LocationManager 返回空值

Question

我正在使用 GPS 在 Android 应用中获取我的位置。但是当我创建一个 LocationManager 它返回空指针异常。即使在 androidMainFest.xml 和应用程序上设置了权限，它仍然返回以下异常。谁能提前帮忙。谢谢！

代码：

package com.example.pavsaranga.scat;

import android.content.Context;
import android.content.pm.PackageManager;
import android.location.Criteria;
import android.location.Location;
import android.location.LocationManager;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v4.app.FragmentActivity;
import android.widget.Toast;

import com.google.android.gms.location.LocationListener;
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;

public class Map extends FragmentActivity implements OnMapReadyCallback, LocationListener {

    private GoogleMap mMap;
    double longitude, latitude;
    String bestProvider;
    LocationManager lm;
    Location location;
    Criteria criteria;
    LocationListener locList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_map);
        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);
    }

    @Override
    public void onMapReady(GoogleMap googleMap) {
        mMap = googleMap;
        mMap.setMapType(GoogleMap.MAP_TYPE_NORMAL);
        mMap.getUiSettings().setZoomGesturesEnabled(true);
        try {
            boolean permissionGranted = ActivityCompat.checkSelfPermission(this, android.Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED;
            if (permissionGranted) {
                setMyLocation();
            } else {
                ActivityCompat.requestPermissions(this, new String[]{android.Manifest.permission.ACCESS_FINE_LOCATION}, 200);
            }
        } catch(Exception e){
            e.printStackTrace();
        }
    }

    private void setMyLocation() {
        try {
            lm = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);
            if (ActivityCompat.checkSelfPermission(this, android.Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, android.Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
                Toast.makeText(Map.this, "Please Grant Permission.", Toast.LENGTH_SHORT).show();
            } else {
                criteria = new Criteria();
                bestProvider = String.valueOf(lm.getBestProvider(criteria, true)).toString();
                location = lm.getLastKnownLocation(bestProvider);
                if (location != null) {
                    latitude = location.getLatitude();
                    longitude = location.getLongitude();
                }
                else{
                    lm.requestLocationUpdates(bestProvider, 2000, 0, (android.location.LocationListener) locList);
                }
                LatLng myLocation = new LatLng(latitude, longitude);
                mMap.addMarker(new MarkerOptions().position(myLocation).title("You Are Here!"));
                mMap.moveCamera(CameraUpdateFactory.newLatLng(myLocation));
            }
        } catch(Exception e){
            e.printStackTrace();
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode) {
            case 200: {
                if(grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    Toast.makeText(Map.this, "Thank You, Permission Granted!.", Toast.LENGTH_SHORT).show();
                    setMyLocation();
                }
            }
        }
    }

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

例外：

java.lang.IllegalArgumentException: invalid listener: null

MainFest.xml

<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

Answer 1

LocationManager 没有返回 null。错误是您传递的是null LocationListener。

您需要先初始化loclist，然后再将其传递给requestLocationUpdates

Answer 2

您正在声明 locList 并将其传递给 lm.requestLocationUpdates(bestProvider, 2000, 0, (android.location.LocationListener) locList); 方法。它看起来不像你正在初始化它。 这就是它告诉您的异常，您在期望侦听器的地方传入 null。

您可以删除locList 声明并尝试进行以下更改：

改变

lm.requestLocationUpdates(bestProvider, 2000, 0, (android.location.LocationListener) locList);

到

lm.requestLocationUpdates(bestProvider, 2000, 0, new LocationListener(){
    // @Override
    public void onStatusChanged(String provider, int status, Bundle extras) {
        /*
        * Called when the provider status changes. 
        * This method is called when a provider is unable to fetch a location 
        * or if the provider has recently become available after a period of unavailability.
        */
    }

    // @Override
    public void onLocationChanged(Location location) {
        /*
        * Called when the location has changed. 
        */
    }
});