Saving Data on Android

Dele/ng data
One last bit of functionality that you'll add in this chapter is post deleting. Open the 
CloudFirestoreManager
 class and replace the 
TODO
 inside 
deletePost
 function 
with the following:
database.collection(POSTS_COLLECTION) 
  .document(key) 
  .delete() 
  .addOnSuccessListener { onSuccessAction() } 
  .addOnFailureListener { onFailureAction() }
To delete a post you first get a reference to the collection of the post by calling the 
collection
 method on a database instance and passing in the path to the posts 
collection. Then you get a reference to the specific post document that you want to 
delete by passing in the key which is the ID of the post. Finally, you call 
delete
 
method on a document reference which deletes the document referred to by the 
reference. The 
delete
 method deletes data asynchronously, so you also attach the 
listeners that get triggered when the deleting succeeds or fails.
Now, open the 
PostDetailsActivity
 class and inside the 
initializeClickListener
 function replace the 
TODO
 inside the delete button click 
listener with the following:
cloudFirestoreManager.deletePost(post.id, ::onPostSuccessfullyDe
leted, ::onPostDeleteFailed)
Now when you open post details you can delete post by tapping the delete button. 
You'll also test this functionality when you implement reading logic in the "Chapter 
18: Read data from Cloud Firestore".
One important thing to mention here is when you delete a post document, if that 
document contained a subcollection, that subcollection would not have been 
deleted. When you delete a document only that document is deleted; Firestore does 
not delete the documents inside the subcollections.