java - LatLng GPS 位置的 NullPointerException

Question

我正在使用 Google map 获取我的当前位置并在我的位置上显示红色标记。到目前为止，只有 mMap.setMyLocation(true); 适用于我的位置。我在代码末尾收到对象 userLocation 的 NullPointerException，并且 userLocation 控制台的输出也为 null。

如何在加载 map 后立即设置 userLocation？

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback {

    private GoogleMap mMap;

    LocationManager locationManager;

    LocationListener locationListener;
    LatLng userLocation;
    private FusedLocationProviderClient mFusedLocationClient;
    public  float DEFAULT_ZOOM = 15f;
    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);

        if (requestCode == 1) {

            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {

                if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED) {
                    {

                        locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);

                    }

                }

            }

        }

    }

    @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);
            }

    @Override
    public void onMapReady(GoogleMap googleMap) {
        mMap = googleMap;
        locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);
        locationListener = new LocationListener() {
            @Override
            public void onLocationChanged(Location location) {

                userLocation = new LatLng(location.getLatitude(), location.getLongitude());

                mMap.clear();
                mMap.addMarker(new MarkerOptions().position(userLocation).title("Your Location"));
                mMap.moveCamera(CameraUpdateFactory.newLatLng(userLocation));
                System.out.println("the latitude onLocationChanged is "+ userLocation.latitude);


            }

            @Override
            public void onStatusChanged(String s, int i, Bundle bundle) {

            }

            @Override
            public void onProviderEnabled(String s) {

            }

            @Override
            public void onProviderDisabled(String s) {

            }
        };



        if (Build.VERSION.SDK_INT < 23) {
        try {
            locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);
        }
        catch (SecurityException e){
            e.printStackTrace();
        }
        } else {

            if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED) {

                ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, 1);

            } else {

                locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 1, 1, locationListener);
              //  LatLng latLng = new LatLng(40.741895,-73.989308);
                mMap.setMyLocationEnabled(true);
            //outputs the latitude is null 
                System.out.println("the latitude is "+userLocation);
              mMap.addMarker(new MarkerOptions().position(userLocation).title("Your Location"));
                mMap.moveCamera(CameraUpdateFactory.newLatLng(userLocation));


            }


        }


    }


}

Answer 1

你不能。获得位置需要时间。在调用回调之前它不可用，这意味着在此之前您无法加载它。您可以尝试调用 getLastKnownLocation，但大多数情况下它也会返回 null。最好的解决方案是在回调完成之前不要依赖它。