How can I chain Firestore queries to populate trips with driver details without nested listeners?

Below is an example of a better approach. Instead of nesting listeners (which leads to managing asynchronous timing manually), you can chain the asynchronous calls using the Tasks API. You start by retrieving the trips, create a list of tasks (one for each driver lookup), and then wait for all driver lookups to finish before updating the adapter. This approach eliminates the need for mutable final lists scattered across callbacks.

Below is an example in Java:

JAVACopied
private void retrieveTrips() {
    // First, get all trips.
    database.collection("trips")
        .get()
        .continueWithTask(task -> {
            if (!task.isSuccessful() || task.getResult() == null) {
                throw task.getException();
            }

            List<Trip> trips = new ArrayList<>();
            List<Task<QuerySnapshot>> driverTasks = new ArrayList<>();

            // For each trip document, create a Trip object and a task to load its driver.
            for (QueryDocumentSnapshot tripDoc : task.getResult()) {
                Trip trip = new Trip();
                trip.setTripUid(tripDoc.getId());
                trip.setStartTime(tripDoc.getString("startTime"));
                trip.setStartLocation(tripDoc.getString("startLocation"));
                // ... set other trip attributes

                String driverId = tripDoc.getString("driverId");

                // Create a task to get the driver data.
                Task<QuerySnapshot> driverTask = database.collection("users")
                        .whereEqualTo("userId", driverId)
                        .get()
                        .addOnSuccessListener(driverResult -> {
                            if (!driverResult.isEmpty()) {
                                // Assuming one user result per driverId.
                                QueryDocumentSnapshot userDoc = driverResult.getDocuments().get(0);
                                String driverUsername = userDoc.getString("username");
                                String driverEmail = userDoc.getString("email");
                                // ... retrieve more attributes
                                User driver = new User(driverId, driverUsername, driverEmail /*, ...*/);
                                trip.setDriver(driver);
                            } else {
                                // Handle driver not found (if needed).
                            }
                        })
                        .addOnFailureListener(e -> {
                            // Handle any errors in loading driver data.
                        });
                driverTasks.add(driverTask);
                trips.add(trip);
            }

            // Wait for all driver tasks to complete.
            return Tasks.whenAllSuccess(driverTasks).continueWith(successTask -> trips);
        })
        .addOnSuccessListener(trips -> {
            // At this point, each trip object has been updated with its driver details.
            if (!trips.isEmpty()) {
                TripAdapter tripAdapter = new TripAdapter((List<Trip>) trips, this);
                binding.availableTripsRecyclerView.setAdapter(tripAdapter);
            } else {
                showErrorMessage();
            }
        })
        .addOnFailureListener(e -> {
            // Handle errors in the trips query.
            Log.e("retrieveTrips", "Error loading trips", e);
        });
}

Explanation

• Chain the Tasks:
  The trips query returns a Task from which we create a list of driver lookup tasks.
• Parallel Driver Lookups:
  A driver lookup is started for each trip. All tasks are collected in the driverTasks list.
• Wait for All Tasks:
  We use Tasks.whenAllSuccess so that only after all driver lookups complete do we continue to update the UI.
• Single Point to Update UI:
  Once all the tasks finish, the adapter is set on the RecyclerView.

This approach keeps your code cleaner, avoids deeply nested callbacks, and ensures that the RecyclerView is updated only once all necessary data is available.