位置对象在Google Maps Android中返回null

Question

I am working on a project where the current latitude and longitude are taken from another class.When i switch off the phone or open the app the next day , the current latitude and longitude are shown as 0.0, which is because the location object is becoming null.But as soon as i open the app again the map is shown correctly.

The code in the main class:

public void getcurrentlocation()
{
    gps=new GPSTracker(MainActivity.this);
    if(gps.canGetLocation())
    {
        current_latitude=gps.getLatitude();
        current_longitude=gps.getLongitude();
        placemarkersonmap();
    }
    else
    {
    gps.showSettingsAlert();
    }
}
public void placemarkersonmap()
{

    googleMap.setMyLocationEnabled(true);
    googleMap.getUiSettings().setMyLocationButtonEnabled(true);

    LatLng latLng = new LatLng(current_latitude, current_longitude);

    Toast.makeText(getApplicationContext(),current_latitude+" "+current_longitude,Toast.LENGTH_LONG).show();

    googleMap.moveCamera(CameraUpdateFactory.newLatLngZoom(latLng, 12f));
    googleMap.animateCamera(CameraUpdateFactory.newLatLngZoom(latLng, 12f));

    MarkerOptions marker = new MarkerOptions().position(new LatLng(8.505439, 76.971293));


 googleMap.addMarker(marker);

}

The helper class for retrieving current latitude and longitude ,GPSTracker class:

package com.example.marinamapseg;

import android.app.AlertDialog;
import android.app.Service;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.os.IBinder;
import android.provider.Settings;
import android.util.Log;
import android.widget.Toast;

public class GPSTracker extends Service implements LocationListener {

    private final Context mContext;

    // flag for GPS status
    boolean isGPSEnabled = false;

    // flag for network status
    boolean isNetworkEnabled = false;

    // flag for GPS status
    boolean canGetLocation = false;

    Location location; // location
    double latitude; // latitude
    double longitude; // longitude

    // The minimum distance to change Updates in meters
    private static final long MIN_DISTANCE_CHANGE_FOR_UPDATES = 10; // 10 meters

    // The minimum time between updates in milliseconds
    private static final long MIN_TIME_BW_UPDATES = 1000 * 60 * 1; // 1 minute

    // Declaring a Location Manager
    protected LocationManager locationManager;

    public GPSTracker(Context context) {
        this.mContext = context;
        getLocation();
    }

    public Location getLocation() {
        try {


            locationManager = (LocationManager) mContext
                    .getSystemService(LOCATION_SERVICE);

            // getting GPS status
            isGPSEnabled = locationManager
                    .isProviderEnabled(LocationManager.GPS_PROVIDER);

            // getting network status
            isNetworkEnabled = locationManager
                    .isProviderEnabled(LocationManager.NETWORK_PROVIDER);

            if (!isGPSEnabled && !isNetworkEnabled) {
                Toast.makeText(getApplicationContext(),"NO PROVIDER ENABLED",Toast.LENGTH_LONG).show();
            } else {
                this.canGetLocation = true;
                // First get location from Network Provider
                if (isNetworkEnabled) {
                    locationManager.requestLocationUpdates(
                            LocationManager.NETWORK_PROVIDER,
                            0,
                            0, this);
                    Log.d("Network", "Network");
                    if (locationManager != null) {
                        location = locationManager
                                .getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
                        if (location != null) {
                            latitude = location.getLatitude();
                            longitude = location.getLongitude();
                        }
                    }
                }
                // if GPS Enabled get lat/long using GPS Services
                if (isGPSEnabled) {
                    if (location == null) {
                        locationManager.requestLocationUpdates(
                                LocationManager.GPS_PROVIDER,
                                0,
                                0, this);
                        Log.d("GPS Enabled", "GPS Enabled");
                        if (locationManager != null) {
                            location = locationManager
                                    .getLastKnownLocation(LocationManager.GPS_PROVIDER);
                            if (location != null) {
                                latitude = location.getLatitude();
                                longitude = location.getLongitude();
                            }
                        }
                    }
                }
            }

        } catch (Exception e) {
            e.printStackTrace();
        }

        return location;
    }

    /**
     * Stop using GPS listener
     * Calling this function will stop using GPS in your app
     * */
    public void stopUsingGPS(){
        if(locationManager != null){
            locationManager.removeUpdates(GPSTracker.this);
        }      
    }

    /**
     * Function to get latitude
     * */
    public double getLatitude(){
        if(location != null){
            latitude = location.getLatitude();
        }

        // return latitude
        return latitude;
    }

    /**
     * Function to get longitude
     * */
    public double getLongitude(){
        if(location != null){
            longitude = location.getLongitude();
        }

        // return longitude
        return longitude;
    }

    /**
     * Function to check GPS/wifi enabled
     * @return boolean
     * */
    public boolean canGetLocation() {
        return this.canGetLocation;
    }

    /**
     * Function to show settings alert dialog
     * On pressing Settings button will lauch Settings Options
     * */
    public void showSettingsAlert(){
        AlertDialog.Builder alertDialog = new AlertDialog.Builder(mContext);

        // Setting Dialog Title
        alertDialog.setTitle("GPS is settings");

        // Setting Dialog Message
        alertDialog.setMessage("GPS is not enabled. Do you want to go to settings menu?");

        // On pressing Settings button
        alertDialog.setPositiveButton("Settings", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog,int which) {
                Intent intent = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
                mContext.startActivity(intent);
            }
        });

        // on pressing cancel button
        alertDialog.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int which) {
            dialog.cancel();
            }
        });

        // Showing Alert Message
        alertDialog.show();
    }

    @Override
    public void onLocationChanged(Location location) {
    }

    @Override
    public void onProviderDisabled(String provider) {
    }

    @Override
    public void onProviderEnabled(String provider) {
    }

    @Override
    public void onStatusChanged(String provider, int status, Bundle extras) {
    }

    @Override
    public IBinder onBind(Intent arg0) {
        return null;
    }

}

I tried setting the minimum time between updates and the minimum distance between updates as 0 for the requestLocationUpdates method of locationmanager .But that didnt help either.

So the issue is why does the location object return null when i open the app the next day.It also works fine as soon as the close the app and restart it again.Please help!

Answer 1

For getting updates make use of the LocationListener interface you have implemented instead of using

if (locationManager != null) {
    location = locationManager
                   .getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
    if (location != null) {
        latitude = location.getLatitude();
        longitude = location.getLongitude();
    }
}

do the following in your onLocationChanged()

@Override
public void onLocationChanged(Location location) {
    latitude = location.getLatitude();
    longitude = location.getLongitude();
}

This makes sure that when a new location update is available it is stored in your respective variables.

PS: For getting location updates in the background, make sure that your service is not terminated when the app is closed. You could also make use of SharedPreferences if you want to persist changes even when the phone is restarted.