Cloud Firestore - getting documents from multiple locations

Question

When a user checks their friends list in my application, I want the application to go through each user in the list and retrieve their updated information from Cloud Firestore.

This is my current code:

 final CollectionReference usersRef= FirebaseFirestore.getInstance().collection("users");

            usersRef.document(loggedEmail).collection("friends_list").get().addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
                @Override
                public void onSuccess(QuerySnapshot documentSnapshots) {
                    if (!documentSnapshots.isEmpty()){


                        for (DocumentSnapshot friendDocument: documentSnapshots) {

                            usersRef.document(friendDocument.getString("email")).get().addOnSuccessListener
                                    (new OnSuccessListener<DocumentSnapshot>() {
                                @Override
                                public void onSuccess(DocumentSnapshot documentSnapshot) {
                                    User friend=documentSnapshot.toObject(User.class);
                                friendsList_UserList.add(friend);

                                }
                            });

                        }


                        ///...

                    }

                    else
                        noFriendsFound();

                }

And this is an illustration of my desired process:

As you can see, I can get each user's information in this way, but I can’t find a way to listen to this process and continue when I have information about all the friends in the user list.

Is there a way that I can immediately get all the information about friends?

Answer 1

Firestore , .

, getDocumentChanges QuerySnapshot, , .

, -,

Map<String, ListenerRegistration> friendListeners = new HashMap<>();

- :

usersRef.document(loggedEmail).collection("friends_list")
    .addSnapshotListener(new EventListener<QuerySnapshot>() {
      @Override
      public void onEvent(QuerySnapshot snapshot, FirebaseFirestoreException error) {
        for (DocumentChange change : snapshot.getDocumentChanges()) {
          DocumentSnapshot friend = change.getDocument();
          String friendId = friend.getId();
          ListenerRegistration registration;
          switch (change.getType()) {
            case ADDED:
            case MODIFIED:
              if (!friendListeners.containsKey(friendId)) {
                registration = usersRef.document(friendId).addSnapshotListener(null);
                friendListeners.put(friendId, registration);
              }
              break;

            case REMOVED:
              registration = friendListeners.get(friendId);
              if (registration != null) {
                registration.remove();
                friendListeners.remove(friendId);
              }
              break;
          }
        }
      }
    });

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