invoke a method when app is closed

I'm developing bluetooth low energy application, using bluetoothgatt, I would like to have a connection going as long as application is running.

which function can I relay on to be called on application being closed? (I understand onTerminate isn't assured to be called)

I want to invoke the bluetoothGatt.close() function.

Currently I'm not using it and I see that the other side keeps on sending data to the android for about 30 seconds.. leading the next app connection to have invalid old data.. (I'm thinking it is because I'm currently not calling close..)

any help would be greatly appreciated, also maybe there is a place for me to see source code of working apps? (I'm new to android programming)