What strategy to adopt to monitor the user location in background?

Question

I want to track the user location in background, in the purpose to show him an alert when he is close to one of his friend.

So i start with CLLocationManager. As far as i know their is only one reliable way to let the app know about the location update even if the user reboot the Iphone or kill the app: startMonitoringSignificantLocationChanges. But the problem is that even inside a city with many wifi, startMonitoringSignificantLocationChanges fire the DidUpdateLocations when the user move around 1km and that is really too much for my need

on the other way startUpdatingLocation is firing DidUpdateLocations at good interval (even too much because even when the user do not move it's fire quite often DidUpdateLocations). But startUpdatingLocation not survive to iphone reboot or app being killed by the user. Also I suspect that even with an accuracy of 100m, startUpdatingLocation use lot of battery consumption.

So the question: What strategy i can use in my app to track efficiently without draining too much the battery the user location at full time? I need an accuracy of around 100m and if possible an interval between 2.5 - 5 min for each track (i didn't find any option to specify a delay to wait before to catch a new location)

Actually i think to do something like this :

• 2 locationManager, 1 GPS and 1 Significant Changes
• when app start I do with significantChangesLocationManager: startMonitoringSignificantLocationChanges and startMonitoringVisits
• I also call GPSLocationManager startUpdatingLocation to retrieve the accurate user position. I set up PausesLocationUpdatesAutomatically(true) so that the GPSLocationManager will stop by himself soon or late
• on DidUpdateLocations raise by the GPSLocationManager I start monitoring region enter/exit (100m radius around the obtained latitude/longiture) with significantChangesLocationManager

What do you think of such strategy ?

Answer 1

As mentioned in comments, here are some approach I had to do in order to get the expected result in our app.

Significant Location Updates - is good to wake your app up once it gets finished by some reason that you can't control, so even if you lose some points, it will get back in some seconds/minutes. This is not good if you need a good accuracy, as you cannot control the distance / time or whatever, iOS will update locations when mobile antenna is changed, wifi connection, turn air plane mode off and on but will not give a sequence of gps points.

Start Location Updates - Best accuracy but battery drainer. So you can't just turn it on let it go. You must implement some controls over it.

Background tasks - The only way I've found to keep my app alive.

The way you can combine is:

• Have 2 location managers isolated, one for significant location change and one for start updating location;
• Start your app with both turned on;
• Inside your didUpdateLocations you can create your logic to start and stop your background tasks;
• Create methods to start and pause your location manager and create your timers to control that;
• inside your bg task you will start or pause your update locations, but never stop it, just if for example, your user logs out and you want to stop location;
• keep significant location update location manager alive forever, if for some reason iOS decide to kill your app when in bg, it will ensure that in a given moment your app will come back to life;
• For battery live, try do not use kCLLocationAccuracyBestForNavigation for accuracy as when it starts it eat a LOT of resources, most of time, kCLLocationAccuracyBest is more then enough and if you can use kCLLocationAccuracyNearestTenMeters;
• Your didUpdateLocations will give you mostly a bunch of points, try to get only those with good horizontalAccuracy, around 20 meters for us was good enough, so let it work and once you have a point with good accuracy, you can pause it again.

Below you have some links that helped me a lot to implement our solution, none of those have a "as is" solution, as I said, I had to mix all of them and test a lot to understand its behaviour and make the necessary adjustments.

https://github.com/voyage11/Location

http://mobileoop.com/background-location-update-programming-for-ios-7

http://zaachi.com/2013/09/30/ios-locationmanager-location-update-in-my-own-interval-with-application-in-the-background.html

Answer 2

In addition to Mark's answer I would like you to explore pausesLocationUpdatesAutomatically which can help you save battery from draining when user has stopped Apple Documentation

Allowing the location manager to pause updates can improve battery life on the target device without sacrificing location data. When this property is set to true, the location manager pauses updates (and powers down the appropriate hardware) at times when the location data is unlikely to change. For example, if the user stops for food while using a navigation app, the location manager might pause updates for a period of time. You can help the determination of when to pause location updates by assigning a value to the activityType property.

Answer 3

Even though you will receive more triggers than you need, as you already said, you can use startMonitoringSignificantLocationChanges. It is implemented in a very energy efficient way. It allows the app to be terminated and only be woken up again when iOS thinks the device has moved significantly. Another advantage would be that your app doesn't need the location background mode, which could raise questions during an app review.

The startUpdatingLocation let's the app continuously update the location of the device, even though you only receive a couple didUpdateLocations: events. Also, iOS cannot shutdown the app while updating is active, so it consumes a lot of battery.

You can also consider geofencing, with an exit geofence around the current location. However, significant location updates will be more reliable. Exit geofences won't trigger anymore once you're already out of a geofence, which could happen when the phone is turned off inside a geofence and turned back on outside. This solution has the same advantage of not needing a background mode.

As far as I understand your use case, startMonitoringSignificantLocationChanges sounds as the best option. You won't have control over the exact time interval and distance it triggers, but it is very energy efficient and easy to use.