Cómo devuelvo una lista de usuarios si uso la autenticación simple de nombre de usuario y contraseña de Firebase

Aquí he creado un programa para Android para hacer lo que Rob dijo para firebase principiante (como yo) ahi.este programa primero almacena el nombre de usuario del usuario signedUp o signedIn y luego los muestra en una ListView

SignInActivity.java

public class SignInActivity extends BaseActivity implements View.OnClickListener,View.OnKeyListener{

private DatabaseReference mDatabase;
public static FirebaseAuth mAuth;
private static final String TAG = "MainActivity";

EditText usernameField;
EditText passwordField;
TextView changeSignUpModeTextView;
Button signUpButton;
ImageView logo;
RelativeLayout relativeLayout;

Boolean signUpModeActive;
static ArrayList<String> userList = new ArrayList<>();

@Override
public void onStart() {
    super.onStart();

    // Check auth on Activity start
    if (mAuth.getCurrentUser() != null) {
        onAuthSuccess(mAuth.getCurrentUser());
    }
}
@Override
public boolean onKey(View view, int i, KeyEvent keyEvent) {

    if(i == keyEvent.KEYCODE_ENTER && keyEvent.getAction() == keyEvent.ACTION_DOWN){
        signUpOrLogIn(view);
    }
     return false;
}

@Override
public void onClick(View view) {

    if(view.getId() == R.id.changeSignUpMode){

        if (signUpModeActive == true){

            signUpModeActive = false;
            changeSignUpModeTextView.setText("Sign Up");
            signUpButton.setText("Log In");

        }else{

            signUpModeActive = true;
            changeSignUpModeTextView.setText("Log In");
            signUpButton.setText("Sign Up");
        }

    }else if(view.getId() == R.id.logo || view.getId() == R.id.relativeLayout){

        InputMethodManager inm = (InputMethodManager) getSystemService(INPUT_METHOD_SERVICE);
        inm.hideSoftInputFromWindow(getCurrentFocus().getWindowToken(),0);

    }

}


public void signUpOrLogIn(View view) {

    showProgressDialog();
    String email = usernameField.getText().toString().trim();
    String password = passwordField.getText().toString().trim();

    if (signUpModeActive == true) {
        mAuth.createUserWithEmailAndPassword(email,password)
                .addOnCompleteListener(MainActivity.this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        hideProgressDialog();
                        Toast.makeText(MainActivity.this, "createUserWithEmail:onComplete:" + task.isSuccessful(), Toast.LENGTH_SHORT).show();
                        // If sign in fails, display a message to the user. If sign in succeeds
                        // the auth state listener will be notified and logic to handle the
                        // signed in user can be handled in the listener.
                        if (!task.isSuccessful()) {
                            Toast.makeText(MainActivity.this, "Authentication failed." + task.getException().toString().substring(task.getException().toString().indexOf(" ")),
                                    Toast.LENGTH_SHORT).show();
                            Log.i("Error", task.getException().toString());
                        } else {
                            onAuthSuccess(task.getResult().getUser());
                            showUserList();
                        }
                    }
                });
    } else {
        mAuth.signInWithEmailAndPassword(email,password)
                .addOnCompleteListener(MainActivity.this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        hideProgressDialog();
                        // If sign in fails, display a message to the user. If sign in succeeds
                        // the auth state listener will be notified and logic to handle the
                        // signed in user can be handled in the listener.
                        if (!task.isSuccessful()) {
                            // there was an error

                            Toast.makeText(MainActivity.this, task.getException().toString(),
                                    Toast.LENGTH_LONG).show();
                        } else

                        {
                            onAuthSuccess(task.getResult().getUser());
                            showUserList();
                        }
                    }
                });
    }
}

public void showUserList(){
    startActivity(new Intent(getApplicationContext(), UserList.class));
    finish();
}
private void onAuthSuccess(FirebaseUser user) {
    String username = usernameFromEmail(user.getEmail());

    // Write new user
    writeNewUser(user.getUid(), username, user.getEmail());

    // Go to MainActivity

}
private String usernameFromEmail(String email) {
    if (email.contains("@")) {
        return email.split("@")[0];
    } else {
        return email;
    }
}

private void writeNewUser(String userId, String name, String email) {
    User user = new User(name, email);

    mDatabase.child("users").child(userId).setValue(user);
    ArrayList<String> userNames = new ArrayList<>();
    userNames.add(name);
    mDatabase.child("usernamelist").setValue(userNames);
}


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mAuth = FirebaseAuth.getInstance();
    mDatabase = FirebaseDatabase.getInstance().getReference();


    if(mAuth.getCurrentUser()!=null){
        showUserList();
    }

    usernameField = (EditText) findViewById(R.id.username);
    passwordField = (EditText) findViewById(R.id.password);
    changeSignUpModeTextView = (TextView) findViewById(R.id.changeSignUpMode);
    signUpButton = (Button) findViewById(R.id.signupbutton);
    logo = (ImageView)findViewById(R.id.logo);
    relativeLayout= (RelativeLayout)findViewById(R.id.relativeLayout);

    signUpModeActive = true;

    changeSignUpModeTextView.setOnClickListener(this);

    usernameField.setOnKeyListener(this);
    passwordField.setOnKeyListener(this);

    logo.setOnClickListener(this);
    relativeLayout.setOnClickListener(this);



}



}

Lista de usuarios.java

public class UserList extends AppCompatActivity {

private static final String TAG = "UserList" ;
private DatabaseReference userlistReference;
private ValueEventListener mUserListListener;
ArrayList<String> usernamelist = new ArrayList<>();
ArrayAdapter arrayAdapter;;

ListView userListView;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_user_list);
    userlistReference = FirebaseDatabase.getInstance().getReference().child("usernamelist");
    onStart();
    userListView = (ListView) findViewById(R.id.userlistview);


}

@Override
protected void onStart() {
    super.onStart();
    final ValueEventListener userListener = new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            usernamelist = new ArrayList<>((ArrayList) dataSnapshot.getValue());
            usernamelist.remove(usernameOfCurrentUser());
            Log.i(TAG, "onDataChange: "+usernamelist.toString());
            arrayAdapter = new ArrayAdapter(UserList.this,android.R.layout.simple_list_item_1,usernamelist);
            userListView.setAdapter(arrayAdapter);
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.w(TAG, "onCancelled: ",databaseError.toException());
            Toast.makeText(UserList.this, "Failed to load User list.",
                    Toast.LENGTH_SHORT).show();
        }
    };
    userlistReference.addValueEventListener(userListener);

    mUserListListener = userListener;
}
public String usernameOfCurrentUser()
{
    String email = MainActivity.mAuth.getCurrentUser().getEmail();
    if (email.contains("@")) {
        return email.split("@")[0];
    } else {
        return email;
    }
}
@Override
public void onStop() {
    super.onStop();

    // Remove post value event listener
    if (mUserListListener != null) {
        userlistReference.removeEventListener(mUserListListener);
    }

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch(item.getItemId()) {
        case R.id.action_logout:
            FirebaseAuth.getInstance().signOut();
            startActivity(new Intent(this, MainActivity.class));
            finish();
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}

}

Puede usar Google Identity Toolkit API para obtener una lista de todos los usuarios registrados en su proyecto Firebase, esta API es utilizada por la CLI Firebase a la que se puede acceder ejecutando firebase auth:export results-file

Asegúrese de que la API del Kit de herramientas de identidad esté habilitada

Firebase-lista de usuarios.js

const serviceAccount = require('path/to/firebase-sdk-json-service-account');

const googleapis = require('googleapis');
const identitytoolkit = googleapis.identitytoolkit('v3');

const authClient = new googleapis.auth.JWT(
    serviceAccount.client_email,
    null,
    serviceAccount.private_key,
    ['https://www.googleapis.com/auth/firebase'],
    null
);

authClient.authorize((err) => {
    if (err) {
        return console.error(err);
    }

    let nextPageToken = undefined;
    let users = [];
    const getAccounts = () => identitytoolkit.relyingparty.downloadAccount({
        auth: authClient,
        resource: {
            targetProjectId: serviceAccount.project_id,
            maxResults: 200,
            nextPageToken: nextPageToken
        }
    }, (e, results) => {
        if (e) {
            return console.error(err);
        }

        users = users.concat(results.users);
        if (results.nextPageToken) {
            nextPageToken = results.nextPageToken;
            return getAccounts();
        } else {
            console.log(users);
        }
    });
    getAccounts();
});

Lista de usuarios en python:

from firebase_admin import credentials, db, auth
cred = credentials.Certificate('\path\to\serviceAccountKey.json')
default_app = firebase_admin.initialize_app(cred, {
    "databaseURL": "https://data_base_url.firebaseio.com"
})
users = auth.list_users()