Saving Data on Android

Wri/ng data when offline
Disconnect your mobile device from the network and run your app. Add a new post. 
You'll see that the post appears on the home screen like it was added to the database. 
But how is that possible if you are offline?
Open the Firebase console and check if your post is there. You'll notice that it's not. 
Now, connect your mobile phone to the network and observe the database data in 
the console. You'll see that a few moments after you connect your app back to the 
internet your post appears in the database.
Setting the 
setPersistenceEnabled
 method argument to 
true
 also keeps track of 
all the writes you initiated while you were offline and then, when network 
connection comes back, it resends all the write operations. This makes the user 
experience optimal even if the user loses the network connection for a moment, 
because your app works as if it's connected to the internet because it uses local data 
from the disk for synchronization.