Saving Data on Android

Upda/ng data
Next, you'll add an update feature to your app. You'll see that it's very similiar to 
what you did when adding new data. Open the 
CloudFirestoreManager
 class and 
replace the 
TODO
 inside the 
updatePostContent
with the following:
//1
 
 
val
 updatedPost = HashMap() 
 
 
//2
 
 updatedPost[CONTENT_KEY] = content 
 
//3
 
database.collection(POSTS_COLLECTION) 
  .document(key) 
  .update(updatedPost) 
  .addOnSuccessListener { onSuccessAction() } 
  .addOnFailureListener { onFailureAction() }
1. You start the same way as when adding a new post, by creating a map of data that 
you want to save to the database.
2. When updating an existing document you only need to specify the data that you 
are updating. In this case, this is only the post content.
3. Finally, you get the reference to the posts collection and from there you get the 
reference to the document that you're updating by specifying a key. Then you call 
update
 method on a document reference which updates fields in the document. 
The listener logic stays the same as in the case of writing new data.
Open the 
PostDetailsActivity
 class and inside the 
initializeClickListener
 
function replace the 
TODO
 in the on-click listener, so the 
update post
 button will 
update the database with the update content when clicked:
cloudFirestoreManager.updatePostContent( 
  post.id, 
  postText.text.
toString
().trim(), 
  ::onPostSuccessfullyUpdated, 
  ::onPostUpdateFailed 
)
Now you can update posts in the database as well. You'll be able to test this 
functionality when you implement reading logic in the "Chapter 18: Read data from 
Cloud Firestore".
Saving Data on Android
Chapter 17: Managing Data with Cloud Firestore
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