Archives for May 2014

iOS Region Monitoring and Location Manager

One interesting feature in the new version of Watch Over Me app is Crime Zone. The main purpose of the crime zones is to notify the user of Watch Over me when he/she is passing the area. Watch Over Me app will ask if the user wants to be watched over during that time.

We have accumulated some trustable and reliable resources from various websites from the Internet to gather the crime zone areas in 7 major cities in 5 different countries (Malaysia, USA, UK, Australia, Thailand). These 7 cities are our test regions and we will be adding more regions in the future.

iOS Region Monitoring and Location Manager

I am using Region Monitoring from the Location Manager to implement this feature on the iOS Watch Over Me app. This particular module is not hard compare with getting the location in iOS 7 in background mode.

The maximum regions that an iOS device can monitor is only limited to 20. So, I have to come out with a good strategy to only get and monitor the regions that are useful to the users.

This is the logic we use for the region monitoring in iOS:-
1. Getting the current location from the user.
2. Send the current location to the server.
3. The server will return all the regions within a 20 km radius from the current location
4. If there are more than 20 regions, the server will only send the closest 20 regions from the current location.
5. After receiving the regions, the app will start to monitor the regions.

A Potential Bug for locationManager delegate method didEnterRegion

After the app has been released on the app store, we are still constantly in testing on the region monitoring on Kuala Lumpur (KL). We have 8 monitored regions in KL at the time of typing this article.

On one fine day, my colleague saw a bug with multiple region notifications triggered at the same time. The location of my colleague is at least 3000 meters away from the nearest region that we monitored. I have double checked on the radius and also the coordinate for all the regions that we monitor in KL, there wasn’t any discrepancy of the data.

Region Monitoring Bug
iOS Region Monitoring Bug
Region MonitoringiOS Region Monitoring


The screen on the Left shows the bug when 4 notifications triggered at the same time when the device is NOT even close to any of the regions.

The screen on the Right is a simple app that I made to stimulate the location of the device (Green Marker) and also the location (Red Marker) and the radius of each region (Red Circle). The Blue Circle is the estimation of the radius of the device when it triggered the notification for all 4 regions. The estimated radius for the device is about 4700 meters! It should not happen.

I have searched through various websites (StackOverFlow, Apple Developer Forum and etc.), I couldn’t find other developer faced the same issue like mine. I saw a lot of questions on NOT getting any notification when passing through a region, but my case is the entire opposite.

I am not sure if this is the fault on my code, on iOS system level or the problem on the local Telco in my country. I have posted a question on StackOverFlow for more than 1 week but I didn’t get any answer as well.

How to Fix this Bug?

Without much reliable information, I can only do trial and error in order to fix this bug. I have been trying a few different method on trying to fix the bug. This includes setting the distanceFilter = 10 for the locationManager. Previously, it was kCLDistanceFilterNone because I want the app to detect every single movement.

My colleague still saw this bug after a few days of testing. So, the above solution is no good.

After experimenting for sometimes, I found a solution that works good.

1. Whenever a user enters a monitored region, he/she will get a notification in the locationManager delegate method didEnterRegion. But due to the bug, this method is not reliable. I have to use a smart way to filter out the fake location.
2. I found a way to get the recent location of the user from locationManager.
3. After the app gets the user location, the app will make sure that the location is within the region before deciding to send the location notification to the device.

From my research, this location is not 100% reliable, especially if there isn’t any location based running at the background. But generally, it is good enough to filter out the bad location which may trigger multiple notifications for different regions.

Here are the example code: