Android Wifi Scan - BroadcastReceiver for SCAN_RESULTS_AVAILABLE_ACTION not getting called

问题

Here is my code:

public class FloatWifiManager implements IWifiManager {

    private WifiManager wifiManager;

    private BroadcastReceiver wifiScanReceiver;

    public FloatWifiManager(Context context) {
        ...
        wifiManager = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);

        // Registering Wifi Receiver
        wifiScanReceiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context c, Intent intent) {
                if (intent.getAction().equals(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION)) {
                    // not getting called, only after running app and manually going to the wifi settings in android
                }
            }
        };

        IntentFilter intentFilter = new IntentFilter();
        intentFilter.addAction(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION);
        context.registerReceiver(wifiScanReceiver, intentFilter);
        wifiManager.startScan();
    }

I registered the BroadcastReceiver exactly like I saw in all the examples, and did startScan.

What happens is, the wifi list is changing (for sure, I tested), but onReceive is not called if I just stay in the app.

What makes onReceive finally to be called - is to launch the app, leave it running, and going in the android phone to Settings -> Wifi settings. when going there, all of the sudden the List is updating and onReceive is called.

What's the problem here?

1. Does wifiManager.startScan(); runs the scan only once? or it is a function that keeps listening to incoming "Scan Results"?

2. And obviously, why does the receiver doesn't get called?

回答1:

Yes, startScan() requests only one single scan.

You can get rid of the if (intent.getAction().equals(..)) condition. Anything else seems to be ok.

just to make it clear - my goal to have a receiver that will get called every time the Wifi networks list are changing, without having to click a "start scan" button.

AFAIK it is not possible to get notified whenever any of the wifi networks change. You can only request a scan with startScan - and of course you can call startScan repeatedly using a Thread or Handler.

The docs say that SCAN_RESULTS_AVAILABLE_ACTION is called when "an access point scan has completed, and results are available from the supplicant". How and when a scan is proceeded depends on the implemention of the supplicant. Elenkov writes, that "Android devices rarely include the original wpa_supplicant code; the included implementation is often modified for better compatibility with the underlying SoC".

---

Scan for access points

This example scans for available access points and ad hoc networks. btnScan activates a scan initiated by the WifiManager.startScan() method. After the scan, WifiManager calls the SCAN_RESULTS_AVAILABLE_ACTION intent and the WifiScanReceiver class processes the scan result. The results are displayed in a TextView.

public class MainActivity extends AppCompatActivity {

    private final static String TAG = "MainActivity";

    TextView txtWifiInfo;
    WifiManager wifi;
    WifiScanReceiver wifiReceiver;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        wifi=(WifiManager)getSystemService(Context.WIFI_SERVICE);
        wifiReceiver = new WifiScanReceiver();

        txtWifiInfo = (TextView)findViewById(R.id.txtWifiInfo);
        Button btnScan = (Button)findViewById(R.id.btnScan);
        btnScan.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.i(TAG, "Start scan...");
                wifi.startScan();
            }
        });
    }

    protected void onPause() {
        unregisterReceiver(wifiReceiver);
        super.onPause();
    }

    protected void onResume() {
        registerReceiver(
            wifiReceiver, 
            new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION)
        );
        super.onResume();
    }

    private class WifiScanReceiver extends BroadcastReceiver {
        public void onReceive(Context c, Intent intent) {
            List<ScanResult> wifiScanList = wifi.getScanResults();
            txtWifiInfo.setText("");
            for(int i = 0; i < wifiScanList.size(); i++){
                String info = ((wifiScanList.get(i)).toString());
                txtWifiInfo.append(info+"\n\n");
            }
        }
    }
}

Permissions

The following permissions need to be defined in AndroidManifest.xml:

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

android.permission.ACCESS_WIFI_STATE is necessary for calling WifiManager.getScanResults(). Without android.permission.CHANGE_WIFI_STATE you cannot initiate a scan with WifiManager.startScan().

When compiling the project for api level 23 or greater (Android 6.0 and up), either android.permission.ACCESS_FINE_LOCATION or android.permission.ACCESS_COARSE_LOCATION must be inserted. Furthermore that permission needs to be requested, e.g. in the onCreate method of your main activity:

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    String[] PERMS_INITIAL={
            Manifest.permission.ACCESS_FINE_LOCATION,
    };
    ActivityCompat.requestPermissions(this, PERMS_INITIAL, 127);
}

来源：https://stackoverflow.com/questions/39455722/android-wifi-scan-broadcastreceiver-for-scan-results-available-action-not-gett