如何在Firebase數據庫中添加Google地圖標記

Question

我想從Firebase數據庫中檢索位置標記，但運行此代碼後無法看到任何標記。我試圖製作EventListener，但地圖運行時沒有任何標記。這是我的代碼：

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback { 

    private GoogleMap mMap; 

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

    } 


    /** 
    * Manipulates the map once available. 
    * This callback is triggered when the map is ready to be used. 
    * This is where we can add markers or lines, add listeners or move the camera. In this case, 
    * we just add a marker near Sydney, Australia. 
    * If Google Play services is not installed on the device, the user will be prompted to install 
    * it inside the SupportMapFragment. This method will only be triggered once the user has 
    * installed Google Play services and returned to the app. 
    */ 
    @Override 
    public void onMapReady(GoogleMap googleMap) { 
     mMap = googleMap; 
     DatabaseReference ref = FirebaseDatabase.getInstance().getReference(); 




        String rightLocation = ref.child("location_right").toString(); 
        String leftLocation = ref.child("location_left").toString(); 

        double location_left = Double.valueOf(leftLocation); 
        double location_right = Double.valueOf(rightLocation); 
        LatLng sydney = new LatLng(location_left, location_right); 
        mMap.addMarker(new MarkerOptions().position(sydney).title("Marker in Sydney")); 
        mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(sydney,18)); 











     // Add a marker in Sydney and move the camera 

    } 
} 

這裏有什麼問題？

Answer 1

您需要使用ValueEventListener才能從Firebase數據庫中獲取數據。這在the documentation中有解釋。

@Override 
public void onMapReady(GoogleMap googleMap) { 
    mMap = googleMap; 
    DatabaseReference ref = FirebaseDatabase.getInstance().getReference(); 

    ref.addListenerForSingleValueEvent(new ValueEventListener() { 
     @Override 
     public void onDataChange(DataSnapshot dataSnapshot) { 
      if (dataSnapshot.exists()) { 
       double location_left = dataSnapshot.child("location_left").getValue(Double.class); 
       double location_right = dataSnapshot.child("location_right").getValue(Double.class); 
       LatLng sydney = new LatLng(location_left, location_right); 
       mMap.addMarker(new MarkerOptions().position(sydney).title("Marker in Sydney")); 
       mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(sydney,18)); 
      } else { 
       Log.e(TAG, "onDataChange: No data"); 
      } 

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