android - 在 Firebase onDataChange 中返回一个值

Question

我正在尝试编写一个 firebase 应用程序，其中包含一个简单的 ArrayList，它将用作 ListView 的提要。为此，我有一个单例来处理 FirebaseConnection 并将获取数据。然后将此数据提供给列表。问题是数据首先作为 null 提供给列表，然后数据完成检索。您可能对我有什么解决方案吗？

这是我的代码:

public ArrayList< Event > getAllEventsOnFirebase() {
    final ArrayList< Event > events = new ArrayList<Event>();

    DatabaseReference eventsTable = getDatabaseTableWith(Constants.tableEvents);

    eventsTable.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {

                HashMap<String, Object> event = (HashMap<String, Object>) postSnapshot.getValue();

                String eventId = (String) event.get(Constants.taEventUUID);
                String eventName = (String) event.get(Constants.taEventName);
                String eventAddress = (String) event.get(Constants.taEventAddress);


                Event newEvent = new Event(eventId, eventName, eventAddress);

                events.add(newEvent);

            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

    return events;
}

此外，我填充列表的方式如下:

   eventsArray = FirebaseConnection.getInstance().getAllEventsOnFirebase();

    EventListAdapter adapter = new EventListAdapter(this, eventsArray);
    eventsListView.setAdapter(adapter);

Answer 1

数据从 Firebase 异步加载。幸运的是，您可以随时填充 ArrayList，只要您通过调用 adapter.notifyDataSetChanged() 告诉适配器您更改了它。

public ArrayList< Event > getAllEventsOnFirebase(final EventListAdapter adapter, 
                                                 final ArrayList<Event> events) {
  DatabaseReference eventsTable = getDatabaseTableWith(Constants.tableEvents);
  eventsTable.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot postSnapshot: dataSnapshot.getChildren()) {
            String eventId = postSnapshot.child(Constants.taEventUUID).getValue(String.class);
            String eventName = event.child(Constants.taEventName).getValue(String.class);
            String eventAddress = event.child(Constants.taEventAddress).getValue(String.class);

            Event newEvent = new Event(eventId, eventName, eventAddress);

            events.add(newEvent);
        }
        adapter.notifyDataSetChanged();
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException();
    }
  });
}

现在你可以称它为:

ArrayList< Event > events = new ArrayList<Event>();
EventListAdapter adapter = new EventListAdapter(this, eventsArray);
eventsListView.setAdapter(adapter);

FirebaseConnection.getInstance().getAllEventsOnFirebase(adapter, events);