I am developing an application using the RxAndroidBle library that performs BLE scans regularly about every 30 seconds, and some BLE operations every minute or so. After a couple of hours, usually between 5 and 24h, the scan stops working. Every time a scan is supposed to be started, I get:
09-05 09:08:37.160 8160-8160/myapp D/BluetoothAdapter: startLeScan(): null
09-05 09:08:37.165 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:37.165 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:37.165 8160-8160/myapp D/BluetoothLeScanner: Start Scan
09-05 09:08:37.165 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:37.165 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:37.170 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:37.170 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:37.210 8160-12850/myapp D/BluetoothLeScanner: onClientRegistered() - status=133 clientIf=0
09-05 09:08:37.210 8160-12850/myapp D/BluetoothLeScanner: Registration failed, unregister clientIf = 0
09-05 09:08:37.215 8160-8160/myapp D/BluetoothLeScanner: Scan failed, reason app registration failed for UUID = 4c321920-a2b7-449a-bc24-ea4361f7a255
09-05 09:08:44.150 8160-8160/myapp V/myapp.debug: unsubscribing scan
09-05 09:08:44.150 8160-8160/myapp V/myapp.debug: Clearing scan subscription
09-05 09:08:44.150 8160-8160/myapp D/BluetoothAdapter: stopLeScan()
09-05 09:08:44.150 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:44.155 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:44.155 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:44.155 8160-8160/myapp D/BluetoothAdapter: STATE_ON
09-05 09:08:44.155 8160-8160/myapp D/BluetoothLeScanner: could not find callback wrapper
Does anyone have any idea of what causes this problem or what can be done to fix it?
On the older implementations of Android there seems to be a race condition between the time a bluetooth adapter is enabled and how quickly you can do a scan. You can trigger this error by either trying to scan with a disabled bluetooth adapter or one that is transitioning (or has open connections and is trying to read). The underlying issue that was causing it in my app was the inability of the bluetooth sub system to get a new bluetooth socket. The answer above (running out of GATT resources) could be part of it. The overall logic in an older Android device to avoid this issue is: 1. Make sure you disable/enable the bluetooth adapter about every 5 scans. This seems to help clear out older cached data. 2. Make sure you don't try to initiate a scan while the bluetooth adapter is not enabled (this is made possible if you are disabling/enabling on a regular basis). 3. Make sure you have a delay between disconnecting from GATT interfaces and doing your next scan. 4. Don't try to read the GATT characteristics of any more than about 3 devices at a time.
Overall, its somewhat unavoidable in an older Android device to completely avoid the problem but you can mitigate it by carefully timing your scan/stop scan/connect/disconnect/ cycle.