Android: Problem with BLE scanning stopped while screen is off on newer phones

[RobinDev]

Hello,

We're using the Scanner Compat (v1.6.0) library for Android in our app, and it has helped us tremendously.

Recently we've observed that the Xiaomi 12T has stopped delivering scan results with the screen off. This means that users cannot reliably connect to our device anymore, unless the user briefly activates the screen on their phone, in which case the scan results pop in instantly, leading to a prompt successful connection with our device as expected. This hasn't always been a problem, and I can probably trace it back to an Android 14 update.

For context, we're using PendingIntent scan strategy to scan continuously, scan filters for our devices, scan settings of SCAN_MODE_BALANCED, and we're restarting the scan every 15min via a scheduled job. We also have the batteryOptimization disabled, as well as the ACCESS_BACKGROUND_LOCATION permission.

In my investigation of the problem, I stumbled upon the reportDelay scan setting, which we havn't been using before. Re-reading the README page of the Scanner Compat library, there is a small mention of this, saying that it should be used with PendingIntent background scanning. Also, in a repo of Googles samples for working with BLE, I fould a small comment saying something along the lines of "this [reportDelay] enables scanning while the screen off"

Thinking that I've found the solution to our problems, I set up a test with the Xiaomi12T with our previously mentioned scan setup but also a reportDelay of 10000ms.

While testing, inspecting the phone log in LogCat with the filter `tag:BtGatt.ScanManager` shows that scan results - with screen off - are being "flushed" (`MSG_FLUSH_BATCH_RESULTS`) on what seems to be a strict timer of 5 minutes. I interpret this "flushing" as the phone buffering scan results, to be released after 5 minutes *only* to apps having the reportDelay (batching?) enabled.

Together with the reportDelay, when the flushing happens, we CAN actually connect to our devices from background with the screen off - unfortunately with the added inconvenience of up to 5 minutes delay from the assumed phone built-in timer.

This wouldn't be so bad if it was isolated to just one phone, but our users report what seems to be the same behavior on newer OnePlus models, running Android 15. I am hesitant to go ahead and set this reportDelay for all of our users across the board, as I have no idea how other phones react. At the very least, users will see an increased delay in bluetooth conection time, which isn't exactly an improvement.

Do you have any thoughts about or insights into this issue? Like, is this the expected behavior for newer phones? Is there some combination of scan settings, phone settings, or other settings that will mitigate this behavior?

Kind regards,

Robin

[snoopy20]

With the screen off I believe your App has to be set to run in the background or it gets paused. I haven't looked into it though.

[RobinDev]

Hello.

Thank you for the prompt answer.

As to my understanding, the PendingIntent based scanning should allow for scanning in the background while the app isn't active. This approach has also worked for us with other phone models, and, if my memory serves me right, also the Xiaomi12T before an update. Also, we have background_location permissions, as well as the ignoreBatteryOptimization, so the app should be (relatively) uninhibited by power saving mechanisms, even in background.

Lately, however, as described in my oroginal post, it seems a new behavior has implemented in at least some newer phones, where non-batched scanning is entirely suspended while the screen is off. And in the case of the Xiaomi12T at least, batched scan results are only propagated to apps once every 5 minuts.