title: How to monitor network type and get notified if it changes
body: <p>I'm trying to make an app to both show the state of the network type (2G, 3G, or 4G), which I have done by using TelephonyManager, and also notify the user whenever the network type changes. 
  This is the part I have problem in, how can I monitor the network type and get notified whenever it changes?</p>