if i may ask, what is your purpose of this event? why do you need it ?
based on your question and the details provided are very little, but in android and in B4A there is :
-----------------------------------------------------------
in b4a
Sub Activity_Pause (UserClosed As Boolean)
End Sub
*it fires when the activity losses the focus (Paused) + check if the user has closed the activity
In Android :
public void onPause() {
}
-----------------------------------------------------------
-----------------------------------------------------------
in b4a
Sub Activity_Resume
End Sub
*it fires when the activity regains the focus (Resumed)
In android :
public void onResume() {
}
-----------------------------------------------------------
hope that helps