从Firebase Messenger应用程序下载用户

Question

I'm trying to write messenger app using Firebase.

In database I have a few entries, which are User.class objects. I'm trying to write function which can download User object from database. I though that it'd be better to build separate class (UserManager) for this task, because I don't like making mess in code. But there is a problem, because in onCreate method I need to use User object to download some additional info from database to create conversation list, so downloading user from server should be done before that. Also if user is not in database, it should create and push User to database using FirebaseAuth (I've got that working).

Should I build class extending AsynchTask, and there put downloading user, and then updating UI with the data downloaded after user ? How do I know if the user was already downloaded. Probably I should build some listener but I don't know how to do that.

Additional question:

If I use this reference with value listener, do i get a user object or some value from inside of the object?

DatabaseReference userReference = FirebaseDatabase.getInstance().getReference().child("users/" + mUserID);

Here is my database:

Each entry key is userID from FirebaseAuth for easier implementation.

I've been cracking my head on this for a few days and tried different approaches. I'll apriciate any help. I think, that some code or a scheme would be a huge help.

Answer 1

Maybe this part of my code will help you figure out:

DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("users")
.child(mUserID);

ref.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        User user = dataSnapshot.getValue(User.class);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        Log.e(TAG, "DatabaseError: " + databaseError.getMessage());
    }
});

Answer 2

How do I know if the user was already downloaded?

You can add a flag to each user with the value of false and once you have downloaded the user object, to set the value to true but this is not how things are working with Firebase. You cannot know when a user from the database is completed downloaded becase Firebase is a realtime database and getting data might never complete. That's why is named a realtime database because in any momemnt the data under that User object can be changed, properties can be added or deleted.

You can use a CompletionListener only when you write or update data and you'll be notified when the operation has been acknowledged by the Database servers but you cannot use this interface when reading data.

If I use this reference with value listener, do i get a user object or some value from inside of the object?

If the value that you are listening to is a User object, then you'll get a User object. If the value is another type of object, which can also be a String (which is also an object) then you'll get that type of object, which can also be a String object. Remember, that only the keys in a Firebase database are always strings.