数据快照循环未执行

Question

我正在开发一个位置跟踪程序，我正在使用Firebase使其工作，所以在这样做时我使用了数据快照，即addValueEventListener并覆盖方法onDataChange并应用了一个循环即为（ DataSnapshot postSnapshot : dataSnapshot.getChildren()）但是这个循环的内容没有被执行/遍历。附加代码。请帮忙。

DatabaseReference locations;
Double lat, lng;
private GoogleMap mMap;
private String email;

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_map_tracking);
    // 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);

    locations = FirebaseDatabase.getInstance().getReference("Locations");

    if(getIntent() != null){
        email = getIntent().getStringExtra("email");
        lat = getIntent().getDoubleExtra("lat", 0);
        lng = getIntent().getDoubleExtra("lng", 0);
    }

    if(!TextUtils.isEmpty(email)){
       // Toast.makeText(this, " location for this user ", Toast.LENGTH_SHORT).show();
        loadLocationForThisUser(email);


    }

}

private void loadLocationForThisUser(String email) {
    Query user_location = locations.orderByChild("email").equalTo(email);

    user_location.addValueEventListener(new ValueEventListener() {
        //locations.addValueEventListener(new ValueEventListener() {
        @RequiresApi(api = Build.VERSION_CODES.N)
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            Toast.makeText(MapTracking.this, "ondatachange", Toast.LENGTH_SHORT).show();
            for (DataSnapshot postSnapshot : dataSnapshot.getChildren()) {
                Toast.makeText(MapTracking.this, "ondatachange1223", Toast.LENGTH_SHORT).show();
                Tracking tracking = postSnapshot.getValue(Tracking.class);
                Toast.makeText(MapTracking.this, tracking.getLat() + " friend's location " + tracking.getLng(), Toast.LENGTH_SHORT).show();

                LatLng friendlocation = new LatLng(Double.parseDouble(tracking.getLat()),
                        Double.parseDouble(tracking.getLng()));

                Location currentuser = new Location("");
                currentuser.setLatitude(lat);
                currentuser.setLongitude(lng);

                Location friend = new Location("");
                friend.setLatitude(Double.parseDouble(tracking.getLat()));
                friend.setLongitude(Double.parseDouble(tracking.getLng()));

                Toast.makeText(MapTracking.this, tracking.getLat() + " friend's location " + tracking.getLng(), Toast.LENGTH_SHORT).show();
                mMap.clear();

                distance(currentuser, friend);

                mMap.addMarker(new MarkerOptions()
                        .position(friendlocation)
                        .title(tracking.getEmail())
                        .snippet("Distance" + new DecimalFormat("#.#").format((currentuser.distanceTo(friend)) / 1000) + " km ")
                        .icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_BLUE)));
                mMap.animateCamera(CameraUpdateFactory.newLatLngZoom(new LatLng(lat, lng), 12.0f));
                }

            LatLng current = new LatLng(lat, lng);
                //Toast.makeText(MapTracking.this, current.latitude + " current location " + current.longitude, Toast.LENGTH_SHORT).show();
            mMap.addMarker(new MarkerOptions().position(current).title(FirebaseAuth.getInstance().getCurrentUser().getEmail()).icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_BLUE)));

        }
        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}

数据库结构

Answer 1

不幸的是，这可能是由于一个错字。您的简化数据库结构如下：

df1$col_new <- df1[1:3][cbind(seq_len(nrow(df1)), df1$ID)]
df1$col_new
#[1] 12  6 21

但是您正在将- Location - pushId - email: "someone@example.com" - lat: "25.0" - lon: "15.0" - uid: userId 附加到ValueEventListener节点，而该节点不存在，因此不会从数据库中检索到任何数据：

Locations

如果您将其更改为指向“位置”节点，则locations = FirebaseDatabase.getInstance().getReference("Locations"); 方法应该有效：

onDataChange()