onCellInfoChanged() callback is always null

i am trying to get a list of all available cells the device can find. But i am stuck, as my CellInfo is always null and i don't figure why. Can someone give me a hint? There is pretty few info on onCellInfoChanged() at google.

MainActivity:

 CellListener cellListener = new CellListener(this);
 cellListener.start();

CellListener:

public class CellListener extends PhoneStateListener {

private static final String TAG = "CellListener";
private TelephonyManager telephonyManager = null;
private PhoneStateListener listener = null;
private String newCell = null;  
private int events = PhoneStateListener.LISTEN_CELL_LOCATION | PhoneStateListener.LISTEN_CELL_INFO;
private Context context = null;

public CellListener(Context context) {
    this.context = context;
}

public void start() {

    telephonyManager = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
    CellLocation.requestLocationUpdate();

    telephonyManager.listen(this, events);
}

@Override
public void onCellInfoChanged(List<CellInfo> cellInfo) {
    Log.i("CellListener","onCellInfoChanged(List<CellInfo> cellInfo) ");
    super.onCellInfoChanged(cellInfo);

     if(cellInfo == null) return;     // this always null here

     for (CellInfo c : cellInfo) {          
        Log.i("CellListener"," c = "+c);
    }       
 }

 @Override
    public void onCellLocationChanged(CellLocation location) {
        if (!(location instanceof GsmCellLocation)) {
            return;
        }
        GsmCellLocation gsmCell = (GsmCellLocation) location;
        String operator = telephonyManager.getNetworkOperator();
        if (operator == null || operator.length() < 4) {
            return;
        }
        newCell = operator.substring(0, 3) + ':' + operator.substring(3) + ':'
                + gsmCell.getLac() + ':' + gsmCell.getCid();

        Log.i(TAG,"newCell = "+newCell);     
    }
}

Logcat:

11-18 14:50:23.806: I/CellListener(4953): newCell = 262:02:4311:99031735
11-18 14:50:23.814: I/CellListener(4953): onCellInfoChanged(List<CellInfo> cellInfo)

As you can see both events (onCellInfoChanged & onCellLocationChanged) get triggered once and the latter is correctly returning the current cell the device is using.

Solution

The real reason you are only being called exactly once and with null as an argument is the following: there appears to be a rate-limiting setting in place by default, as can be observed in the "testing" app, which can be accessed by dialing *#*#INFO#*#* (i.e. *#*#4636#*#*).

In the testing app, choose "Phone Information" and scroll down to the button CELLINFOLISTRATE xxxx, in your case presumably CELLINFOLISTRATE 2147483647. As 2147483647 == MAX_INT, this probably means no calls at all

For me (stock android 6.0, nexus 6), there's the a choice between MAX_INT (one call with null), 0 and 1000.

I'm not 100% sure what these values mean, but presumably the 0 stands for instant (and thus very many) calls, and 1000 for something like at least a second between calls. Keep in mind though, this is pure speculation.

I will edit this answer as I find out more, for example by looking at the implementation of said testing app.