Saving Data on Android

Caching data locally
To enable disk persistence, you only need one line of code. Open 
WhatsUpApplication
 class and  enable persistence on the 
FirebaseDatabase
 
instance in the 
onCreate
 method:
override
 fun onCreate() { 
    
super
.onCreate() 
    
FirebaseDatabase
.getInstance().setPersistenceEnabled(
true
) 
}
Turn on the network connection on your device. Build and run your app. Your posts 
will appear on the home screen. Now, disconnect your mobile phone from the 
network, close the app and kill the app process from the 
Recent apps
 menu. Run 
your app again. You'll see the posts appearing on the home screen.
Setting the 
setPersistenceEnabled
 method argument to 
true
 enables the app to 
store the data to the devices local storage — the disk. That is what makes the data 
available even after you kill the app.
Saving Data on Android
Chapter 14: Realtime Database Offline Capabilities
raywenderlich.com
233

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The reason you had to enable disk persistence in the 
Application
 class, instead of 
the 
RealtimeDatabaseManager
, is that 
setPersistenceEnabled
 method needs to 
be called once per app and before creating the first database reference.