iOS stops waking up the app upon incoming BLE connection from peripheral

Question

we have a BLE peripheral that connects to the phone every hour and passes some data. Here is how the process works:

Upon launch with key UIApplicationLaunchOptionsBluetoothCentralsKey in

1. application(didFinishLaunchingWithOptions launchOptions) app re-initializes CBCentralManager with ID that was passed to it.
2. Then it goes through the regular restoration cycle and reads data off the BLE peripheral.
3. Performs REST request to the service in the cloud.

Assuming that app has been launched at least once after phone reboot everything works well for a few days (if app isnt running or been forced out of memory, iOS properly starts it up again, assuming user didnt do forced close manually).

However every few days iOS stops waking up the app when there is an incoming request from BLE device. If user relaunches app everything works properly for a few days and then stops agains. Given the nature of our product, it's critical to have our app / peripheral working together in the most reliable way possible.

Theories as to why it might be happening:
(upon closer examination all of them were dismissed)

• Users restart the phone and forget to relaunch the app.
  We've added logging of the uptime and it showed that phone didnt restart in between app launches.

• Memory warnings lead to app being booted out.
  Once again, added logging, they showed that there was no applicationDidReceiveMemoryWarning

• Bad connection leads to app running for longer period than 10s when uploading results and iOS terminates it and gets upset
  We artificially delayed server response by 15s to test this and everything continues to work properly during testing.

Any ideas on what is happening and why iOS stops notifying app about incoming BLE connection?

One of the problems is that we cannot figure out how to reliably reproduce the issue So any suggestions there will be much appreciated as well!

Thank you!

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UPDATE 1: Here is how we initialize CBCentralManager:

self.centralManager = CBCentralManager(delegate: self, queue: nil, options: [
    CBCentralManagerOptionRestoreIdentifierKey : MyCentralManagerID,
    CBCentralManagerOptionShowPowerAlertKey : 0])

I saw some suggestions that queue parameter should not be nil. Given that I'm unable to reliably reproduce issue I'm hesitant to make that change until I can confidently observe its effects.

Answer 1

I wanna start by saying that I have been working with CoreBluetooth for a long time now and from what I have noticed CoreBluetooth State Preservation and Restoration does not work reliably at all. You can get it working sort of "ok", but you will never get it to reconnect reliably unless Apple fixes it some day.

There are so many bugs that causes this to not work properly, but I will give you one that I believe is causing your problems:

State restoration will only relaunch your app due to bluetooth related activity if the event originates from a peripheral accessory that you are communicating with, such as connect/disconnect events and characteristics notifications. For other events, most importantly general bluetooth state change events, your app will not be relaunched and notified of this. The reason why this is so bad is because all bluetooth state change events will cancel all pending or current connections, meaning that pending connections will be dropped and your application will not be notified of it. This effectively means that your application will still believe that the connections are still pending when in fact they are not. Since your application is terminated at this time, the only way for it to wake up again is by having the user manually launch it again (or alternatively “hack” other background modes for this purpose, which does not work very reliably either).

This thing happens if the user toggles Flight Mode, toggles Bluetooth, power cycles the iOS device, or any other undefined reasons that many cause state changes…

But this is only one bug. Many other exists as well, such as the XPC connection being interrupted at different times for no apparent reason. I have also noticed that the pending connection can go into “limbo” mode where the peripheral state gets set to Connecting, but in fact it will never connect unless you cycle the connection state.

Anyhow, I am sad to say it, but if you are developing an app that must rely on the peripheral being reconnected in the background then I would not recommend doing it. You will be frustrated. I could probably write an essay about all the bugs in Core Bluetooth that Apple does not want to fix. Even more strange is that you can pretty easily ruin the bluetooth connectivity globally on the device from one single app so that no app can use bluetooth until the device is rebooted. This is pretty bad since it goes against Apple's own Sandboxing principle.