Using a backgrounded iOS app to track multiple BLE beacons with the same UUID and differing major and minor values is challenging. (Ranging normally times out after 10 seconds in the background.) Two basic approaches:
Solution #1: Constant Ranging While in Region
For this approach, you need to set up your app to unlock unlimited ranging in the background while you are inside the beacon region. This is a bit tricky, and will use significant battery if you are around beacons for long periods of time. But is legal to do for App Store distribution provided that your app makes it clear that it uses location in the background for an approved purpose.
I documented this in a blog post here: http://www.davidgyoungtech.com/2023/02/10/forever-ranging
The basic steps are:
Setup:
- Put the following declaration in your Info.plist
<key>UIBackgroundModes</key>
<array>
<string>location</string>
</array>
- Obtain NSLocationAlways permission from the user.
- Start monitoring for a
CLBeaconRegion with the ProximityUUID and a nil major and nil minor.
- Start ranging for the same region. There is no real reason to turn ranging off as logic below will handle throttling whether the OS allows it to do work in the background.
- In your
didRange callback, add logic to process all the detected beacons.
On Region Entry:
- Start location updates at the lowest power setting (basically just using cell radio) with:
locationManager.pausesLocationUpdatesAutomatically = false
locationManager.desiredAccuracy = kCLLocationAccuracyThreeKilometers
locationManager.distanceFilter = 3000.0
if #available(iOS 9.0, *) {
locationManager.allowsBackgroundLocationUpdates = true
} else {
// not needed on earlier versions
}
// start updating location at beginning just to give us unlimited background running time
self.locationManager.startUpdatingLocation()
- Start a background task as described in my blog post here: https://developer.radiusnetworks.com/2014/11/13/extending-background-ranging-on-ios
On Region Exit:
- Stop the background task from the previous step
- Stop the location updates with
self.locationManager.stopUpdatingLocation(
The two changes above will effectively make it so you no longer get ranging updates in the background when no beacons are visible, which will save battery.
Solution #2: Use One Extra Region Per Beacon
This solution is not foolproof, but has the advantage of not requiring a constant background task and background updates. Depending on the placement of your beacons and how users move between them, you will not guarantee detecting each one. But it works for many cases where beacons are not typically placed close together.
- Obtain NSLocationAlways permission from the user.
- Start monitoring for a
CLBeaconRegion with the ProximityUUID and a nil major and nil minor.
- Start ranging for the same region. There is no real reason to turn ranging off as logic below will handle throttling whether the OS allows it to do work in the background.
- In your
didRange callback, add logic to process all the detected beacons.
Each time you detect a new beacon (with a different major and minor) do the following:
- Start monitoring a new
CLBeaconRegion with the UUID, major and minor of the known beacon. This will allow you to get a didExit callback when it disappears. Because iOS only allows you to monitor 20 regions, you can only do this for 19 additional regions at the same time.
On Region Exit for a CLBeaconRegion for a specific major and minor:
- Stop monitoring that region.
The advantage of this approach is that if you have a large number of beacons in overlapping transmitter range, you will get an additional didExit callback as you go out of range of each one. Each time there is a region transition (entry or exit) you get another 10 seconds of background ranging time. This allows you an opportunity to look for new beacons in the area periodically.
This second solution is not perfect. If you encounter beacon A, then it stays in range for 20 seconds before you encounter beacon B, you won't get a ranging callback for beacon B because ranging times out after 10 seconds. In this scenario, you might detect beacon B later on if beacon A goes out of range before beacon B does (the region exit from beacon A gives you more ranging time), but if beacon A does not go out of range before beacon B, then you will never detect beacon B.
Which Solution Should You Use?
Use solution #1 if you must have perfect detections, you app obviously provides a location-specific benefit to the user, and you are OK with the battery drain of constant ranging while beacons are around.
Use solution #2 if you don't need perfect detections, if beacon placement is tolerant of the technique's shortcomings, or if you can't live with the battery drain or constant background ranging for other reasons.
