无法使用 Firebase 更新谷歌地图位置

Question

我正在尝试在我的 android 应用程序中将位置从 firebase 数据库更新为谷歌地图。我已经使用了 firebase 的 onDataChange 方法来做到这一点。我的问题是，即使在 Firebase 数据库中更改了海拔和经度的位置，我仍然无法更新谷歌地图上的位置。下面是 MapsActivity.java 类的代码。

import android.support.v4.app.FragmentActivity;
import android.os.Bundle;
import android.util.Log;

import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback {

    private GoogleMap mMap;
    String value;
    @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);

        FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference myRef = database.getReference("Location");

      //  myRef.setValue("Hello, World!");

        myRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
              value = dataSnapshot.getValue(String.class);
                String [] separated = value.split(",");
                String latiPos= separated[0].trim();
                String longiPos =separated[1].trim();
                String TAG ="VAL";
                double dLat = Double.parseDouble(latiPos);
                double dLong = Double.parseDouble(longiPos);
                // Add a marker in Sydney and move the camera
                LatLng sydney = new LatLng(dLat, dLong);
                mMap.addMarker(new MarkerOptions().position(sydney).title(latiPos+" "+longiPos));


            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
    }


    /**
     * 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;

        // Add a marker in Sydney and move the camera
     //   LatLng sydney = new LatLng(-34, 151);
      //  mMap.addMarker(new MarkerOptions().position(sydney).title(value));
        //mMap.moveCamera(CameraUpdateFactory.newLatLng(sydney));
    }
}

Answer 1

我建议有两件事来解决您的问题：

1. 将存储在FirebaseDatabase 中的数据包装在Java 对象中，以便轻松存储和检索数据。现在 您正在做很多工作来处理存储在数据库中的数据。所以， 定义一个类，它代表您作为 Java 对象存储的数据。然后你的读/写 到数据库会容易得多。这是可用于表示地图标记的类的示例 根据我在上面的代码中看到的。您可以对其进行自定义以满足您的需求：

   public class LocationMarker {
   
       double latitude;
       double longitude;
   
   
       /**
        * Required Empty Constructor used by Firebase
        */
       public LocationMarker() {
       }
   
       public LocationMarker(double latitude, double longitude) {
           this.latitude = latitude;
           this.longitude = longitude;
   
       }
   
       public double getLatitude() {
           return latitude;
       }
   
       public void setLatitude(double latitude) {
           this.latitude = latitude;
       }
   
       public double getLongitude() {
           return longitude;
       }
   
       public void setLongitude(double longitude) {
           this.longitude = longitude;
       }
   
   
       /**
        * These getters will be ignored when the object is serialized into a JSON object.
        * Since they will be ignored this means that they don't need a corresponding property (field)
        * in the Database. This is how we can return a value without having them declared as an instance variable.
        */
   
       @Exclude
       public LatLng getPosition(){
           return new LatLng(latitude, longitude);
       }
   
       @Exclude
       public String getTitle(){
           return Double.toString(latitude) + " " + Double.toString(longitude);
       }
   }
   

   现在您可以更轻松地将位置写入数据库。

   DatabaseReference myRef = FirebaseDatabase.getInstance().getReference();
   double latitude = -34;
   double longitude = 151;
   LocationMarker locationMarker = new LocationMarker(latitude,longitude);
   myRef.child("Location").setValue(locationMarker);
   

   然后阅读：

   myRef.addValueEventListener(new ValueEventListener() {
       @Override
       public void onDataChange(DataSnapshot dataSnapshot) {
   
            LocationMarker locationMarker = dataSnapshot.getValue(LocationMarker.class);
   
            if(locationMarker != null){
                LatLng sydney = locationMarker.getPosition();
                mMap.addMarker(new MarkerOptions()
                                       .position(sydney)
                                       .title(locationMarker.getTitle));
            }
        }
   
        @Override
        public void onCancelled(DatabaseError databaseError) {
   
        }
   });
   

2. 对onMapReady() 和onDataChange() 的调用是异步发生的。 这意味着您的挑战是您无法控制何时调用 onMapReady()。 您也无法控制您的ValueEventListener 何时调用onDataChange()。 正如现在所写，对 FirebaseDatabase 的调用可能会在您拥有有效的 GoogleMap 对象之前返回。所以在对onMapReady()的调用中添加你的ValueEventListener：

   @Override
   public void onMapReady(GoogleMap googleMap) {
        mMap = googleMap;
        addValueEventListener();
   }
   
   //....
   
   private void addValueEventListener(){
   
       if(mMap == null) return;
   
        myRef.addValueEventListener(new ValueEventListener() {
             @Override
             public void onDataChange(DataSnapshot dataSnapshot) {
   
                  LocationMarker locationMarker = dataSnapshot.getValue(LocationMarker.class);
   
                  if(locationMarker != null){
                  LatLng sydney = locationMarker.getPosition();
                  mMap.addMarker(new MarkerOptions()
                                       .position(sydney)
                                       .title(locationMarker.getTitle));
                  }
             }
   
             @Override
             public void onCancelled(DatabaseError databaseError) {
   
             }
        });
   }
   

希望这有助于向您的数据库显示存储和检索地图标记。这应该在“位置”下存储一个标记。您必须针对多个标记对数据库结构进行调整。另外，请务必删除所有添加的侦听器:)