Android Long Running Wi-Fi Scanning Service

Question

I have been following https://developer.android.com/guide/topics/connectivity/wifi-scan guide in order to create a Wi-Fi scanner. I am using a foreground service with a handler to call wifiMananger.startScan(); every 30 seconds (I tried with 15 minutes).

Everything works great for about 2 hours then suddenly WifiManager.EXTRA_RESULTS_UPDATED boolean returns false and the wifiManager.getScanResults() are not getting updated. Then as soon as the phone is plugged in it starts sending results again. (No, it is not low on battery)

I have battery optimization turned off. I have all of the required permissions allowed. Locations is turned on with Wi-Fi scanning enabled. The device I'm testing on is a Samsung S7 Edge running Android 8. So I know it's not the new OS. (I also tested with a Nokia 5.1 running Android 10 with pretty much the same results).

Does anyone know why this is happening or has anyone encountered this issue before?

Thanks in advance.

Anility

Answer 1

Updated: In case of your problem, since you are using exactly 30 seconds due to some problem there might be more than 4 time in a 2 minutes period, make it like 35 seconds and test the result.

Original Answer: From this WifiManager startScan throttled in P Beta (Developer Preview 2) :

"Call Limitations - Throttling

We are further limiting the number of scans apps can request to improve network performance and improve battery life.

The WifiManager.startScan() usage is limited to:

• Each foreground app is restricted to 4 scans every 2 minutes.
• All background apps combined are restricted to one scan every 30 minutes."

It is said that this restriction is due to the battery drain, so it is normal to remove the restriction while charging. Read more about throttling in official documentation.

Doldrums

Hi, I initially thought the same thing that somehow 30 seconds was too short. So I tried 3, 10 and 15 minute interval but ended up with the same result. The odd thing is, it works great for about 2 hours. So I'm pretty sure it not related to the background service restriction of 30 minutes. I even tried restarting the service every half an hour just be sure but no progress. Appreciate the help though. – Anility

Hi. You are correct if that is the case. Since you have a foreground service, your process is not in background and the 30 minute background restriction does not apply to you. And if you are not running on android 9 and higher or not scanning more than 4 times in a 2 minutes period the 2-min-4-scan restriction does not apply to your app either. Let me investigate it more. – Doldrums

Answer 2

This is because Android OS enters the infamous "Doze mode". I recommend you to look through the official documentation/explanation.

Unfortunately, there is no way around this. It will also affect any kind of a foreground service and pause it for longer periods. Doze mode will only trigger if the screen is locked and the device is not plugged in.

I encountered the same issue on my project and we implemented a periodic check. If the screen is locked and device not plugged in for a longer period of time, we show a notification asking the user to either plug the device in or open the app so the service could keep working. The implementation is a bit lengthy, but if it is what you decide to go with, I can update the answer with some of the code (using the AlarmManager within a BroadcastReceiver to achieve this).

Kwok

Hi, I doubt doze mode is the problem here since I am receiving broadcast from the Wifi-scan. It just doesn't do the scan. I have also removed the app from battery optimization list. Perhaps Wifi-scanner itself is going into doze mode. Thanks for the answer. – Anility

It still seems consistent with Doze mode, though. The docs explicitly say that Wi-Fi scans will be stopped, which is why your WifiManager.EXTRA_RESULTS_UPDATED returns false. Just adding the app to the battery optimization whitelist isn't enough - you also need to acquire a PARTIAL_WAKE_LOCK to do something with it, see developer.android.com/training/monitoring-device-state/… – Kwok