最近在做一个 Android 项目，需要用到GPS获取位置信息，从 API 查了一下，发现获取位置信息仅需极其简单的一句即可：
getLastKnownLocation(LocationManager.GPS_PROVIDER),
于是高兴地不得了。可是一写进代码里，返回值（Location 类型）居然一直为null..郁闷的不得了。在网上查了好久，发现好多人都和我一样纠结于这个问题上，有人说是因为GPS没打开，也有人说是相关权限没加上..可是我的明明已经在设置里打开，权限自然也加上了。在api上纠结了半天，终于找出原因了，原来要打开GPS其实在于这句：
setTestProviderEnabled("gps",true);
而跟手机上的设置没多大关系（起码在我的手机上测是这样的）。手机上的设置关闭了，这一句照样能打开；而即使手机设置打开了，没这一句也是白搭。与这句对应的是
setTestProviderEnabled("gps",false);
用来关闭GPS.
GPS打开后可以用上面的方法获取Location了吗？还是不可以！确切地说是有时候可以，因为这个函数获取的是上次已经获得的位置信息，设想如果此程序第一次跑，先前并没有获取过位置信息，当然返回值为null了。经仔细查看api，在
requestLocationUpdates (String provider, long minTime, float minDistance, LocationListener listener)
里发现了这样一句话：It may take a while to receive the most recent location. If an immediate location is required, applications may use the getLastKnownLocation(String) method. 因此为了获取位置信息，应该用此方法为manager设置监听器，在监听器中onLocationChanged(Location location)里获取。
测试代码如下：
public void onLocationChanged(Location location)
{
Log.i("onLocationChanged", "come in");
if (location != null)
{
Log.w("Location","Current altitude = "+ location.getAltitude());
Log.w("Location","Current latitude = "+ location.getLatitude());
}
}
经过测试，经过一段时间后可以获取Location（获取时间与minTime、minDistance相关）。还需注意的一个问题是在设置了监听器后，删除监听器之前不能用上面的方法关闭gps，否则会报错。因此关闭gps的方法是
manager.removeUpdates (listener);//listener 即为监听器实例
manager.setTestProviderEnabled("gps",false);
以下是测试代码，所需权限有：
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"></uses-permission>
<uses-permission android:name="android.permission.ACCESS_MOCK_LOCATION"></uses-permission>
<uses-permission android:name="android.permission.UPDATE_DEVICE_STATS"></uses-permission>
2 import android.content.Context;
3 import android.location.Criteria;
4 import android.location.Location;
5 import android.location.LocationListener;
6 import android.location.LocationManager;
7 import android.os.Bundle;
8 import android.util.Log;
9
10 public class audio extends Activity
11 {
12
13 /** Called when the activity is first created. */
14 LocationManager locationManager;
15 LocationListener llistener;
16 String provider;
17
18 public void onCreate(Bundle savedInstanceState)
19 {
20 super .onCreate(savedInstanceState);
21 setContentView(R.layout.main);
22
23 Criteria criteria = new Criteria();
24 criteria.setAccuracy(Criteria.ACCURACY_FINE);
25 criteria.setAltitudeRequired( false );
26 criteria.setBearingRequired( false );
27 criteria.setCostAllowed( true );
28 criteria.setPowerRequirement(Criteria.POWER_LOW);
29 String serviceName = Context.LOCATION_SERVICE;
30 locationManager = (LocationManager) getSystemService(serviceName);
31 locationManager.setTestProviderEnabled( " gps " , true );
32 provider = locationManager.getBestProvider(criteria, true );
33 Log.d( " provider " , provider);
34
35 llistener = new LocationListener() {
36 @Override
37 public void onLocationChanged(Location location)
38 {
39 // TODO Auto-generated method stub
40 Log.i( " onLocationChanged " , " come in " );
41 if (location != null )
42 {
43 Log.w( " Location " , " Current altitude = "
44 + location.getAltitude());
45 Log.w( " Location " , " Current latitude = "
46 + location.getLatitude());
47 }
48 locationManager.removeUpdates( this );
49 locationManager.setTestProviderEnabled(provider, false );
50 }
51
52 @Override
53 public void onProviderDisabled(String provider)
54 {
55 // TODO Auto-generated method stub
56 Log.i( " onProviderDisabled " , " come in " );
57
58 }
59
60 @Override
61 public void onProviderEnabled(String provider)
62 {
63 // TODO Auto-generated method stub
64 Log.i( " onProviderEnabled " , " come in " );
65 }
66
67 @Override
68 public void onStatusChanged(String provider, int status,
69 Bundle extras)
70 {
71 // TODO Auto-generated method stub
72 Log.i( " onStatusChanged " , " come in " );
73
74 }
75
76 };
77 locationManager.requestLocationUpdates(provider, 1000 , ( float ) 1000.0 , llistener);
78 }
79
80 protected void onDestroy()
81 {
82 locationManager.removeUpdates(llistener);
83 locationManager.setTestProviderEnabled(provider, false );
84 super .onDestroy();
85 }
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