為什么函數getLastKnownLocation在Android SDK上返回Null？

Question

我在eclipse上使用android sdk，我想從設備獲取位置，所以我在其他類中編寫了一個程序（與main不同），並從主類中調用mContext函數。

package com.example.deomanapp;

import java.io.IOException;
import java.util.List;
import java.util.Locale;

import com.example.deomanapp.MainActivity.PlaceholderFragment;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.location.Address;
import android.location.Geocoder;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.widget.TextView;
import android.widget.Toast;

public class Mhelper {

    public void mContext(Context context)
    {
        LocationManager lm;
        lm   = (LocationManager)context.getSystemService(context.LOCATION_SERVICE); 
        Location location = lm.getLastKnownLocation(LocationManager.GPS_PROVIDER);

        double longitude = location.getLongitude();
        double latitude = location.getLatitude();
        String slongitude = String.valueOf((long) longitude);
        String slatitude = String.valueOf((long) latitude);

        Toast.makeText(context, slongitude, Toast.LENGTH_SHORT).show();

    }

}

問題是getLongitude或getLatitude返回null，因此該程序在此行與此日志崩潰：

04-20 04:30:30.410: E/AndroidRuntime(5151): java.lang.NullPointerException
04-20 04:30:30.410: E/AndroidRuntime(5151):     at com.example.deomanapp.Mhelper.mContext(Mhelper.java:29)

此代碼有什么問題？

PS：我讀了其他標題相同且主題沒有幫助的問題（它們都不具有實際答案），因為：

1-我在啟用GPS且可以正常工作的真實設備（非仿真器）上測試了該程序，但是盡管Device之前已獲得其位置，但該程序無法獲取位置，並且必須顯示LastKnownLocation 。

2-我給程序ACCESS_FINE_LOCATION和ACCESS_COARSE_LOCATION權限。

3-我什至使用了一項功能來查看GPS是否打開，當我將其關閉時，程序會提醒我。

Answer 1

問這是一個愚蠢的問題，因為它為NULL，所以為null，我通過對主要問題的評論來理解它，我的解決方案是在sdk管理器中下載android api圖像 ，運行一次Google地圖 ，然后通過DDMS將GPS位置發送給它運行程序。

Answer 2

我試圖修改一些代碼，希望它能滿足您的需求。

您將在答案的底部找到我的LocationsCoreModule代碼的實現：

LocationsCoreModule locationService;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    LocationRequest mLocationRequest = LocationRequest.create();
    // Use high accuracy
    mLocationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
    // Set the update interval to 5 seconds
    mLocationRequest.setInterval(5000);
    // Set the fastest update interval to 1 second
    mLocationRequest.setFastestInterval(1000);

    locationService = new LocationsCoreModule(this, mLocationRequest);
    locationService.setLocationsListener(new LocationsCoreModuleCallback() {

        @Override
        public void locationClientConnected() {
              Location location = locationService.getLastLocation();
              double longitude = location.getLongitude();
              double latitude = location.getLatitude();
              String slongitude = String.valueOf((long) longitude);
              String slatitude = String.valueOf((long) latitude);

              Toast.makeText(getApplicationContext(), slongitude, Toast.LENGTH_SHORT).show();
        }
    });
}

如果您希望應用程序立即開始偵聽新的GPS位置：

    @Override
protected void onStart() {
    super.onStart();

    locationService.start(new LocationListener() {

        @Override
        public void onLocationChanged(Location arg0) {
            // TODO Auto-generated method stub

        }
    }, new OnConnectionFailedListener() {

        @Override
        public void onConnectionFailed(ConnectionResult arg0) {
            // TODO Auto-generated method stub

        }
    });
}

@Override
protected void onStop() {
    super.onStop();

    locationService.stop();
}

最后，LocationsCoreModule：

public class LocationsCoreModule implements
        GooglePlayServicesClient.ConnectionCallbacks {

    private static final String TAG = "LocationsCoreModule";

    public interface LocationsCoreModuleCallback {
        public void locationClientConnected();
    }

    private LocationsCoreModuleCallback locationsCoreModuleCallback;

    private com.google.android.gms.location.LocationListener locationListener;

    private Location lastLocation;

    private LocationClient mLocationClient;

    private final LocationRequest mLocationRequest;
    private final Context context;


    @Inject 
    public LocationsCoreModule(Context context, LocationRequest locationRequest) {
        this.context = context;
        this.mLocationRequest = locationRequest;


    }

    public void setLastLocation(Location lastLocation) {
        this.lastLocation = lastLocation;
    }

    public void setLocationsListener(
            LocationsCoreModuleCallback locationsCoreModuleCallback) {
        this.locationsCoreModuleCallback = locationsCoreModuleCallback;
    }

    public void start(
            com.google.android.gms.location.LocationListener locationListener,
            GooglePlayServicesClient.OnConnectionFailedListener connectionFailedListener) {
        this.locationListener = locationListener;
        mLocationClient = new LocationClient(context, this,
                connectionFailedListener);
        mLocationClient.connect();
    }

    public void stop() {
        if (mLocationClient != null) {
            // If the client is connected
            if (mLocationClient.isConnected() && locationListener != null) {
                /*
                 * Remove location updates for a listener. The current Activity
                 * is the listener, so the argument is "this".
                 */
                mLocationClient.removeLocationUpdates(locationListener);
            }

            // Disconnecting the client invalidates it.
            mLocationClient.disconnect();
        }
    }


    public boolean isConnected() {
        if (mLocationClient == null) return false;
        return mLocationClient.isConnected();
    }

    public Location getLastLocation() {

        if (lastLocation != null) {
            return lastLocation;
        }

        if (mLocationClient != null) {
            if (mLocationClient.isConnected()) {
                return lastLocation = mLocationClient.getLastLocation();
            }

            if (!mLocationClient.isConnecting())
                mLocationClient.connect();
        }

        return null;

    }

    @Override
    public void onConnected(Bundle connectionHint) {
        Log.d(TAG, "GooglePlayServices connected!");

        Location lastLocation = mLocationClient.getLastLocation();

        if (lastLocation == null)
            Log.e(TAG, "Lastlocation is null even after connected!!!!");

        if (locationsCoreModuleCallback != null) {
            locationsCoreModuleCallback.locationClientConnected();
            locationsCoreModuleCallback = null; // single shot
        }

    }

    public void requestLocationUpdates() {
        if (mLocationClient != null && mLocationClient.isConnected()) {
            Log.d(TAG, "Requesting location updates");
            mLocationClient.requestLocationUpdates(mLocationRequest,
                    locationListener);
        }
    }

    public void stopLoactionUpdates() {
        if (mLocationClient != null && mLocationClient.isConnected()) {
            mLocationClient.removeLocationUpdates(locationListener);
        }
    }

    @Override
    public void onDisconnected() {
        Log.d(TAG, "GooglePlayServices disconnected!");
    }

}