How to use results from a Firebase query to ultimately populate a ListView in Android Studio

问题

I found that my ListView is not updated when the app is run because the adapter is set before the Firebase query occurs.

First Question: Is there anyway to set a listener for query completion? This way I can set the adapter there instead. I know some Firebase methods auto-generate onComplete()/onSuccess() listeners.

For debugging purposes I can trigger an update to the ListView by re-entering onResume(). This is where I receive an error message:

The content of the adapter has changed but ListView did not receive a notification. Make sure the content of your adapter is not modified from a background thread, but only from the UI thread. Make sure your adapter calls notifyDataSetChanged() when its content changes. [in ListView(16908298, class android.widget.ListView) with Adapter(class android.widget.ArrayAdapter)]

To my understanding this error means I cannot add items to my ArrayList "usernames" inside the onChildAdded() method if I use usernames in my ArrayAdapter.

Second Question: How else can I store the values returned from my Firebase query and use those to ultimately populate a ListView without directly changing its contents from a "background thread"?

My attempt:

public class EditFriendsActivity extends AppCompatActivity {

    protected ArrayList<String> usernames = new ArrayList<>();

    @Override
    protected void onResume() {
        super.onResume();

        Firebase usernameRef = ref.child("users");
        Query queryRef = usernameRef.orderByKey(); 

        queryRef.addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            String username = (String) dataSnapshot.child("username").getValue(); 
            usernames.add(username); 
            Log.v(TAG, usernames + "");
        }
        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {
            Log.v(TAG, "Inside ChildChanged");
        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {
            Log.v(TAG, "Inside ChildRemoved");
        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {
            Log.v(TAG, "Inside ChildMoved");
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

            Log.e(TAG, firebaseError.getMessage());

            AlertDialog.Builder builder = new AlertDialog.Builder(EditFriendsActivity.this);
            builder.setTitle(R.string.error_title)
            .setMessage(firebaseError.getMessage())
            .setPositiveButton(android.R.string.ok, null);

            AlertDialog dialog = builder.create();
            dialog.show();
            }
        });

        // Array adapter
        ArrayAdapter<String> adapter = new ArrayAdapter<>(
                EditFriendsActivity.this,
                android.R.layout.simple_list_item_multiple_choice, 
                usernames);

        mFriendsList.setAdapter(adapter); 
    }
}  

Updated Code: Fixed typo in listener from marked answer and cleared list each time.

Firebase userRef = ref.child("users");
userRef.addListenerForSingleValueEvent(new ValueEventListener() {

    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        usernames.clear(); // Clear list before updating
        for (DataSnapshot child : dataSnapshot.getChildren()) {
            String username = (String) child.child("username").getValue();
            usernames.add(username);
            Log.v(TAG, usernames + "");
        }        

Follow Up Question Here

回答1:

Is there anyway to set a listener for query completion? This way I can set the adapter there instead. I know some Firebase methods auto-generate onComplete()/onSuccess() listeners.

Firebases synchronizes data from a backend to your application. So unlike a traditional database that has a request/response flow, Firebase queries don't really complete. That's why in the FirebaseUI library we keep an open listener and notify the ListView of any changes.

That said: if you just want to load the data from a location once, you can use addSingleValueEventListener:

queryRef.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        for (DataSnapshot child: snapshot.getChildren()) {
            String username = (String) child.child("username").getValue(); 
            usernames.add(username); 
        }
        ArrayAdapter<String> adapter = new ArrayAdapter<>(
            EditFriendsActivity.this,
            android.R.layout.simple_list_item_multiple_choice, 
            usernames);

        mFriendsList.setAdapter(adapter); 
    }

    @Override
    public void onCancelled(FirebaseError firebaseError) {
        Log.e(TAG, firebaseError.getMessage());
    }
});

There are probably typos in the above, but this is the general gist of it.

回答2:

You can simply use a Query

Query ref = FirebaseDatabase.getInstance().getReference()
            .child("users").orderByChild("name");

来源：https://stackoverflow.com/questions/32511133/how-to-use-results-from-a-firebase-query-to-ultimately-populate-a-listview-in-an