See https://forums.garmin.com/developer/connect-iq/b/news-announcements/posts/would-you-like-some-raspberry-pi-with-your-connect-iq

onConnectionStateChanged it called when a sensor pairs/connects or disconnects and which happened is indicated by the state passed

In the github code you reference, it doesn't look like it's paying attention to state .  Look at the blog post I referenced, in the BLE delegate code and onConnectedStatechanged.

If it connects, it sets up the app.  If it disconnects, resetConnection is called, and one of the things that does is to start scanning again.

I've used this logic for years and it works. 

To test it, it depends a bit on the sensor you are using.  Some you can just turn off, breaking the connection.  Some, you get far enough away from the device, which for some Garmin devices and be 20+ feet

Thanks for your reply.

Even I checked state，the onConnectedStateChanged has never been invoked when disconnecting.

I suspect the occasion is different between a datafield and widget. 

In this thread 

https://forums.garmin.com/developer/connect-iq/f/discussion/365544/onconnectedstatechanged-not-being-called, they are using a datafield too. Without a solution finally.

I put together a DF where I connect/pair with a BLE device.. It stays connected as long as the sensor is on and within range.

If the sensor is lost, maybe not that very second but soon, the DF unpairs, starts scanning. and reconnects.  I do this based on the state in onConnectionStateChanged.  It works fine in the 7.3.1 sim as well as on a real device.  I've tested for hours with numerous disconnects/reconnects.  Here's the function:

Looks like you may need to do the unPairDevice...