title: Flutter websocket disconnect listening
body: <p>In Flutter, I wanna listen to websocket disconnect event, how to achieve that? 
  <strong>The websocket connect will be drop when app goes to background</strong>, I still not found a method to let it continuesly running in background (does anyone have solution?), So I have to detect if a websocket connect is lost or something, so that I can re-connect when lost connection.
  Pls help if anyone knows how to achieve that.</p>