Android应用中怎么监听手机GPS的打开状态

package com.yiba.core;import android.content.BroadcastReceiver;import android.content.Context;import android.content.Intent;import android.content.IntentFilter;import android.location.LocationManager;public class GPS_Presenter {  private Context mContext ;  private Receiver receiver ;  private GPS_Interface mInterface ;  private String GPS_ACTION = "android.location.PROVIDERS_CHANGED" ;  public GPS_Presenter(Context context , GPS_Interface mInterface ){    this.mContext = context ;    this.mInterface = mInterface ;    observeWifiSwitch();  }  private void observeWifiSwitch(){    IntentFilter filter = new IntentFilter();    filter.addAction( GPS_ACTION );    receiver = new Receiver() ;    mContext.registerReceiver(receiver, filter);  }    public void onDestroy(){    if ( receiver != null ){      mContext.unregisterReceiver( receiver );    }    if (mContext!=null){      mContext = null;    }  }  class Receiver extends BroadcastReceiver {    @Override    public void onReceive(Context context, Intent intent) {      if (intent.getAction().matches( GPS_ACTION )) {         if ( mInterface != null ){           mInterface.gpsSwitchState( gpsIsOpen( context ));         }      }    }  }    public boolean gpsIsOpen(final Context context) {    LocationManager locationManager        = (LocationManager) context.getApplicationContext().getSystemService(Context.LOCATION_SERVICE);    // 通过GPS卫星定位，定位级别可以精确到街（通过24颗卫星定位，在室外和空旷的地方定位准确、速度快）    boolean gps = locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER);    // 通过WLAN或移动网络(3G/2G)确定的位置（也称作AGPS，辅助GPS定位。主要用于在室内或遮盖物（建筑群或茂密的深林等）密集的地方定位）    boolean network = locationManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER);    if (gps || network) {      return true;    }    return false;  }}