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使用Google Play服务和FusedLocationProviderClient进行位置更新

androidgoogle-play-servicesandroid-locationfusedlocationproviderapigoogle-location-services
7
我希望使用最新的融合位置提供程序客户端在“后台服务”上获取位置更新。我不想使用所有使用位置监听器和Google API客户端的功能。我还需要使用由Google Play服务提供的位置设置API来检查该“后台服务”上的位置设置是否被禁用或启用。请帮忙。
1
你目前尝试了什么，卡在哪里了？ - azizbekian
我已经使用位置监听器获取了位置更新 - LocationServices.FusedLocationApi.requestLocationUpdates(mGoogleApiClient, mLocationRequest, locationListener); 但是我想按照谷歌的建议使用以下代码 - mFusedLocationClient.requestLocationUpdates(mLocationRequest, mLocationCallback, callback)。 - Akash Bisariya
1
这里的实际问题是什么？你尝试过什么？ 如果你的应用程序针对Marshmallow及以上版本，你需要显示权限屏幕，没有其他方式，这是Android权限策略。 - Dus
我在谈论位置设置回调，而不是权限 @Dus - Akash Bisariya
你的问题非常模糊。你能否重新构思你的问题，明确表达你想要做什么以及你已经完成了什么？因为每个评论你帖子的人都不理解你想要什么。 - Aman Jain
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5个回答
4
如果我们需要使用融合位置API，则需要使用Google API客户端。对于后台服务来说，使用它并不是问题。以下是我用于在后台获取位置更新的示例，但有一件事情就是自从6.0版本以后谷歌推出了“Doze”模式，所以我不能保证在你的设备处于“Doze”模式时能够成功更新。
import android.Manifest;
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.location.Location;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.os.IBinder;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;
import android.util.Log;

import com.google.android.gms.common.ConnectionResult;
import com.google.android.gms.common.GoogleApiAvailability;
import com.google.android.gms.common.api.GoogleApiClient;
import com.google.android.gms.location.LocationListener;
import com.google.android.gms.location.LocationRequest;
import com.google.android.gms.location.LocationServices;

/**
 * Created By Dhaval Solanki
 */

public class ConnectionService extends Service implements GoogleApiClient.ConnectionCallbacks, GoogleApiClient.OnConnectionFailedListener, LocationListener {
    String TAG = "ConnectionService";


    private GoogleApiClient mGoogleApiClient;
    private LocationRequest mLocationRequest;
    private Location previousLocation;
    public static final long UPDATE_INTERVAL_IN_MILLISECONDS = 30000;
    public static final long FASTEST_UPDATE_INTERVAL_IN_MILLISECONDS =
            UPDATE_INTERVAL_IN_MILLISECONDS / 2;


    private Location mLastLocation;
    private Context context;

    @Override
    public IBinder onBind(Intent intent) {
        Log.i(TAG, "onBind");
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        context = getApplicationContext();
        Log.i(TAG, "onStartCommand");
        if (checkPermsion(context)) {
            setupLocationService(context);
        }
        return Service.START_STICKY;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        ApplicationCLass.isServiceRunning = true;
    }

    @Override
    public boolean onUnbind(Intent intent) {
        Log.i(TAG, "onUnbind");
        ApplicationCLass.isServiceRunning = false;
        return super.onUnbind(intent);
    }

    @Override
    public void onDestroy() {
        ApplicationCLass.isServiceRunning = false;
        super.onDestroy();
    }

    private void setupLocationService(Context context) {
        if (checkPlayServices()) {
            mGoogleApiClient = new GoogleApiClient.Builder(context)
                    .addApi(LocationServices.API)
                    .addConnectionCallbacks(this)
                    .addOnConnectionFailedListener(this)
                    .build();
            createLocationRequest();
        }
    }

    protected void createLocationRequest() {
        mLocationRequest = new LocationRequest().create();
        mLocationRequest.setInterval(UPDATE_INTERVAL_IN_MILLISECONDS);
        mLocationRequest.setFastestInterval(FASTEST_UPDATE_INTERVAL_IN_MILLISECONDS);
        mLocationRequest.setPriority(LocationRequest.PRIORITY_BALANCED_POWER_ACCURACY);
        mGoogleApiClient.connect();
    }

    public boolean checkPermsion(Context context) {
        int MyVersion = Build.VERSION.SDK_INT;
        if (MyVersion > Build.VERSION_CODES.LOLLIPOP_MR1) {
            if (ContextCompat.checkSelfPermission(context, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
                return false;
            } else if (ContextCompat.checkSelfPermission(context, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
                return false;
            } else {
                return true;
            }
        } else {
            return true;
        }
    }

    private boolean checkPlayServices() {
        GoogleApiAvailability googleAPI = GoogleApiAvailability.getInstance();
        int result = googleAPI.isGooglePlayServicesAvailable(this);
        if (result != ConnectionResult.SUCCESS) {
            return false;
        }
        return true;
    }


    private void startLocationUpdates() {
        if (mGoogleApiClient.isConnected()) {
            if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
                return;
            }
            if (Prefs.getUserLat(context).trim().length() > 0 && Prefs.getUserLong(context).trim().length() > 0) {
            } else {
                mLastLocation = LocationServices.FusedLocationApi.getLastLocation(
                        mGoogleApiClient);
                if (mLastLocation != null) {
                    Prefs.setUserLat(context, String.valueOf(mLastLocation.getLatitude()));
                    Prefs.setUserLong(context, String.valueOf(mLastLocation.getLongitude()));
                }
            }
            LocationServices.FusedLocationApi.requestLocationUpdates(
                    mGoogleApiClient, mLocationRequest, this);
        }
    }

    @Override
    public void onConnected(@Nullable Bundle bundle) {
        Log.i(TAG, "Connected to onConnected");
        startLocationUpdates();
    }

    @Override
    public void onConnectionSuspended(int i) {
        Log.i(TAG, "Connected to onConnectionSuspended");
    }

    @Override
    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {
        Log.i(TAG, "Connected to onConnectionFailed");
    }

    @Override
    public void onLocationChanged(Location location) {
        try {
            Logger.print("onLocationChanged", "latitued :" + location.getLatitude() + " ,, Longitude " + location.getLongitude());
        } catch (Exception e) {
            e.printStackTrace();
        }

    }


}

检查另一个警报服务是否正在运行

import android.app.ActivityManager;
import android.app.IntentService;
import android.content.Context;
import android.content.Intent;
import android.util.Log;

public class AlarmService extends IntentService {
    @Override
    protected void onHandleIntent(Intent intent) {
        boolean isServiceRunning = false;
        ActivityManager manager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
        for (ActivityManager.RunningServiceInfo service : manager.getRunningServices(Integer.MAX_VALUE)) {
            if (ConnectionService.class.getName().equals(service.service.getClassName())) {
                isServiceRunning = true;
                break;
            }
        }
        Log.i("AlarmService", "Service is running " + isServiceRunning);
        if(!isServiceRunning) {
            startService(new Intent(getApplicationContext(),ConnectionService.class));
        } else {
            ConnectionService.checkConnectionAndConnect(getApplicationContext());
        }
    }

    /**
     * Creates an IntentService.  Invoked by your subclass's constructor.
     */
    public AlarmService() {
        super("AlaramService");
    }
}

最后启动服务

private void checkAndStartService() {
        final ActivityManager manager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
        new AsyncTask<Void, Void, Boolean>() {
            boolean isServiceRunning = false;

            @Override
            protected Boolean doInBackground(Void... params) {
                for (ActivityManager.RunningServiceInfo service : manager.getRunningServices(Integer.MAX_VALUE)) {
                    if (ConnectionService.class.getName().equals(service.service.getClassName())) {
                        isServiceRunning = true;
                        break;
                    }
                }
                return isServiceRunning;
            }

            @Override
            protected void onPostExecute(Boolean aBoolean) {
                super.onPostExecute(aBoolean);
                Log.i("onPostExecute", "Service running = " + aBoolean);
                if (!aBoolean) {
                    startService(new Intent(ActivitySplash.this, ConnectionService.class));
                    Intent i = new Intent(ActivitySplash.this, AlarmService.class);
                    PendingIntent pi = PendingIntent.getService(ActivitySplash.this, 0, i, 0);
                    AlarmManager am = (AlarmManager) ActivitySplash.this.getSystemService(Context.ALARM_SERVICE);
                    am.cancel(pi); // cancel any existing alarms
                    am.setRepeating(AlarmManager.RTC_WAKEUP, System.currentTimeMillis(), 60000, pi);
                }
            }
        }.execute();
    }
只有一个问题，如果用户在您的服务正在运行时关闭了GPS，那么您如何获取位置更新@Dhaval Solanki - Akash Bisariya
在切换后，无法进入待机模式，也就是说只是锁定了手机。 - Dhaval Solanki
如果您关闭并重新启动开关，则需要重新启动服务，您可以通过使用启动完成接收器来实现。 - Dhaval Solanki
那么我们如何向用户请求位置设置，并从服务中获取该位置设置的回调。 - Akash Bisariya
答案中提到的“Prefs”类是什么？ - Brady Sheehan
这只是一个偏好设置自定义类，您可以使用自己的类来存储数据。 - Dhaval Solanki
1
您可以扩展服务类，如下所示：

public class BackgroundLocationService extends Service implements
GooglePlayServicesClient.ConnectionCallbacks,
GooglePlayServicesClient.OnConnectionFailedListener,
LocationListener

你可以在这里查看完整的源代码 BackgroundLocationService.java 为了启用位置设置
   private void displayLocationSettingsRequest() {
    GoogleApiClient googleApiClient = new GoogleApiClient.Builder(context)
            .addApi(LocationServices.API).build();
    googleApiClient.connect();

    LocationRequest locationRequest = LocationRequest.create();
    locationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
    locationRequest.setInterval(10000);
    locationRequest.setFastestInterval(10000 / 2);

    LocationSettingsRequest.Builder builder = new LocationSettingsRequest.Builder().addLocationRequest(locationRequest);
    builder.setAlwaysShow(true);

    PendingResult<LocationSettingsResult> result = LocationServices.SettingsApi.checkLocationSettings(googleApiClient, builder.build());
    result.setResultCallback(new ResultCallback<LocationSettingsResult>() {
        @Override
        public void onResult(@NonNull LocationSettingsResult result) {
            final Status status = result.getStatus();
            switch (status.getStatusCode()) {
                case LocationSettingsStatusCodes.SUCCESS:
                    break;
                case LocationSettingsStatusCodes.RESOLUTION_REQUIRED:

                    try {
                        // Show the dialog by calling startResolutionForResult(), and check the result
                        // in onActivityResult().
                        status.startResolutionForResult(context, REQUEST_CHECK_SETTINGS);
                    } catch (IntentSender.SendIntentException ignored) {
                    }
                    break;
                case LocationSettingsStatusCodes.SETTINGS_CHANGE_UNAVAILABLE:
                    break;
            }
        }
    });
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    switch (requestCode) {
   // Check for the integer request code originally supplied to 
    startResolutionForResult().
        case REQUEST_CHECK_SETTINGS:
            switch (resultCode) {
                case Activity.RESULT_OK:
                    //startLocationUpdates();
                    break;
                case Activity.RESULT_CANCELED:
                    displayLocationSettingsRequest();//keep asking
                    showToast("Location permission needed");
                    break;
            }
            break;
    }
}

您可以从服务中检查位置设置，如果位置关闭，

LocationManager lm = (LocationManager)context.getSystemService(Context.LOCATION_SERVICE);
  boolean gps_enabled = false;
      boolean network_enabled = false;

   try {
gps_enabled = lm.isProviderEnabled(LocationManager.GPS_PROVIDER);
 } catch(Exception ex) {}

   try {
network_enabled = lm.isProviderEnabled(LocationManager.NETWORK_PROVIDER);
  } catch(Exception ex) {}

   if(!gps_enabled && !network_enabled) {
// notify user
 // Either you can display a Notification(Recommended way) or you can show dialog with 

dialog.getWindow().setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);
//It needs SYSTEM_ALERT_WINDOW permission. Add this permission in Manifest.

   <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>     

 }
你正在检查一个活动的设置，我的问题是当没有前台活动时如何检查服务的设置。 - Akash Bisariya
在哪里可以获取位置设置窗口的结果，以便在位置设置开启时请求位置更新？@Ameer - Akash Bisariya
1

您可以按照以下方式编写IntentService的代码。考虑到它在后台运行，我们不会显示权限对话框。

public class GetLocationService extends IntentService
    implements
    GoogleApiClient.ConnectionCallbacks,
    GoogleApiClient.OnConnectionFailedListener, LocationListener {
public static String TAG = GetLocationService.class.getSimpleName();
private GoogleApiClient mGoogleApiClient;


public GetLocationService() {
    super(TAG);
}

/**
 * Creates an IntentService.  Invoked by your subclass's constructor.
 *
 * @param name Used to name the worker thread, important only for debugging.
 */
public GetLocationService(String name) {
    super(name);
}

@Override
protected void onHandleIntent(Intent intent) {
    if (mGoogleApiClient != null && mGoogleApiClient.isConnected()) {
        Log.d(TAG, "disconnecting");
        mGoogleApiClient.disconnect();
    }
    Log.d(TAG, "onHandleIntent");
    mGoogleApiClient = new GoogleApiClient.Builder(this)
            .addApi(LocationServices.API)
            .addConnectionCallbacks(this)
            .addOnConnectionFailedListener(this)
            .build();
    mGoogleApiClient.connect();
}

@Override
public void onConnected(@Nullable Bundle bundle) {
    LocationRequest locationRequest = LocationRequest.create();
    locationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);

    LocationRequest locationRequestLow = LocationRequest.create();
    locationRequestLow.setPriority(LocationRequest.PRIORITY_LOW_POWER);
    if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) ==
            PackageManager.PERMISSION_GRANTED) {
        // TODO: Consider calling
        //    ActivityCompat#requestPermissions
        // here to request the missing permissions, and then overriding
        //   public void onRequestPermissionsResult(int requestCode, String[] permissions,
        //                                          int[] grantResults)
        // to handle the case where the user grants the permission. See the documentation
        // for ActivityCompat#requestPermissions for more details.
        LocationServices.FusedLocationApi.requestLocationUpdates(mGoogleApiClient, locationRequest, this);
    } else {
        Log.d(TAG, "permission denied");
    }
}

@Override
public void onConnectionSuspended(int i) {

}

@Override
public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {

}

@Override
public void onLocationChanged(Location location) {
    Log.d(TAG, location.toString());    
}

}

1

你好，我对位置服务及其完美使用有很好的理解。下面我会给你介绍融合API的想法。

请确保按照以下步骤操作。这很不错，也很简单。 第一步。创建这个类GoogleLocationService.java

public class GoogleLocationService {
private GoogleServicesCallbacks callbacks = new GoogleServicesCallbacks();
LocationUpdateListener locationUpdateListener;
Context activity;
protected GoogleApiClient mGoogleApiClient;
protected LocationRequest mLocationRequest;

public static final long UPDATE_INTERVAL_IN_MILLISECONDS = 30000;


public GoogleLocationService(Context activity, LocationUpdateListener locationUpdateListener) {
    this.locationUpdateListener = locationUpdateListener;
    this.activity = activity;
    buildGoogleApiClient();
}

protected synchronized void buildGoogleApiClient() {
    //Log.i(TAG, "Building GoogleApiClient");
    mGoogleApiClient = new GoogleApiClient.Builder(activity)
            .addConnectionCallbacks(callbacks)
            .addOnConnectionFailedListener(callbacks)
            .addApi(LocationServices.API)
            .build();
    createLocationRequest();
    mGoogleApiClient.connect();
}

protected void createLocationRequest() {
    mLocationRequest = new LocationRequest();
    mLocationRequest.setInterval(UPDATE_INTERVAL_IN_MILLISECONDS);
    mLocationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);

}

private class GoogleServicesCallbacks implements GoogleApiClient.ConnectionCallbacks, GoogleApiClient.OnConnectionFailedListener, LocationListener {

    @Override
    public void onConnected(Bundle bundle) {
        startLocationUpdates();
    }

    @Override
    public void onConnectionSuspended(int i) {
        mGoogleApiClient.connect();
    }

    @Override
    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {

        if (connectionResult.getErrorCode() == ConnectionResult.SERVICE_VERSION_UPDATE_REQUIRED) {
            Toast.makeText(activity, "Google play service not updated", Toast.LENGTH_LONG).show();

        }
        locationUpdateListener.cannotReceiveLocationUpdates();
    }

    @Override
    public void onLocationChanged(Location location) {
        if (location.hasAccuracy()) {
            if (location.getAccuracy() < 30) {
                locationUpdateListener.updateLocation(location);
            }
        }
    }
}

private static boolean locationEnabled(Context context) {
    boolean gps_enabled = false;
    LocationManager lm = (LocationManager) context.getSystemService(Context.LOCATION_SERVICE);
    try {
        gps_enabled = lm.isProviderEnabled(LocationManager.GPS_PROVIDER);
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    return gps_enabled;
}

private boolean servicesConnected(Context context) {
    return isPackageInstalled(GooglePlayServicesUtil.GOOGLE_PLAY_STORE_PACKAGE, context);
}

private boolean isPackageInstalled(String packagename, Context context) {
    PackageManager pm = context.getPackageManager();
    try {
        pm.getPackageInfo(packagename, PackageManager.GET_ACTIVITIES);
        return true;
    } catch (PackageManager.NameNotFoundException e) {
        e.printStackTrace();
        return false;
    }
}


public void startUpdates() {
    /*
     * Connect the client. Don't re-start any requests here; instead, wait
     * for onResume()
     */
    if (servicesConnected(activity)) {
        if (locationEnabled(activity)) {
            locationUpdateListener.canReceiveLocationUpdates();
            startLocationUpdates();
        } else {
            locationUpdateListener.cannotReceiveLocationUpdates();
            Toast.makeText(activity, "Unable to get your location.Please turn on your device Gps", Toast.LENGTH_LONG).show();
        }
    } else {
        locationUpdateListener.cannotReceiveLocationUpdates();
        Toast.makeText(activity, "Google play service not available", Toast.LENGTH_LONG).show();
    }
}

//stop location updates
public void stopUpdates() {
    stopLocationUpdates();
}

//start location updates
private void startLocationUpdates() {

    if (checkSelfPermission(activity, ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && checkSelfPermission(activity, ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
        return;
    }
    if (mGoogleApiClient.isConnected()) {
        LocationServices.FusedLocationApi.requestLocationUpdates(mGoogleApiClient, mLocationRequest, callbacks);
    }
}

public void stopLocationUpdates() {
    if (mGoogleApiClient.isConnected()) {
        LocationServices.FusedLocationApi.removeLocationUpdates(mGoogleApiClient, callbacks);
    }
}

public void startGoogleApi() {
    mGoogleApiClient.connect();
}

public void closeGoogleApi() {
    mGoogleApiClient.disconnect();
}

 }

步骤2. 将此界面制作为LocationUpdateListener.java

 public interface LocationUpdateListener {

/**
 * Called immediately the service starts if the service can obtain location
 */
void canReceiveLocationUpdates();

/**
 * Called immediately the service tries to start if it cannot obtain location - eg the user has disabled wireless and
 */
void cannotReceiveLocationUpdates();

/**
 * Called whenever the location has changed (at least non-trivially)
 * @param location
 */
void updateLocation(Location location);

/**
 * Called when GoogleLocationServices detects that the device has moved to a new location.
 * @param localityName The name of the locality (somewhere below street but above area).
 */
void updateLocationName(String localityName, Location location);
}

第三步：创建这个位置服务类LocationService.java

public class LocationService extends Service {

private GoogleLocationService googleLocationService;

@Override
public void onCreate() {
    super.onCreate();
    //start the handler for getting locations
    //create component

    updateLocation(getApplicationContext());
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    return Service.START_STICKY;
}

//get current location os user
private void updateLocation(Context context) {
    googleLocationService = new GoogleLocationService(context, new LocationUpdateListener() {
        @Override
        public void canReceiveLocationUpdates() {
        }

        @Override
        public void cannotReceiveLocationUpdates() {
        }

        //update location to our servers for tracking purpose
        @Override
        public void updateLocation(Location location) {
            if (location != null ) {
                Timber.e("updated location %1$s %2$s", location.getLatitude(), location.getLongitude());

            }
        }

        @Override
        public void updateLocationName(String localityName, Location location) {

            googleLocationService.stopLocationUpdates();
        }
    });
    googleLocationService.startUpdates();
}


IBinder mBinder = new LocalBinder();


public class LocalBinder extends Binder {
    public LocationService getServerInstance() {
        return LocationService.this;
    }
}

@Override
public IBinder onBind(Intent intent) {
    return mBinder;
}

//stop location updates on stopping the service
@Override
public void onDestroy() {
    super.onDestroy();
    if (googleLocationService != null) {
        googleLocationService.stopLocationUpdates();
    }
}
}

编辑后的答案：

如何使用服务：

在您的主活动中启动服务，像这样：

startService(new Intent(context, LocationService.class));

停止循环，

stopService(new Intent(context, LocationService.class));

并在清单文件中声明如下：
<service
 android:name=".service.location.LocationService"
 android:enabled="true"></service>

注意: 我之所以这样做，是因为我需要在应用程序关闭后获取位置信息。如果您不需要服务，请直接在需要更新位置的类中调用下面的代码并删除位置服务。

 private GoogleLocationService googleLocationService;

 googleLocationService = new GoogleLocationService(context, new LocationUpdateListener() {
    @Override
    public void canReceiveLocationUpdates() {
    }

    @Override
    public void cannotReceiveLocationUpdates() {
    }

    //update location to our servers for tracking purpose
    @Override
    public void updateLocation(Location location) {
        if (location != null ) {
            Timber.e("updated location %1$s %2$s", location.getLatitude(), location.getLongitude());

        }
    }

    @Override
    public void updateLocationName(String localityName, Location location) {

        googleLocationService.stopLocationUpdates();
    }
});
googleLocationService.startUpdates();


and call this onDestroy 
if (googleLocationService != null) {
    googleLocationService.stopLocationUpdates();
}

记得在清单文件中声明Locationservice。我认为这是最好的解决方案。谢谢，希望能对你有所帮助。
0

这个问题似乎不是很具体。看起来你在使用位置服务方面遇到了麻烦。

最好的想法可能是查看谷歌示例（https://github.com/googlesamples/android-play-location），以更好地理解机制。我可能会从实现最简单的示例（LocationUpdates）开始。它使用FusedLocationProvider并检查设置是否启用，因此应该符合您的需求，或者至少让您入门。

如果您确实需要使用服务，可以查看LocationUpdatesForegroundService或LocationUpdatesPendingIntent。但我强烈建议在执行此操作之前先阅读和理解第一个示例。

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