Android BLE handling disconnect timeout

Question

Referencing blog posts like this one, and SO questions like this one. I am going to assume that this is a general behavior (and not a bug on my side). The common answer seems to be something to the effect of: "Change the BLE firmware so it actively disconnects."

The question which is not well addressed is how Android apps handle what must be a very common occurrence? Connection is lost unexpectedly due to "range", I.E. radio signal strength.

Is there a way for an app to be notified "immediately" on the loss of connection?

It does seem unrealistic that all apps just sit there for something like the 20 seconds which is mentioned as a core OS timeout value. Is that what we should all be doing even though my equivalent app on iOS knows about the loss of connections in less than 1s?

Example 1

One common type of BLE device is the "Find my keys" type. Many of them have a feature to alert the user when you leave the "keys" unintentionally. I assume that this uses the connection going down as an indicator of you walking too far away. Right?

Example 2

Your app is supposed to be notified of of value changes from a characteristic on the device. This would be any kind of sensor data where you have some threshold is being crossed for example. I can think of plenty of examples where you'd want to know immediately that your "sensor" is out or range.

Known Workarounds

I have seen one workaround amounting to constantly monitoring the RSSI to the BLE device but that seems like it would eat up a lot of battery. Similarly any failure to write to a characteristic (that normally succeeds) could also be used, again with battery life paying a price.

Something approaching a definitive answer to this questions seems like a good resource.

Answer 1

In latest versions of Android they lowered the default timeout to 5 seconds, which is of course much better.

I guess most peripherals send their own Connection Update request where they can set a different timeout value, which solves that issue (except for the small time between connection setup and until the connection update has gone through).

So called "out of range" detector apps are according to me pretty useless and hard to make good because BLE devices will probably sometimes temporarily disconnect anyway, even though they are in range.

For your actual question if it's possible to "immediately" get notified on the loss of the connection, the basic answer is no, because the Bluetooth controller doesn't send anything to the main cpu that packets were lost (that's the whole purpose of having a timeout so you are allowed to have packet losses). But you can of course try to poll the rssi or set up a flow where the peripheral every constant interval sends a notification and then you can in your app detect if you don't get the notification after some amount of time. But in this case it's smarter to just set your own timeout (from the peripheral) using the connection parameter update procedure.