java - 从 Firestore 下载数据较晚

Question

我想将一个数组与一个 fragment 打包在一起。 Parcelable 数组依赖于 firestore 数据检索。我的意思是数组的元素是从 Firestore 获取的。但是从 Firestore 检索数据已经很晚了，并且正在执行下一行代码并且正在打包空数组。如何使下一行等待从 Firestore 检索数据？

public class MainActivity extends AppCompatActivity implements OnMapReadyCallback {



private static final String MAPVIEW_BUNDLE_KEY = "MapViewBundleKey";
private static final int PERMISSIONS_REQUEST_ENABLE_GPS = 9001;
private static final int PERMISSIONS_REQUEST_ACCESS_FINE_LOCATION = 9002;
private static final String TAG = "MainActivity";
private static final int ERROR_DIALOG_REQUEST = 9003;
private boolean mLocationPermissionGranted = false;

private List<User>mUserList=new ArrayList<>();
private ArrayList<UserLocation>mUserLocations=new ArrayList<>();

private FusedLocationProviderClient mFusedLocationProviderClient;
FirebaseFirestore mDb;

private GoogleMap mGoogleMap;
private UserLocation mUserPosition=null;
private LatLngBounds latLngBoundary;

private ClusterManager mClusterManager;
private MyClusterManagerRenderer mClusterManagerRenderer;
private ArrayList<ClusterMarker> mClusterMarkers=new ArrayList<>();


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mDb = FirebaseFirestore.getInstance();

    initUser();//in this method the data retrieving is implemented

    if (findViewById(R.id.fragment_container) != null) {
        if (savedInstanceState != null) {
            return;
        }            
        MapFragment mapFragment = new MapFragment();
        Bundle bundle=new Bundle();
        bundle.putParcelableArrayList(getString(R.string.userlocations_array),  mUserLocations);
        mapFragment.setArguments(bundle);


        getSupportFragmentManager().beginTransaction()
                .add(R.id.fragment_container, mapFragment).commit();
    }

}

private void initUser() {
    User user=new User();
    user.setEmail("nobeld@gmail.com");
    user.setResponse("ok");
    user.setUser("student");
    user.setUserId("5");
    user.setUserName("nobel");
    ((UserClient)(getApplicationContext())).setUser(user);
    mUserList.add(user);
    User user1=new User();
    user1.setEmail("rahuld@gmail.com");
    user1.setResponse("ok");
    user1.setUser("student");
    user1.setUserId("6");
    user1.setUserName("rahul");
    User user2=new User();
    user2.setEmail("milond@gmail.com");
    user2.setResponse("ok");
    user2.setUser("student");
    user2.setUserId("7");
    user2.setUserName("milon");
    mUserList.add(user1);
    mUserList.add(user2);
    for(User u: mUserList){
        getUserLocation(u);
        //firestore is implemented inside this method
        Log.d(TAG, "initUser: in user array");
    }



}

private void setCameraView(){
    if(mUserPosition!= null){
        Log.d(TAG, "setCameraView: user position got");
        double bottomboundary=mUserPosition.getGeo_point().getLatitude()-.05;
        double leftboundary = mUserPosition.getGeo_point().getLongitude()-.05;
        double upboundary = mUserPosition.getGeo_point().getLatitude()+.05;
        double rightboundary = mUserPosition.getGeo_point().getLongitude()+.05;
        latLngBoundary=new LatLngBounds(new LatLng(bottomboundary,leftboundary),
                new LatLng(upboundary,rightboundary));
        mGoogleMap.moveCamera(CameraUpdateFactory.newLatLngBounds(latLngBoundary,0));

    }else {
        Log.d(TAG, "setCameraView: user position is null");
    }
}
private void getUserLocation(User user){
    Log.d(TAG, "getUserLocation: ");
    DocumentReference locationRef=mDb.collection(getString(R.string.collection_user_location_student))
            .document(user.getUserId());
    locationRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
        @Override
        public void onComplete(@NonNull Task<DocumentSnapshot> task) {
            if(task.isSuccessful()){
                if(task.getResult().toObject(UserLocation.class)!= null){
                    Log.d(TAG, "Location onComplete: ");
                    UserLocation u=task.getResult().toObject(UserLocation.class);
                    mUserLocations.add(u);
                    //here adding the elements to array.

                }else {
                    Log.d(TAG, "onComplete: result is empty");
                }
            }
        }
    });

}

}

Answer 1

因为数据仅在 onComplete() 方法内可用，因为它是异步行为，所以当您尝试将 mUserLocations 列表添加到 Bundle 对象，数据尚未完成从数据库的加载，这就是不可访问的原因(列表为空)。解决此问题的一个快速方法是移动以下代码行:

Bundle bundle=new Bundle();
bundle.putParcelableArrayList(getString(R.string.userlocations_array),  mUserLocations);
mapFragment.setArguments(bundle);

在以下代码行之后的 onComplete() 方法内部:

//here adding the elements to array.

What to do to make the next lines wait until the data is retrieved from the Firestore??

如果您想在该方法之外使用mUserLocations，我建议您查看此post中我的答案的最后部分。 其中我解释了如何使用自定义回调来完成它。您还可以看看这个video 为了更好地理解。