Saving Data on Android

Keeping data in sync
Realtime database stores a copy of the data, locally, only for active listeners. To 
understand this, first, delete your app's data by going to your device's 
Settings 
▶︎
 
Apps & notifications 
▶︎
 WhatsUp 
▶︎
 Stor
age and finally tap on t
he Clear Stor
age 
butt
on.
Next, build and run your app while making sure you're connected to the internet. 
When the home screen opens and posts show up disconnect the device from the 
Internet, once again. Next, open any post that you know has comments. You'll notice 
that there are no comments displayed even if you instructed the app to store data 
locally by setting 
setPersistenceEnabled(true)
. Since Realtime database stores 
data locally only for active listeners your comments weren't saved, because you 
haven't accessed them yet.
If you want to save data locally for the location that has no active listeners attached, 
you can use the 
keepSynced
 method on a database reference, which you'll do in a 
moment.
Saving Data on Android
Chapter 14: Realtime Database Offline Capabilities
raywenderlich.com
234

---

First, open 
RealtimeDatabaseManager
 class and remove 
private
 modifier from the 
COMMENTS_REFERENCE
 constant.
Now, open 
WhatsUpApplication
 class and call 
keepSynced
 on comments reference 
passing in the 
true
 as an argument:
FirebaseDatabase
.getInstance().apply { 
      setPersistenceEnabled(
true
) 
      getReference(
COMMENTS_REFERENCE
).keepSynced(
true
) 
}
Now, the Realtime database will download the comments and keep them in sync, 
even if there are no active listeners at that location. Whatever happens at this 
location — either data gets deleted or updated, you'll receive an update locally, as 
well.
Connect your mobile device to the network, and build and run your app. When posts 
appear on the home screen disconnect your app from the network. Now open any 
post that you know has comments. You'll see your comments there, this time.
Default cache size is 
10MB
, which allows you to store a substantial amount of data 
locally, and, in most cases, this should be enough. If you exceed that limit, any data 
that hasn't been used for a long time will be deleted. So it's basically an LRU-cache-
kind-of mechanism.
In a multi-user app, if there are two users that are not connected to the internet, and 
both write a post, one later than the other, when they finally connect to the internet, 
they will end up in a race-condition. Whichever user has a better and a faster 
connection will write to the database first. This is important to know because, in 
some cases, this may not be the desired behavior.