate has a CLLocationManager instance near the top of the class, as shown below:

class AppDelegate: UIResponder, UIApplicationDelegate {
  var window: UIWindow?

  let locationManager = CLLocationManager() // Add this statement
  ...
}

Replace application(_:didFinishLaunchingWithOptions:) with the following implementation:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey : Any]? = nil) -> Bool {
  locationManager.delegate = self
  locationManager.requestAlwaysAuthorization()
  return true
}

You’ve set up your AppDelegate to receive geofence-related events. But you might wonder, “Why did I designate the AppDelegate to do this instead of the view controller?”

Geofences registered by an app are monitored at all times, including when the app isn’t running. If the device triggers a geofence event while the app isn’t running, iOS automatically relaunches the app directly into the background. This makes the AppDelegate an ideal entry point to handle the event, as the view controller may not be loaded or ready.

Now you might also wonder, “How will a newly-created CLLocationManager instance be able to know about the monitored geofences?”

It turns out that all geofences registered by your app for monitoring are conveniently accessible by all location managers in your app, so it doesn’t matter where the location managers are initialized. Pretty nifty, right? :]

Now all that’s left is to implement the relevant delegate methods to react to the geofence events. Before you do so, you’ll create a method to handle a geofence event.

Add the following method to AppDelegate.swift:

func handleEvent(forRegion region: CLRegion!) {
  print("Geofence triggered!")
}

At this point, the method takes in a CLRegion and simply logs a statement. Not to worry—you’ll implement the event handling later.

Next, add the following delegate methods in the CLLocationManagerDelegate extension of AppDelegate.swift, as well as a call to the handleRegionEvent(_:) function you just created, as shown in the code below:

extension AppDelegate: CLLocationManagerDelegate {
  
  func locationManager(_ manager: CLLocationManager, didEnterRegion region: CLRegion) {
    if region is CLCircularRegion {
      handleEvent(forRegion: region)
    }
  }
  
  func locationManager(_ manager: CLLocationManager, didExitRegion region: CLRegion) {
    if region is CLCircularRegion {
      handleEvent(forRegion: region)
    }
  }
}

As the method names aptly suggest, you fire locationManager(_:didEnterRegion:) when the device enters a CLRegion, while you fire locationManager(_:didExitRegion:) when the device exits a CLRegion.

Both methods return the CLRegion in question, which you need to check to ensure it’s a CLCircularRegion, since it could be a CLBeaconRegion if your app happens to be monitoring iBeacons, too. If the region is indeed a CLCircularRegion, you accordingly call handleRegionEvent(_:).

Note: A geofence event is triggered only when iOS detects a boundary crossing. If the user is already within a geofence at the point of registration, iOS won’t generate an event. If you need to query whether the device location falls within or outside a given geofence, Apple provides a method called requestStateForRegion(_:).

Now that your app is able to receive geofence events, you’re ready to give it a proper test run. If that doesn’t excite you, it really ought to, because for the first time in this tutorial, you’re going to see some results. :]

The most accurate way to test your app is to deploy it on your device, add some geotifications and take the app for a walk or a drive. However, it wouldn’t be wise to do so right now, as you wouldn’t be able to verify the print logs emitted by the geofence events with the device unplugged. Besides, it would be nice to get assurance that the app works before you commit to taking it for a spin.

Fortunately, there’s an easy way do this without leaving the comfort of your home. Xcode lets you include a hardcoded waypoint GPX file in your project that you can use to simulate test locations. Lucky for you, the starter project includes one for your convenience. :]

Open up TestLocations.gpx, which you can find in the Supporting Files group, and inspect its contents. You’ll see the following:

<?xml version="1.0"?>
<gpx version="1.1" creator="Xcode">
  <wpt lat="37.422" lon="-122.084058">
    <name>Google</name>
  </wpt>
  <wpt lat="37.3270145" lon="-122.0310273">
    <name>Apple</name>
  </wpt>
</gpx>

The GPX file is essentially an XML file that contains two waypoints: Google’s Googleplex in Mountain View and Apple’s Headquarters in Cupertino.

To begin simulating the locations in the GPX file, build and run the project. When the app launches the main view controller, go back to Xcode, select the Location icon in the Debug bar and choose TestLocations:

Back in the app, use the Zoom button on the top-left of the navigation bar to zoom to the current location. Once you get close to the area, you’ll see the location marker moving repeatedly from the Googleplex to Apple, Inc. and back.

Test the app by adding a few geotifications along the path defined by the two waypoints. If you added any geotifications earlier in the tutorial before you enabled geofence registration, those geotifications will obviously not work, so you might want to clear them out and start afresh.

For the test locations, it’s a good idea to place a geotification roughly at each waypoint. Here’s a possible test scenario:

  • Google: Radius: 1000m, Message: “Say Bye to Google!”, Notify on Exit
  • Apple: Radius: 1000m, Message: “Say Hi to Apple!”, Notify on Entry

Once you’ve added your geotifications, you’ll see a log in the console each time the location marker enters or leaves a geofence. If you activate the home button or lock the screen to send the app to the background, you’ll also see the logs each time the device crosses a geofence, though you obviously won’t be able to verify that behavior visually.

Note: Location simulation works both in iOS Simulator and on a real device. However, the iOS Simulator can be quite inaccurate in this case; the timings of the triggered events do not coincide very well with the visual movement of the simulated location in and out of each geofence. You would do better to simulate locations on your device, or better still, take the app for a walk!

Notifying the User of Geofence Events

You’ve made a lot of progress with the app. At this point, it simply remains for you to notify the user whenever the device crosses the geofence of a geotification—so prepare yourself to do just that.

To obtain the note associated with a triggering CLCircularRegion returned by the delegate calls, you need to retrieve the corresponding geotification that was persisted in NSUserDefaults. This turns out to be trivial, as you can use the unique identifier you assigned to the CLCircularRegion during registration to find the right geotification.

In AppDelegate.swift, add the following helper method at the bottom of the class:

func note(fromRegionIdentifier identifier: String) -> String? {
  let savedItems = UserDefaults.standard.array(forKey: PreferencesKeys.savedItems) as? [NSData]
  let geotifications = savedItems?.map { NSKeyedUnarchiver.unarchiveObject(with: $0 as Data) as? Geotification }
  let index = geotifications?.index { $0?.identifier == identifier }
  return index != nil ? geotifications?[index!]?.note : nil
}

This helper method retrieves the geotification note from the persistent store, based on its identifier, and returns the note for that geotification.

Now that you’re able to retrieve the note associated with a geofence, you’ll write code to trigger a notification whenever a geofence event is fired, using the note as the message.

Add the following statements to the end of application(_:didFinishLaunchingWithOptions:), just before the method returns:

application.registerUserNotificationSettings(UIUserNotificationSettings(types: [.sound, .alert, .badge], categories: nil))
UIApplication.shared.cancelAllLocalNotifications()

The code you’ve added prompts the user for permission to enable notifications for this app. In addition, it does some housekeeping by clearing out all existing notifications.

Next, replace the contents of handleRegionEvent(_:) with the following:

func handleEvent(forRegion region: CLRegion!) {
  // Show an alert if application is active
  if UIApplication.shared.applicationState == .active {
    guard let message = note(fromRegionIdentifier: region.identifier) else { return }
    window?.rootViewController?.showAlert(withTitle: nil, message: message)
  } else {
    // Otherwise present a local notification
    let notification = UILocalNotification()
    notification.alertBody = note(fromRegionIdentifier: region.identifier)
    notification.soundName = "Default"
    UIApplication.shared.presentLocalNotificationNow(notification)
  }
}

If the app is active, the code above simply shows an alert controller with the note as the message. Otherwise, it presents a location notification with the same message.

Build and run the project, and run through the test procedure covered in the previous section. Whenever your test triggers a geofence event, you’ll see an alert controller displaying the reminder note:

Send the app to the background by activating the Home button or locking the device while the test is running. You’ll continue to receive notifications periodically that signal geofence events:

And with that, you have a fully functional, location-based reminder app in your hands. And yes, get out there and take that app for a spin!

Note: When you test the app, you may encounter situations where the notifications don’t fire exactly at the point of boundary crossing.

This is because before iOS considers a boundary as crossed, there is an additional cushion distance that must be traversed and a minimum time period that the device must linger at the new location. iOS internally defines these thresholds, seemingly to mitigate the spurious firing of notifications in the event the user is traveling very close to a geofence boundary.

In addition, these thresholds seem to be affected by the available location hardware capabilities. From experience, the geofencing behavior is a lot more accurate when Wi-Fi is enabled on the device.

Geofencing in iOS with Core Location Swift 3