在onDataChange之后，ArrayList元素会丢失

Question

我100％保证正在填充ArrayList，因为当我进入调试模式时，我可以看到正确添加了元素。

但在完成onDataChange块位置后，ArrayList为空。

这是我的代码，我将不胜感激任何想法：

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback {

private GoogleMap mMap;
public ArrayList<LocationObject> locations = null;
FirebaseDatabase database;
@Override
protected void onCreate(Bundle savedInstanceState) {
    database = FirebaseDatabase.getInstance();
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_maps);
    locations = new ArrayList<>();
    // Obtain the SupportMapFragment and get notified when the map is ready to be used.
    SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
            .findFragmentById(R.id.map);
    mapFragment.getMapAsync(this);
    final DatabaseReference ref = database.getReference("venues");
    ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot d : dataSnapshot.getChildren()) {
                locations.add(d.getValue(LocationObject.class));
            }
            Log.d("ELEMENT", String.valueOf(locations.size()));
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.e("ERROR", "Connection failed");
        }
    });
}
@Override
public void onMapReady(GoogleMap googleMap) {
    Log.d("SIZE", String.valueOf(locations.size()));
    //Log.d("SIZE:", String.valueOf(locations.size()));
    mMap = googleMap;
    if(locations.size() != 0) {
        for (LocationObject o : locations) {
            double lat = o.getLat();
            double lng = o.getLoong();
            LatLng place = new LatLng(lat, lng);
            mMap.addMarker(new MarkerOptions().position(place).title(o.getDesc()));
            mMap.moveCamera(CameraUpdateFactory.newLatLng(place));
        }
    }
    else{
        LatLng sydney = new LatLng(-34, 151);
        mMap.addMarker(new MarkerOptions().position(sydney).title("Marker in Sydney"));
        mMap.moveCamera(CameraUpdateFactory.newLatLng(sydney));
    }
  }

}

Answer 1

addValueEventListener和onMapReady是异步函数，onMapReady可以在addValueEventListener之前完成。