Saving Data on Android

Other offline scenarios and network 
connec/vity features
Firebase has many features that can help you when offline mode and connectivity 
are an important part of your app. The features you're about to learn apply to your 
app regardless if the local offline persistence is enabled, or not, in your app.
Saving Data on Android
Chapter 14: Realtime Database Offline Capabilities
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Real-/me presence system
The 
real-time presence
 system allows your app to know the status of your users — 
are they 
online
, 
offline
, 
away
, or some other status. This feature is inevitable for 
chat applications for example, because you want to know if the person you are 
texting is online. This feature may seem simple, but to build an entire app 
infrastructure or a mechanism, which handles this for you, can be quite troublesome.
Firebase has this infrastructure implemented and it allows you to use it out of the 
box. Firebase saves the user presence status info to the 
/.info/
connected
 location that you can observe just like any other location in the database. 
The 
.info/connected
 reference just contains a 
boolean
 which indicates if the 
client is connected or not. The problem appears if you want to write something to 
the database when the user status changes to the offline status. For this case, you 
can use the special 
onDisconnect()
 method which Firebase provides, that tells the 
Firebase server to do something when it notices that the client isn't online anymore. 
The 
onDisconnect()
 method works properly, even in cases when the app crashes, or 
the connection is lost, or any other nasty edge case.
On Android, Firebase automatically manages connection state to