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I want to run a specific task in the background. This task takes few seconds to complete (it writes some GPS location data to a file). This task should run once every 1 hour.

As I understand from the SDK, I do not have a way to INITIATE something in the background unless I run Location Services forever. When this service runs, it gives me an event from time to time, and I'm able to run my code during these calls. I tried it, and even with the minimal precision possible, my battery goes down very quickly.

So, I'm looking for a way to run Location Services for few seconds every hour. All the rest of the time I do NOT want to run Location Services and I do NOT want to use the battery.

Can you help here please? Thanks,

Gena

Gena
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2 Answers2

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What do you actually want to know every hour? Are you trying to determine the location accurately, or just determine if the phone has moved significantly? startMonitoringSignificantLocationChanges will notify you when the phone moves "significantly" (which generally means changing cell towers). In cities this can happen quite often. startMonitoringForRegion:desiredAccuracy: will only notify you when you move outside of a given radius.

startMonitoringSignificantLocationChanges is pretty cheap. It mostly relies on the cell antenna, which is generally on anyway. If it fires more than once an hour, you could always just skip processing and return. But there's no guarantee that you'll be called once an hour, particularly in rural areas.

If you are a VoIP app, then you may register a periodic "checkin" interval when you may run code. But you cannot do this for location apps.

Have you profiled your code using the Energy Diagnostics instrument? What's actually eating time and power? Are you chewing on the radio, the CPU, the disk?

EDIT

When the app dies, Significant Changes still continue to run and leave a small direction arrow icon next to the percentage on the top toolbar of the iPhone. My users complain that the icons remains after killing the app

After the app dies, if you were registered for significant changes, you would expect to be relaunched in response to them. If you want to unregister in cases where you are terminated, you can add stopMonitoring... to your applicationWillTerminate: method. This isn't guaranteed to be called, but it is a good thing to do, and I believe it will be called if the user terminates you directly and you were not suspended (haven't tested that).

Again, significant location changes should be extremely cheap in terms of battery life as long as there is cell coverage. They should not force the GPS to stay on.

Rob Napier
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  • I've tried it and came to the following 2 conclusions: 1. SignificantLocationChanges service does NOT allow me to run tasks in background, as an opposite to the regular Location Services. 2. There is no way to turn the SignificantLocationChanges service off. It always remains on even when the application is killed. – Gena Sep 11 '11 at 17:09
  • You have `location` in UIBackgroundModes, but you don't get woken up for significant location changes? Calling `stopMonitoringSignificantLocationChanges` doesn't unregister you? – Rob Napier Sep 11 '11 at 18:21
  • 1. I have location in background modes. I do get woken up for significant location changes. But I cannot run a timer in background and to perform operations when this timer fires. The timer does not fire in background as the opposite to using Location Services (timer does fire in background). 2. Stop does unregister me if I can run it. But I cannot run it if my app was killed. When the app dies, Significant Changes still continue to run and leave a small direction arrow icon next to the percentage on the top toolbar of the iPhone. My users complain that the icons remains after killing the app – Gena Sep 18 '11 at 06:57
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Your best bet is to use significant location changes ... here are some links you can get help from -

http://developer.apple.com/library/ios/#documentation/CoreLocation/Reference/CLLocationManager_Class/CLLocationManager/CLLocationManager.html

http://www.switchonthecode.com/tutorials/getting-your-location-in-an-iphone-application

http://longweekendmobile.com/2010/07/22/iphone-background-gps-accurate-to-500-meters-not-enough-for-foot-traffic/#more-480

Saurabh
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  • Thank you for the answer. This is exactly what I AM doing now and what I do NOT want to do :) Too much battery. – Gena Sep 08 '11 at 14:30
  • Well.. if you are using significant location change then there is no better way to improve battery life... – Saurabh Sep 08 '11 at 15:55