Saving Data on Android

Saving data to the database
Open the 
RealtimeDatabaseManager
 class. Add the 
database
 field to the class:
private
 val database = 
FirebaseDatabase
.getInstance()
FirebaseDatabase
 object is the main entry point to the database. The  
getInstance()
  method gets you the default 
FirebaseDatabase
 instance. There are 
overloads of 
getInstance()
 methods if you want to get the database for the specific 
URL or specific app.
Saving Data on Android
Chapter 13: Reading to & Writing from Realtime Database
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215

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Add 
POSTS_REFERENCE
 constant above the class declaration:
private
 const val 
POSTS_REFERENCE
 = 
"posts"
Next, add the function for creation of the 
Post
 object to the 
RealtimeDatabaseManager
 class:
private
 fun createPost(key: 
String
, content: 
String
): 
Post
 { 
    val user = authenticationManager.getCurrentUser() 
    val timestamp = getCurrentTime() 
    
return
 
Post
(key, content, user, timestamp) 
}
This function uses the 
AuthenticationManager
 class to get the current logged in 
user name, gets the current time and then returns newly created 
Post
 instance.
Next, add the function for saving the post to the database:
 
fun
 
addPost
(content: String, onSuccessAction: ()
 -> 
Unit
, 
onFailureAction: () -> 
Unit
) { 
    
//1
 
    
val
 postsReference = database.getReference(POSTS_REFERENCE) 
    
//2
 
    
val
 key = postsReference.push().key ?: 
""
 
    
val
 post = createPost(key, content) 
 
    
//3
 
    postsReference.child(key) 
      .setValue(post) 
      .addOnSuccessListener { onSuccessAction() } 
      .addOnFailureListener { onFailureAction() } 
  }
Here's what happens here:
1.
DatabaseReference
 class represents a particular location in the database and it's 
used to refer to the location in the database to which you want to write to or read 
from. 
getReference()
 method returns a reference to the database root node. 
You won't save posts to the root node. Instead, you'll create a new node for the 
posts, which is why you added 
POSTS_REFERENCE
 constant earlier. You pass that 
constant to the 
getReference()
 and this returns a reference for the provided 
path. Now you can use 
postsReference
 to read or write data to this location.
2. Posts will be added as a child of the 
posts
 node. To add a post as a child it needs 
to have a unique key which will be used as a path to the specific post. The key 
needs to be unique because setting a value to the existing path would overwrite 
previous value on that path. You don't want that. You can use the 
push()
 method
Saving Data on Android
Chapter 13: Reading to & Writing from Realtime Database
raywenderlich.com
216

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to create an empty node with an auto-generated key. The 
push()
 method returns 
a database reference to the newly created node. You can call 
getKey()
 on the 
database reference to get the key to that reference. Next, you create a 
Post
 
instance that you'll save to the database and you store the key of that post so you 
can refer to it later.
3. To access the newly created location you can use the 
child()
 method that 
returns a reference to the location relative to the calling reference. Finally, you 
use the 
setValue(post)
 method to save the post to this location. The Realtime 
Database accepts multiple data types to store the data: 
String
, 
Long
, 
Double
, 
Boolean
, 
Map
, and 
List