如何从firebase数据库添加谷歌地图标记 [英] how to add google map marker from firebase database

问题描述

我想从Firebase数据库检索位置标记，但运行此代码后看不到任何标记。我试图制作 EventListener ，但是地图运行时没有任何标记。这是我的代码：

$ p $ public class MapsActivity扩展FragmentActivity实现OnMapReadyCallback {

私有GoogleMap mMap;

@Override
protected void onCreate（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）;
setContentView（R.layout.activity_maps）;
//获取SupportMapFragment并在地图准备好使用时得到通知。
SupportMapFragment mapFragment =（SupportMapFragment）getSupportFragmentManager（）
.findFragmentById（R.id.map）;
mapFragment.getMapAsync（this）;

}


/ **
*操作一次可用的地图。
*当地图准备好被使用时，这个回调被触发。
*这是我们可以添加标记或线条，添加听众或移动相机的地方。在这种情况下，
*我们只在澳大利亚悉尼附近添加一个标记。
*如果设备上没有安装Google Play服务，系统会提示用户在SupportMapFragment中安装
* it。只有当用户安装了
* Google Play服务并返回到应用后，此方法才会被触发。
* /
@Override
public void onMapReady（GoogleMap googleMap）{
mMap = googleMap;
DatabaseReference ref = FirebaseDatabase.getInstance（）。getReference（）;




字符串rightLocation = ref.child（location_right）。toString（）;
字符串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））;











// b在悉尼添加一个标记并移动相机


$ b code





$ b <

解决方案

您需要使用 ValueEventListener 从Firebase数据库获取数据。这在文档中进行了解释。

  @Override 
 public void onMapReady（GoogleMap googleMap）{
 mMap = googleMap; 
 DatabaseReference ref = FirebaseDatabase.getInstance（）。getReference（）; 
 
 ref.addListenerForSingleValueEvent（new ValueEventListener（）{
 @Override $ b $ public void onDataChange（DataSnapshot dataSnapshot）{$ b $ 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（）; 
} 
}）; 
  




I want to retrieve location markers from firebase database but I can't see any markers after running this code. I tried to make EventListener but the map runs without any markers. This my code:

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

    }
}

What might be wrong here?

解决方案

You need to use a ValueEventListener to obtain data from the Firebase database. This is explained in the documentation.

@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();
        }
    });

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