我需要定期执行Wifi扫描.当时间间隔设置为1-2秒时,我遇到了问题.好像我没有得到任何ScanResult.是否有最短的时间设置,以便WifiManager能够执行成功的WiFi扫描？
这是代码.我正在使用服务进行Wifi扫描：
public class WifiScanning extends Service{
private static final String TAG = "WifiScanning";
private Timer timer;
public int refreshRate,numberOfWifiScan,wifiScanGranularity;
WifiReceiver receiverWifi = new WifiReceiver();
WifiManager wifi;
StringBuilder sb;
List<ScanResult> wifiList;
List<APData> apdataList;
List<List<APData>>surveyData;
private TimerTask updateTask = new TimerTask() {
@Override
public void run() {
Log.i(TAG,"Timer task doing work");
wifi.startScan();
}
};
@Override
public IBinder onBind(Intent intent) {
// TODO Auto-generated method stub
return null;
}
@Override
public void onCreate() {
super.onCreate();
Log.i(TAG,"Service creating");
//retrieve the mapRefreshRate from config.xml
XMLOperations test = new XMLOperations();
Configuration config = new Configuration();
config = test.saxXmlParsing(this,1);
if(config==null)
config = test.saxXmlParsing(this,2);
refreshRate = Integer.parseInt(config.getMapRefreshRate());
numberOfWifiScan = Integer.parseInt(config.getNumberOfWifiScan_Positioning());
wifiScanGranularity = Integer.parseInt(config.getWifiScanGranularity_Positioning());
timer = new Timer();
Log.i(TAG,"Refresh Rate: "+ String.valueOf(refreshRate));
timer.schedule(updateTask,refreshRate);
wifi = (WifiManager) getSystemService(Context.WIFI_SERVICE);
registerReceiver(receiverWifi,new IntentFilter(
WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
}
@Override
public void onDestroy() {
super.onDestroy();
Log.i(TAG,"Service destroying");
unregisterReceiver(receiverWifi);
if (timer != null){
timer.cancel();
timer.purge();
timer = null;
}
}
class WifiReceiver extends BroadcastReceiver {
public void onReceive(Context c,Intent intent) {
sb = new StringBuilder();
wifiList = wifi.getScanResults();
String ap_ssid;
String ap_mac;
Double ap_RSSi;
for(int i = 0; i < wifiList.size(); i++){
ap_ssid = wifiList.get(i).SSID;
ap_mac = wifiList.get(i).BSSID;
ap_RSSi = Double.valueOf(wifiList.get(i).level);
APData ap = new APData(ap_ssid,ap_mac,ap_RSSi);
apdataList.add(ap);
sb.append(" " + (wifiList.get(i).SSID).toString());
sb.append(" " + (wifiList.get(i).BSSID).toString());
sb.append((" " + String.valueOf(wifiList.get(i).level)));
sb.append("\n");
}
Log.d(TAG,sb.toString());
for(int i=1; i<=numberOfWifiScan; i++){
surveyData.add(apdataList);
}
}
}
}
但是,我似乎在这一行得到Nullpointer：apdataList.add(ap);.所以我想知道间隔是否太短,导致ScanResult为空？
解决方法
发布代码后编辑：
apdataList似乎没有在onCreate()中初始化
将此添加到onCreate()：
apdataList = new List<APData>();
最小扫描延迟
我认为没有绝对的最小扫描延迟.这在很大程度上取决于硬件性能.
我的建议是,在首选项中添加“尽可能快”选项,然后使用异步循环,一旦找到新结果就重新启动扫描(请参阅下面的代码片段,它已根据您的需要进行了更新).这样,它只受硬件性能的限制.
您也可以使用WifiManager.getScanResults()轮询ScanResults建议的方法是启动WifiManager.startScan()并为WifiManager.SCAN_RESULTS_AVAILABLE_ACTION设置BroadcastReceiver,以便在扫描结果准备好后立即通知.
这是一个示例代码(借用from here并根据您的需要调整)：
IntentFilter i = new IntentFilter();
i.addAction (WifiManager.SCAN_RESULTS_AVAILABLE_ACTION);
registerReceiver(new BroadcastReceiver(){
public void onReceive(Context c,Intent i){
// Code to execute when SCAN_RESULTS_AVAILABLE_ACTION event occurs
WifiManager w = (WifiManager) c.getApplicationContext().getSystemService(Context.WIFI_SERVICE); //Use getApplicationContext to prevent memory leak
myScanResultHandler(w.getScanResults()); // your method to handle Scan results
if (ScanAsFastAsPossible) w.startScan(); // relaunch scan immediately
else { /* Schedule the scan to be run later here */}
}
},i );
// Launch wifiscanner the first time here (it will call the broadcast receiver above)
WifiManager wm = (WifiManager)getApplicationContext.getSystemService(Context.WIFI_SERVICE);
boolean a = wm.startScan();
